A few months ago, we interviewed Bronwyn Barry of Quantum Builders on the Passive House standard. When I called her again to ask about ventilation, she recommended Zehnder, a Swiss company, because their products are Passive House certified and if there’s anyone who knows about ventilation, it’s the Passive House folks. Passive Houses need state-of-the-art ventilation because they rely so heavily on an airtight building envelope, and their stringent energy budgets also mandate the most energy-efficient motors available.

I spoke with Barry Stephens of Zehnder America, the U.S. subsidiary. Zehnder has over 100 projects all over the U.S., including some larger residential complexes. Two-thirds of their projects are Passive House projects, but the company also works with other types of energy retrofits. You can see a great animation of their system here. Barry then referred me to his brother Charlie Stephens of the Northwest Energy Efficiency Alliance, who’s an energy research expert.

We asked both of them some general questions about ventilation principles, along with specific questions for Barry about the Zehnder product line. Most of the discussions here reference the Passive House standard, because it’s so far ahead on ventilation. In fact, if you’re looking for a good ventilation consultant, finding someone with Passive House certification wouldn’t be a bad place to start.

WHY does a home need a ventilation system?

In other words, why is “just opening a window” or “just running the bathroom fan” not enough?

There’s a definite knowledge gap in the residential design and build communities about the need for mechanical ventilation systems. The newest version of California’s energy code mandates whole-house ventilation for new construction based on ASHRAE Standard 62.2, “Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings”. This standard includes some specifics on output, such as the number of cubic feet per minute of air moved. However, Title 24 gives little direction on ventilation system design, installation, or operation. The old theory was that a simple exhaust fan in the bathroom would create negative pressure inside the home which would, in turn, draw fresh air in directly through the walls of the home. In older times, homes were more air-permeable (i.e., drafty) and this was a simple, low-tech solution. Today, however, it’s no longer enough.

Old-school ventilation methods from left to right: bathroom exhaust fan, opening a window, and kitchen range hoods. For today's energy-efficient, airtight homes, these are not enough.

The main reason to ventilate include comfort and health – prevent stale air from building up, and also avoid bringing in outside pollutants such as dust, mold, or pollen. But why the sudden focus on ventilation? Several reasons, among them the following:

• Homes are far more airtight nowadays, leading to a new emphasis on the need for indoor air quality.

• Poor circulation within a home means fresh air doesn’t get to where it’s most needed, like bedrooms. If you’re relying solely on exhaust vented from a bathroom, what happens when doors are closed?

• Exhaust-only ventilation causes pressure imbalance inside the home, not always desirable.

• Air coming in through any old crack in the house means you’re not controlling the source very well. You could be bringing in all sorts of pollutants and dust. Better to choose your intake points and add filtering.

Leaving a window open in a major urban area could be the WORST thing for your indoor air quality, not to mention your peace of mind.

• Drawing air in through the walls introduces the risk of moisture condensation inside the walls, especially if there’s a big temperature differential between outside and inside air.

• Ventilation systems now include heat recovery ventilation units (HRV) so that you’re not continually bringing cold air into the house and then using extra energy to heat it. Instead, heat from the inside air that’s being exhausted outward is transferred directly to the intake air. Essentially, you’re recycling your own heat without sacrificing your fresh air supply.

Thermal imaging shows areas of cold air infiltration. If you're relying on an exhaust fan to draw air through the home for ventilation even in the wintertime, do you really want all this cold air to add to your heating bills?

• Additional features can remove humidity from intake air in climates where that is a concern. One thing to note is that humid air takes more energy to heat or cool than dry air, so managing humidity can save energy, up to 15% on heating in cold climates and up to 45% on cooling in hot, humid climates.

Carbon dioxide buildup creates stuffy rooms and foggy brains

One problem with a poorly ventilated, airtight building is carbon dioxide buildup – stale air. Too much CO2 in the air can cause people to feel sleepy, or worse. This problem doesn’t get as much attention as carbon monoxide poisoning, but it’s still a concern. Carbon dioxide is, of course, a naturally occurring by-product of human respiration. Carbon dioxide levels in fresh air measured in parts per million varies between 0.036% (360 ppm) and 0.039% (390 ppm). It can go higher indoors, perhaps 600-800ppm. Some literature implies that a stuffy auditorium could go as high as 10,000 ppm, which is high enough to cause noticeable drowsiness.

A recent study presented at a Passive House conference showed that carbon dioxide levels in a modern, airtight home could actually accumulate high enough - especially during the night - to make people wake up feeling tired the next morning.

The length of exposure is a factor as well. Barry Stephens mentioned a study he saw presented at a Passive House conference at Brandeis University showing carbon dioxide levels throughout the night inside the bedroom of a modern, airtight home as measured in parts per million. In some of these homes, the CO2 level would rise to 3,000 or even 4,000 PPM by morning, causing occupants to wake up feeling tired.

OSHA’s safety threshold for CO2 level is 5,000 PPM (0.5%) over 8 hours of exposure. At levels over 10,000 or 20,000 ppm more serious symptoms of carbon dioxide poisoning start to appear: headache, lethargy, mental fogginess, confusion, irregular heartbeat, anxiety. Carbon dioxide levels of 100,000 ppm can cause loss of consciousness or even death.

“The correlation between CO2 levels in buildings and health systems in living organisms isn’t well established at this point,” cautioned Charlie Stephens. “[But] it’s the reason why many people have noticed that they sleep better with a window open.”

How do you know if you need a fully pressure-balanced whole-house ventilation system?

Is there some sort of test or threshold for determining whether a home needs the full ventilation treatment? Not yet. A house that passes the California blower door test could potentially benefit from whole-house ventilation. If it is extremely airtight, enough to have under 0.6 air changes per hour (that’s the Passive House standard) it should definitely have one. This still leaves a large gray area of uncertainty.

A pressure balanced ventilation system includes specific controls for both air intake and air exhaust.

Building and energy codes are mandating tighter buildings, increasing the importance of ventilation. Building air infiltration, i.e., air leakage, is usually measured using a blower door test and is often expressed in terms of number of air changes per hour (ACH). So how many ACH does it take to change a ventilation requirement? Well… the U.S. Department of Energy is proposing 5 ACH as the threshold for requiring mechanical ventilation, but since an “average” new home might now be 3 ACH and an “efficient” home might be around 1.5 ACH, all we can say is it’s somewhere in that range.

“There is no defined ACH level that I know of at which ventilation is mandated, or even recommended at this point, because it hasn’t been much of an issue until recently,” said Charlie. “What they would be saying is that for any ACH level below this number, you would be required to have an effective ventilation system, but almost no one knows what an ‘effective’ ventilation system is, except in the case of a properly designed and installed HRV or ERV system.”

And what does ACH measure, anyway?

Air changes per hour is one way to measure the permeability of a building envelope, based on the known physical behavior of gases such as our own air and atmosphere. As we know, gases expand to fill all available space, and this expansiveness creates pressure – air pressure. The more space it has to expand within, the lower the air’s pressure becomes. If there are different air pressures inside vs. outside, air will automatically begin to seep from the higher pressure enclosure into the lower pressure surrounding areas. The higher this differential, the faster the seepage. “The measurements for ACH levels of air leakage are taken at a differential pressure of 50 pascals (0.2 inches water gauge) between the indoors and outdoors,” explained Charlie.

(The measure’s full name is ACH50 – we’re abbreviating.)

Who knew that there were so many ways for air to get in or out of a house?

So what ACH level would be considered “normal”?

Charlie stressed that the proposed 2012 IECC Residential Energy Code upgrades include a provision for a maximum ACH level of 3.0. (Title 24 uses a different calculation, not ACH, and the building must fall within a range and not be TOO tight.) But what is “normal” for ACH as of today? Here are a few random statisics from Northouse and Green Building Advisor:

• An average home’s ACH is 3-3.5 in Alberta and 3.9 in Wisconsin
• The average for new construction in Minnesota is 2.5 ACH
• Older homes might be around 4-9 ACH
• An “efficient” home is around 1.5 ACH

Energy researchers all over the country are wrestling with this question even as we speak, including the Northwest Energy Efficiency Alliance in Portland, OR – Charlie Stephen’s group. “The NEEA ventilation field research will be conducted on recently built homes of recent and reasonably airtight construction during 2011 and 2012,” Charlie informed me. “The monitoring of CO2 levels isn’t so much an end as it is a means to understand how air is moving in a home. It’s essentially a cheap tracer gas.” Even though it’s not the primary purpose of the study, it may help to begin associating CO2 levels with specific ACH numbers.

“We’re also measuring the performance of the mechanical ventilation components (such as fans) installed in the homes, and trying to understand how they’re operated, and whether or not they are effective at providing sufficient ventilation air for the occupants of the home. We’re only looking at homes we consider to be quite airtight – probably 3.0 ACH or lower. We don’t know how many of those we’ll find in our sample set, which is a 4-state sample of existing homes. We’re only looking at 30-50 homes initially. We may look at more after the first phase of the research, depending on what we find. As you suggest, all of our sample homes will have to be blower door-tested to start with.”

But you don’t really know the ACH until you do a blower door test, and you can’t do this until the house is already built.

We have been encouraging our Title 24 clients to plan ahead in the early design stages for things like systems, but how can you plan ahead for something that’s got to be measured in the field after being built? Charlie had an answer, although it still involves measurements taken during construction.

“Blower door tests can be conducted before the home is finished, but not before the air barrier is fully in place. This tends to be after sheetrock, but before interior finish, if the home is new. Many of the sites of air leakage can be accessed from inside the house  in the crawlspace, basement, or attic, and so some corrections are possible even in remodels. But it’s a lot easier in new homes to pay attention to the air barrier for the envelope and to be meticulous about air-sealing as the home is built. Lots of homes are achieving air leakage results under 1.0 ACH50, so it’s not impossible, or even difficult in many cases.”

Do HRV systems make sense in warmer climates, too?

Heat Recovery Ventilation systems make sense in cold climates, but what about hot climates? Actually yes, although in places like Florida you might want an Energy Recovery Ventilation system instead. An ERV will remove some of the humidity from the intake air. Why bother with that? Because it takes more energy to change the temperature of humid air than it does to condition dry air. (It takes more energy to change water temperature than air, and the presence of water in the air changes the air’s thermodynamic properties.)

The difference between the mechanical units is that an ERV uses a membrane that transfers both moisture and heat, while an HRV transfers heat through layers of plastic sheets. As to where exactly to change from one to the other, that’s still up in the air. But you can if you want to; Zehnder units have swappable cores to change from HRV to ERV.

How does the Zehnder ventilation system work in a retrofit?

The Zehnder ventilation system uses smaller, flexible ducts that can be fed through the walls, rather than large metal ducting that requires extensive use of sealing and mastic. Half of Zehnder’s projects are retrofits, usually including other energy features besides heat-recovery ventilation.

One project Barry mentioned was a homeowner named Tad Everhart in Portland. He’s now a Passive House consultant. He did a full Passive House retrofit on his 2,100 SF home. For a Passive House, you need a fully sealed ventilation system with heat recovery, that meets stringent requirements for both energy efficiency and air movement. As Barry Stephens tells it, Everhart was intrigued by Zehnder’s ducting system, and found that he could feed it through the walls just by cutting a small hole. He did the downstairs first using 2 6-tube manifolds, and later did the upstairs the same.

Tad Everhart, a homeowner in Portland, OR, did a full Passive House retrofit on his existing home which also included installing a pressure-balanced, whole-house ventilation system with heat recovery.

Another set of projects that Barry mentioned include a series of gut rehabs of Brooklyn brownstones, where the owners are retrofitting to the Passive House standard. There’s a whole core of Passive House consultants, mainly architects, in the New York area, including Ken Levenson, Jeremy Shannon of Prospect Architecture, and David White of Right Environments. Their work includes both new designs and retrofits.

Tell me more about deep energy retrofits.

“A lot of people start by thinking about the envelope,” said Barry. “They seal it up tight and add insulation, upgrade the windows, maybe re-do their ductwork and heating system. Then they do a blower door test and say, ‘Oh yeah… I guess we have to add ventilation.’”

Barry noted another thing about deep energy retrofits. When you tighten and insulate a building, the heating and cooling loads go way down. In a recent interview, Green Builder Jeff King told us that many HVAC systems are already grossly oversized to begin with. After a retrofit, those systems are REALLY oversized.

You wouldn't need a factory-sized furnace for a tiny cabin, but that's what most home HVAC systems are, essentially - grossly oversized. A furnace that's too big will cycle on and off too frequently, leading to temperature fluctuations rather than maintaining an even, constant temperature.

Can you integrate whole-house heat recovery ventilation with a conventional forced air heating system?

Yes, you can integrate HRV into existing forced-air systems. The existing forced-air heating system does not make whole-house HRV redundant, because the forced-air heating system by itself is just moving air around inside the home, unless you add an extra intake by the furnace to add fresh air. To add a heat-recovery ventilation system in a building with existing forced-air heating, you can install the HRV intake to feed into the furnace return plenum on one end, and to draw exhaust out on the other. This would reduce load on the furnace because it’s no longer having to heat outside air directly. If it’s 20 degrees outside and you add the HRV, then the furnace no longer has to heat up 20-degree air. Instead, it’s getting air that’s already around 65 degrees.

Charlie explained the concept of a hybrid ventilation system that is integrated with an existing forced-air heating system. In this scenario, fresh air would be supplied just before the air handler, in the return duct, while exhaust is through typical bathroom fans. Since this would potentially lead to a pressure imbalance, an additional step is needed. “To create a balanced ventilation system, you’d measure pressure inside the house relative to outdoors, and adjust the capacity of the exhaust fan(s) to match the supply air intake rate at the air handler return. Obviously, the fans would have to be controlled to operate at the same time. And most exhaust fans and air handlers do not have adjustable capacity, so controlling flow is done with dampers – not a particularly efficient way to manage air flow.”

What is the cost of a Zehnder ventilation system, including design, equipment, installation?

For a Zehnder ventilation system, an average 2,500 SF house with heat-recovery, ducting, silencers (a noise abatement option), controls, and installation service would be $6,500 – $7,500. The equipment is around $5,500 and installation another $2,000. Installation takes half the time compared with conventional metal ducting, because it’s smaller, and the connections are simple O-rings and clips rather than requiring extensive use of mastic, sealing, etc. It’s a lot lighter too – one person can carry all the tubing. Barry estimates a typical house project would take 2 people and 2 days.

Note, however, that HVAC subcontractors have bid higher, sometimes $6,000 because they’re unfamiliar with the new technology and that’s how long it might take for them to install a more conventional HVAC system. Barry suggested that you might have to bypass the HVAC guys altogether, or push back to get them down to a more reasonable installation figure.

Can a regular builder or HVAC contractor install a Zehnder ventilation system?

The system isn’t hard to install, in fact it’s lighter and takes less time to install than a conventional forced-air heating system with metal ductwork. Zehnder has been providing the system specifications and design for projects, so all the builder has to do is follow the instructions for installation and commissioning. Builders don’t need to be Zehnder-certified. However, because conventional HVAC contractors have been over-bidding on installation, Barry notes that on many projects it’s the carpenters who’ve been installing the ventilation and ducting, and it’s working fine.

The installers don’t have to design or spec the system. Zehnder has been doing that – taking the plans, doing the layout and specifying the equipment needed.

What does the architect have to do during the design phase to account for a whole-house ventilation system?

Barry/Zehnder: Architects need to plan for the chases. “We can’t just let the HVAC guys figure it out afterwards,” said Barry. Zehnder’s ducts are 3 inches in diameter. For example, consider 5 supplies and 5 returns with 3″ ducts, plus a main intake and a main exhaust going to the outside, each around 6 or 7 inches. You can put the HRV unit in the basement, but don’t forget to plan for chases going to other floors. For example, consider fitting them into stairwells or the back of a closet.

Installation of a Zehnder HRV system in Portland, OR. The components fit within the framing. Photo: Charlie Stephens, Northwest Energy Efficiency Alliance

Barry/Zehnder: One important point is to locate the HRV unit close to an outside wall, to minimize duct length between the air intake and the HRV unit. The 6″ intake duct is surface area that’s exposed to the outside, and although it’s protected, it’s not as well insulated as a solid wall would be. The Passive House standard actually penalizes you for long intake and exhaust runs because they waste energy.

Barry noted a few “gotchas” with new structural systems were noted, particularly laminated veneer lumber beams. You can’t drill through them or thread through them the way you can with a regular truss. This issue comes up sometimes with other structural systems as well, with some materials requiring pre-drilling or fabrication because they can’t be easily modified on site to accommodate last-minute decisions.

These examples of cover grids for room vents are from the Zehnder site. From left to right: floor outlets, wall outlets, ceiling outlets

Does every room need its own intake and exhaust?

No. You can have intake in some rooms and exhaust from others. Barry’s strategy is as follows: get fresh air IN to the bedrooms and living spaces, and get exhaust OUT of bathrooms and kitchens. You want to have constant low-level air movement. This distributes the air quickly and evenly throughout the home.

This diagram from Zehnder America shows one sample schematic of a centralized whole-house ventilation system with pressure balancing and heat recovery. Here, fresh air (blue) is piped in to living spaces and bedrooms while other vents draw out stale air (red) from the bathroom and kitchen.

I’m guessing that the exhaust from the bathroom doesn’t go straight out the window.

The bathroom’s exhaust air doesn’t go straight out the window as with an exhaust fan, but instead that it is drawn back through the central HRV unit, where its heat energy is removed. Only then is the air expelled from the home.

Charlie: All of the air being exhausted by the HRV or ERV leaves the house through the main unit’s HRV or ERV ducting, but almost all of the energy in the air being exhausted is transferred back into the incoming air stream. Thus, the fresh air from the supply intake is only a couple of degrees Fahrenheit different in temperature than the indoor air being exhausted, regardless of the temperature outside. Unlike a conventional heating/cooling system, the air won’t be returning to be recycled inside the home. Instead, it’s being sent outside – exhausted, along with all of the moisture, odors and contaminants in the air.

Is it important to have ducts placed high or low on the wall?

Barry/Zehnder: Where we use register boxes, the supply tends to be low and the return is high. Diffusers can be either.

This diagram from Zehnder America shows how the chases are integrated into a floor.

What’s the maximum recommended duct length for a Zehnder ventilation system?

“I’d say 50 feet, but the number of bends is actually a lot more important. You want to minimize bends. You might be OK with a straight duct that’s as long as 100 feet, but if it’s got several 90-degree bends, the max length might be more like 20 feet,” said Barry.

Minimizing bends and angles in conventional ducting system design is important, because it minimizes air turbulence and maximizes air flow. Left: a very bad example of duct design. Right: this duct design (from another company) includes a fan to assist in moving air around the bends.

How many Zehnder HRV units would you need for a single home?

For Zehnder products in a typical single-family residence, usually just one. It’s based on the total conditioned air volume inside the home, the desired amount of air changes per hour, and the number of people. Sometimes even family pets get counted. Here’s a little known fact – did you know that cats and dogs use more oxygen for their body weight than people? It’s because they breathe faster. If you have a bedroom with two people and two large dogs all sleeping in there, it’s going to collect a lot more CO2.

A heat-recovery ventilation unit from Zehnder. Intake is on one side, exhaust on the other; the heat energy from the exhaust side is transferred to new intake air without actually mixing the air streams.

Can you retrofit a Zehnder ventilation system in a single apartment?

Yes. You can get a smaller unit installed on the wall with vents to the outside. A lot of apartments in Europe are doing this. For an apartment, you don’t need chases in every room, maybe just the main room and the bathroom.

What about maintenance and troubleshooting?

For Zehnder systems, the filters should be changed once a year, and they are externally accessible so the homeowners can do it themselves. Some filters can actually be rinsed with water and reused, but still should be replaced annually. For the HRV unit, you can wash the core with water – you need only a screwdriver to take it off. Ducts can be cleaned, either with a special “roto” type brush or a vacuum. The ducts are smooth with no connectors so it’s easy to brush them out.

Zehnder ventilation system maintenance. Left: removing an air filter. Right: removing the heat exchanger, which can be cleaned with water.

What are the options for filtering?

Zehnder products fall into two main levels, measured using the Minimum Efficiency Reporting Value system, or MERV.  MERV ratings are based on the size of the molecules that are filtered out. The higher the rating, the smaller the molecules that filter can catch. Zehnder filter options are twofold:

• The G4, which is MERV 7-8 – fine for eliminating pollen.
• The F7, which is MERV 13 – best residential dust control; almost good enough for an industrial “clean room”

What about filtering for smog?

Zehnder’s post-filtering solutions include an optional filter box that can go up to MERV 15 and includes charcoal filters. Charcoal is what you want for smog gases such as carbon monoxide or industrial fumes. This box is big, to keep air flowing freely. The filter box is 15-24″ long x 8″ x 10″. Costs $400. Barry noted that filtering outside air won’t necessarily help with dust mites or mold, because those air contaminants are already IN the house. What the filters do is remove outside particulates like pollen, street dust, or soot.

A well-designed home ventilation system can filter out particles that exist in outdoor air, such as diesel soot or pollen. It won't, however, remove contaminants that originate indoors: household dust and such.

Do you recommend installing CO2 sensors too?

Barry/Zehnder: We offer sensors for CO2 levels and relative humidity. They can be tied to our controls so the system automatically adjusts. A karate studio, for example, might have a sensor so that when there are only 2 people in the studio, the system can stay on low – but if there are 25 people working out hard and raising the CO2 level, the sensor automatically kicks the system up to a higher level.

A CO2 monitor like this one shows the concentration of carbon dioxide in the air. Outside air might be 360 parts per million.

All the Zehnder units have a summer bypass feature, too. In the summer, it might be hot in the day but cooler at night. So at night you might want to bypass the HRV to help keep the house cooler. Charlie adds, “the ‘summer bypass’ feature is an automatic one – you push the button to engage the feature, then the HRV decides when to bypass the heat recovery core.”

Does Zehnder provide a commissioning protocol for use by installers?

There is a document on how to test whether the system is properly installed and functioning. Barry recommends using an energy auditor for this, someone who’s got flow hoods, and to have them test room by room for airflow and pressure balance. They can then adjust the air flow at various registers, using what Charlie described as “an ingenious bundle of tubes and washers”.

A Doubting Thomas Chimes In…

Many experienced residential architects will be questioning the utility of requiring yet another system to be incorporated into their home designs. Here we’re posing some questions from the skeptic’s gallery. Readers will have to make up their own minds – we hope we’ve given them enough food for thought, and maybe a few yardsticks, too. The designation of something like “stale air” is pretty vague, but we’ve all been in rooms that were far too stuffy – and if you had the choice, and you’re already spending the money to customize a home, wouldn’t you prefer to be breathing clean air instead?

What about having a bathroom fan with ceiling fans in every room? Won’t that move enough air around?

You’re still dealing with a pressure imbalance, uncontrolled air intake, and if doors between rooms are closed, you’ll still have pockets of stale air in the bedrooms at night – the last place you want it. In homes with radiant heating systems, there isn’t even the air circulation within the home that you would get from a conventional forced-air heating system.

Charlie adds, “in a very airtight house, with all of the windows and doors closed, there’s no guarantee of how much air will come in when the exhaust fan is on, and air that didn’t come in can’t be exhausted out. We suspect that exhaust fan flows in tight houses may be much lower than the fan’s rated capacity, and so total ventilation may be much lower than people think. That’s one reason why we’re doing the field research.”

But that’s ridiculous. I’ve been building efficient homes for 20 years and I think whole-house ventilation adds needless cost.

It does add somewhat to the cost, but it’s still something to consider if you’re already doing a renovation. There’s an opportunity to add in the chases while the walls and floors are already open.

Charlie adds, “People who have been building ‘efficient homes’ for years have almost never been building airtight efficient homes -until recently. Any home that leaks air at more than about 5 ACH is probably leaky enough to keep the builder and the occupants out of serious trouble regarding CO2 buildup, but that doesn’t mean the indoor environment isn’t laden with other contaminants or excessive moisture – and CO2 at times. A properly designed and installed balanced heat recovery ventilation system solves multiple problems and delivers multiple benefits in any home that is substantially airtight.”

OK, so mold is a problem in Florida. Would a house that passes a blower door test but relied only on a bathroom exhaust fan for ventilation still be at risk for mold in a drier climate like California? Why can’t we just use vapor barriers to manage this?

Charlie explained why vapor barriers aren’t enough to manage mold. Mold can exist wherever it can find three things: food, cool temperatures, and high relative humidity or bulk moisture. In a lightly or poorly insulated home, or one where there are thermal bypasses in the insulation system (e.g. where framing members pass all the way from the interior to the exterior), mold can grow on damp surfaces with a food supply such as cellulose.

“An HRV in this circumstance removes excess moisture from the home. When fresh air comes in, its temperature is elevated in the heat recovery core to within a couple of degrees Fahrenheit of the indoor temperature, which significantly lowers its relative humidity, helping to maintain a lower relative humidity level in the home, in spite of elevated levels of cooking or showering activity. There are lots of climate zones in California where mold is an issue. And if indoor relative humidity levels are consistently elevated, for whatever reason, mold can even be a problem in the desert.”

Yes, I know… opening a window is not enough to ventilate every home, but at least I didn't end this article with a whole-house mold photo.

Imagine a home built in the Plains region of the United States that stays warm in the winter without central heating, and cool in the summer without massive air-conditioning. It’s airtight but with an endless supply of fresh air constantly circulating through a filtered, pressure-balanced ventilation system. Every surface is comfortable to the touch, neither too warm nor too cold. Street noise is barely audible through the gasket-sealed, triple-paned windows. 

It sounds futuristic, but so-called Passive Houses have been around for at least 15 years, and it’s yet another strategy for saving energy. Unlike a Net Zero Energy home that might rely on “active” power generation, albeit from renewable sources, a Passive House is just that – passively absorbing heat from its surroundings to release it slowly as it is needed. (In hot climates, Passive Houses are designed to recover and store cooler temperatures.)

Passive House History

The idea of a “passive house” originated in Germany as a result of conversations between two university professors. A Passive House (Passivhaus in German) is a building that requires very little energy for heating and cooling, instead relying on passive energy sources and thermal isolation from its surroundings to achieve temperature stabilization. The notion began around 1988 and is now widely accepted in Germany and Europe. It’s now a standard, with specific and measurable performance requirements that can be field-tested and verified.

Buildings that meet the Passive House standard include public structures such as schools and supermarkets, as well as private residences. In the United States, the Passive House Institute US in Urbana, Illinois is a consulting and research firm working on adaptation and implementation of the Passive House standard within the United States.

This existing home in Ireland has been retrofitted to meet the Passive House standard, and yet it still looks just like every other house on the block. Designer: MOSART Architecture

Here in the Bay Area, Quantum Builders has become a recognized expert in the creation of Passive Houses tailored to our local climate. (Note that there is another builder of that same name in Texas, unrelated). An upcoming project in Tiburon is due to start construction this year, and was designed by award-winning architect Olle Lundberg of Lundberg Design. Bronwyn Barry of Quantum Builders spent considerable time explaining to me how it all works and showing me some of the wall assemblies that Quantum uses in their Passive House projects.

Wall details from above house showing "before" and "after" the Passive House retrofit, from the MOSART web site.

I also spoke with Jonah Stanford of NeedBased Inc., an architect based in New Mexico who’s a Certified Passive House Consultant with a successful track record of completed projects that employ advanced solar design principles in an artful and responsible manner. 

Passive House in a Nutshell

Passive Houses rely on an airtight envelope, lots of insulation, thermal mass, heat-recovery ventilation systems, and a thoroughgoing approach to slowing heat transfer through the walls that leaves no stone unturned. Wall assemblies tend to be thicker – in some retrofits, it’s like wrapping an additional blanket around the existing house – but the most unusual thing about the walls apart from air tightness is the extreme attention paid to eliminating thermal bridging. “A typical home can lose 25% of its heat from thermal bridging,” said Bronwyn.

Thermal bridging can be responsible for 30% of heat loss from a home. Wood frame studs have a lower insulating value than the batt insulation between them, and if the studs are directly in contact with the inside and outside of the wall, they can act to conduct heat out on cold days, resulting in unwanted heat loss.

In a way, it’s not unlike NASA sending up a manned spacecraft and having to account for every last gram of weight, to ensure that there’s enough fuel to get it to its destination. The interior has to be kept warm enough to keep out the cold, which is a chill far more extreme than anything you’d find on the Earth itself. And of course, manned spacecraft have to be airtight, because any air leakage at all would be disastrous.

The idea with a Passive House is to stabilize temperatures by making the thermal mass of the house work for you like a giant hearthstone. You don’t need a conventional furnace at all – even in Northern Europe! Once the house is at the desired temperature, it takes very little energy to keep it there. “Improving a home’s airtightness can result in 25% energy improvement,” said Bronwyn.

Passive Houses have been built in every climate zone. Desert climates of course are more concerned with keeping cool by managing solar heat gain, whereas cold-winter areas are more concerned with staying warm. Tailoring the design for the specific climate and site conditions is of paramount importance.

Clockwise from upper left: Crossway House by Hawkes Architecture in the U.K.; Breezeway House in Salt Lake City, Utah by Brach Design Architecture, the first certified Passive House in the U.S.; zero-emissions research station in Antarctica.

(Note: I’m not sure that they are all “Passive House certified” but they all use the basic Passive House principles. Breezway definitely is certified, and Crossway is “zero carbon” home that has also been accredited by the Passivehaus Institute in Germany. Bronwyn mentioned the Arctic research station as a Passive House, so maybe it’s actually certified as well.)

The Passive House standard doesn’t specifically require the use of non-toxic materials, although the wall assemblies that I saw used materials that were carefully chosen partly for low toxicity – cellulose and rock wool insulation, wood and low-toxicity oriented strand board. Off-gassing isn’t as much of a problem as I had originally thought: the ventilation system has a low but constant rate of air exchange that doesn’t allow stale air to accumulate anywhere in the home. Passive Houses are credited with having excellent indoor air quality; that’s one of their selling points.

Humidity also has to be managed, as with other tight-envelope buildings. Placement of vapor barriers is dependent on climate, similar to other types of construction. The wall assemblies at Quantum Builders showed extensive attention to waterproofing as well as the placement of air and vapor barriers.

Rafter detail showing vapor barrier at interior (blue,) air-barrier at existing siding (red) and bulk moisture barrier at roof sheathing (red.) Arrows show air movement within the roof assembly, allowing air to escape from vents in the roof for passive cooling. Taken from a study for a Passive House retrofit by Quantum Builders.

Like other standards, a Passive House building performance analysis includes the creation of an energy budget. Energy budgets are a key component of many other energy-saving approaches and standards such as passive solar design, GreenPoint Rating, HERS home energy audits, Net Zero Energy homes, or California’s Title 24 energy standard. The Passive House energy budget is thorough and detailed, including occupants, appliances and lighting. Both power consumption and heat generation are considered. 

How do Passive Houses differ from, say, Passive Solar or Net Zero Energy homes?

Passive House is more than a set of principles – and it’s more than a checklist. Passive House is a formalized approach with an associated standard, modeling software, energy budget, and certification/testing process. To be fully certified, each Passive House is verified against actual building performance after the building is completed, as follows:

1. Heating and cooling demand is less than 4.75 kBTU per square foot per year. (A regular house might use 15 times that amount.)
2. An air-tightness rating of less than 0.6 air changes per hour, measured at 50 Pascals. 
3. Energy demand for all uses (called “specific primary energy demand”) including hot water, heating, cooling, auxiliary, and household electricity is less than 38 kBTU per square foot, per year.

The Passive House performance standard.

By comparison, passive solar design isn’t really a “standard” that can be pass or fail. The passive-solar approach looks at building orientation and other principles of solar design, but there’s no specific energy modeling software associated with it (although many software programs can be used to assess most of the solar gains, etc).

Net Zero Energy homes don’t really have a standard or certification other than daily use. They do have a performance goal: use less energy than you produce within a single year, with an annual reckoning every December between you and the utility company. If you’re grid-tied and your energy bill for the year is zero, then yes, you produced more than you consumed, so the house is Net Zero – at least for that year. But there’s plenty of synergy among these approaches. “Passive House gets you super-close to Net Zero Energy”, says Bronwyn. “If you meet the Passive House standard, then Net Zero is easy.” A Passive House requires less energy to begin with, so you’d be able to reduce the size of your renewable-energy systems accordingly.

Title 24 does provide good baseline performance measures, as Bronwyn explained. “The R value needed to meet the Passive House standard varies by climate and is determined per project using the Passive House Planning software. In Minnesota, you might need R30 to R40 walls and R50 roof. Here in California, a Passive House should have around R21 walls, R11 insulated slab, R28 roof, and really great windows.” By comparison, Title 24 mandatory minimums are R30 roof, R13 walls, R19 floor, and Low-E windows. 

The differences lie in the root of each strategy. 

• Passive House is about temperature stabilization as the main focus for reducing the need for actively generated power. 
• Net Zero Energy is about achieving a “net zero” balance between onsite power generation and power consumption, with a strategy that includes active power generation through solar, wind, or other renewable energy sources.
• The GreenPoint Rating system focus is on long-range resource conservation, energy efficiency, community design, and environmental health. So does LEED.
• They all differ from Title 24 in that you can factor in your shade trees for credit, or include these features as part of the energy model.

Tell me about energy budgets in Passive Houses.

Part of the Passive House standard involves setting a specific overall energy budget for the home based on “treated floor area”, or conditioned square footage up to the interior wall area. The Passive House energy budget is imposed purely based on size regardless of activity, and includes all appliances, not just heating and cooling. It’s up to the owners to decide how to use that budget. It’s challenging to meet the standard, but definitely possible. Typically, you have to be very careful when selecting appliances. The Energy Star rating only sets a minimum efficiency; within that, appliances can vary widely in how much power they actually use.

The Passive House software tool, called the Passive House Planning Package, is an elaborate Excel spreadsheet that helps to create a detailed energy model of the home. Although use of this tool isn’t strictly required, it seems to cover every possible angle and takes all the Passive House principles into consideration.

What if you want to light a couple of candles over dinner? Will this throw the house off?

A good ventilation system can take that into account. What most owners do is they open a window! But yes, you do have to be aware of every heat-generating activity that you do. However, Passive Houses can accommodate a wide variety of activities and lifestyles. You don’t have to be afraid of exercising in a Passive House or of hosting large groups of people. In Germany there are entire kindergartens and office buildings that are Passive House certified, even an indoor pool!

What happens if you leave for a long weekend and forget to take out the trash?

The first question everyone asks when they hear about a hermetically sealed, airtight house is “What happens if you fart indoors?” Even though the question itself is crass, the concerns about stale air are reasonable enough, given all we’ve heard about airless offices and flu-laden airliner jets.

“Passive House ventilation systems usually have a ‘flush’ feature nowadays,” said Bronwyn. “If you need to clear the air, you can activate this cycle and then the system resumes normal operation.” As with the above question, you can also open the windows for 10 minutes, air the place out, then shut them again without seriously disturbing the temperature balance inside the home. 

Passive Houses have better air quality than so-called normal buildings. Bronwyn and I spent time discussing the chronic health issues so many urbanites face, from asthma to migraines. She quoted me longitudinal studies from a school in Germany that showed reduced absenteeism, improved occupant health, increased attention span, and reduced CO2 levels. “In a Passive House, the indoor air is constantly being filtered and circulated, while stale air is constantly being expelled.”

Can you use carpeting inside a Passive House? Are there certain conventions for indoor furnishings and materials that need to be re-examined?

Yes, you can, and any dust it generates will be less of a problem because airtight houses are less drafty. There are no stray air currents to kick up dust into the air. There is no reason why you couldn’t use all the same interior design techniques that you would in any other home.

What’s it really like inside a Passive House?

I asked this question of Jonah Stanford. “It’s like being on the moon,” he answered. “It’s amazing, really. The house acts totally different in some ways from what we’ve been conditioned to expect. If you stand near a window on a cold day, you won’t feel a thing. Normally you would feel a thermal draw from the window in cold weather.”

A cross section from a window designed using Passive House principles. It features argon gas-filled triple glazing, thermal breaks, insulation inside the frame, full gasket seals at three places inside the frame, and a waterproofing system on the outside to trap and guide rainwater out and downward.

The evenness and stability of the temperature inside a Passive House eliminates hot and cold zones that we may be used to. “I went to a 2 story office lobby that used Passive House principles and we measured the temperature at the floor, the wall, and the roof. It was all exactly 21.5 degrees Centigrade. Phenomenal.”

Do occupants feel separated from the outdoors?

Not at all. Passive Houses are quieter, but they actually have more fresh air. Occupants can open windows when it’s nice outside as much as they want, just as they would in a conventional house. The only differences is they don’t HAVE to.

Does the Passive House standard REQUIRE that you purchase special building materials all the way from Germany? Can’t you do it using local materials?

Yes you can. Quantum has chosen to work with German manufacturers because they’ve got more experience building to the Passive House standard. Importing assemblies from Germany to the Bay Area actually uses less embodied energy than, say, trucking them from Minnesota. Apparently to be really “carbon compliant” everything trucked by surface has to come from a distance of under 300 miles. 

It’s not the materials, it’s the details that have to be reworked. All thermal bridging must be eliminated, which requires special measures. In addition, airtightness, vapor protection, and waterproofing all need to be addressed.

Insulated roof ridge detail before and after, showing how R-values were improved from a mere 2.2 up to R-50. Taken from study for a Passive House retrofit in Larkspur, by Quantum Builders.

There are no books on typical detailing for Passive Houses – yet. Builders on the East Coast can often use details from German books on Passive Houses, but these details are optimized for a cooler climate and rely more on masonry than on wood frame construction which is most commonly used here in California.

Aren’t the floors cold? Is every surface supposed to be the same temperature?

There is slab insulation under the home to keep the floors from leaking heat out into the ground. A Passive House has a lot of thermal mass partly to keep every surface temperature constant. Thermal imaging via a software package called THERM is a useful supplementary tool. Bronwyn showed me two thermal images, similar to the example image shown below, comparing the effect of placing slab insulation either above or below the slab. 

Imaging example showing a thermal model of a floor to wall assembly, from the software package THERM. Image courtesy of Quantum Builders.

Although both floor surfaces were warm where the floor met the air, the warmth went deeper when the slab was exposed and could warm itself. With the slab underneath, it sucked cold up from the ground and stayed that way.

How NOT to design a foundation. This shows how thermal bridging can effectively drain all the heat out of your home.

What does the ventilation system need to do?

For a Passive House, you need a good mechanical heat-recovery ventilation system with balance between air intake and exhaust, delivering 0.6 air changes per hour. The house should have an even pressure balance between inside and outside air. Air filtration components may be selected based on the location and occupant needs, but are always present. Special attention is paid to the location of openings for air intake, which may vary by climate as well as site. For example, intake in very cold climates may require some form of pre-heating via earth tubes.

“HEPA filters aren’t always necessary,” said Bronwyn. “Passive Houses have lower airborne particulates already, because there are no indoor convection currents (drafts) to stir up dust. The fact is, so-called ‘normal’ indoor air quality is poor to begin with.” 

What additional costs are associated with building a Passive House as opposed to a “regular” one?

The Wikipedia article on Passive Houses contains the statement that overall, Passive Houses cost on average 14% more to build and are more expensive in Northern latitudes above 60 degrees. Other sites claim 10% overall or 7% in Germany. I didn’t get a figure from Quantum, although it’s clear that the additional insulation and thicker walls do add somewhat to the cost. 

“It’s really important to have a fully committed client,” said Bronwyn. “Otherwise they may not want to go all the way.” I observed that most people don’t view their houses as legacy homes to be handed down to their children. It seems that most people stay in their homes about 5 years or so and then move on. They’re not as willing to invest in improvements that you can’t see.

Do Passive Houses have a thermostat?

Yes. Typically this would be set to 68 degrees, and is adjustable to suit occupant preference.

Can you have multiple heating zones for sedentary vs vigorous activity?

If you want to have a small office that’s nice and warm, while the rest of the house is at a cooler temperature, you can use a small portable space heater. This can be accounted for in the home’s energy budget during the early planning stages.

How well do Passive Houses do in extreme climates?

There are Passive Houses built in all seven climate zones in the US. There’s even one in Antarctica, a research station. A desert Passive House will be geared more towards cooling, but the actual wall assembly is similar to what you would use in Minnesota, in both cases well-insulated and protected against thermal bridging, because in either case you want to minimize thermal transfer through the walls.

This desert home by Kendle Design is not Passive House certified, but it uses the same principles of solar design that would also be employed to build to the Passive House standard: massive earth walls and solar shading. It might be challenging to make a glass wall this large using the airtight, triple-paned construction details shown on other Passive House windows, but who knows?

Within the U.S., the most challenging climates seem to be the cold regions around the Canadian border, and the extreme heat and humidity in places like Florida and the Gulf. Sometimes the use of geothermal or earth warming tubes buried in the soil can act as heat exchangers to pre-heat or pre-cool outside air before it goes through the ventilator.

Let’s talk about special building techniques for Passive Houses.

The Passive House standard is performance-related rather than material-specific. Quantum Builder’s South African case study is 100% predesigned and prefabricated from custom-produced wall and roof assemblies. In the Ukraine, according to Jonah Stanford, there are Passive Houses built with monolithic wood walls, although I wasn’t able to find any immediate specifics online. Regardless of material, airtightness, moisture management, and the elimination of thermal bridging are important considerations when designing specific wall assemblies. 

What’s inside this wall assembly here in your office?

From inside to outside: 

1. Drywall
2. Furred-out mechanical chase
3. Oriented strand board layer for air-barrier & structural sheathing
4. Cellulose between the structural framing
5. Insulated fiberboard impregnated with wax
6. Rain-screen furring 
7. Exterior siding

One of the wall assemblies on display at Quantum Builders' offices.

What about the windows in a Passive House?

The one component that isn’t easily obtainable here are the windows. Windows that meet the Passive House standard are hard to come by. They must be airtight, triple glazed, with insulated frames, with a very low U value – .14 or even as low as .11. All moving parts must be precisely fitted, like airlocks in a spaceship.

“R values of typical window are poor. A typical vinyl window is around R2, and the best Marvin windows are around R3.2. The R value of a Passive House window needs to be more around R7 to R9,” said Bronwyn. Considering that the minimum wall insulation in CA is now R13, the windows present the primary avenue of heat loss in a home, and it pays to make them as thermally efficient as possible. Installing the windows presents an opportunity for further insulation. In some cases the window frame can actually be layered behind additional insulation extending from the walls of the house.

The thermal performance of a window is influenced by the performance of the frame, the glass, and the spacer. In addition, the installation method can affect the performance of the entire wall. Each of these components within a window should be as thermally efficient as possible.

Bronwyn had special words about vinyl windows. Although they’re encouraged in Title 24 as being efficient, they have a reputation for off-gassing. And they’re still not airtight enough. The windows used by Quantum are made from wood, sometimes with aluminum or fiberglass cladding. The fiberglass clad windows are enhanced with a special insulating foam: Neopor- a super-insulating carbon impregnated type of EPS, on the outside of the window.  Any gas that escapes can’t penetrate the air barrier to get inside the house.

If you import special materials and such, do these products meet local building codes and standards?

I was especially interested to know if the imported windows were NFRC rated. Bronwyn informed me that their window manufacturers were in the process of getting their products rated, which can take up to a year. 

Why would you choose to go with the imports rather than building it locally, then?

Passive House materials must be built to the most exacting standards possible. Air-tightness must be controlled at every joining, every assembly, every switch box. Rather than try to manually assemble everything onsite, it can be both faster and more quality-enhancing to produce components such as wall systems in a factory that is already set up to achieve these standards. Having vendors and suppliers you can really rely upon is vitally important. Right now, most of these factories are in Germany because the Passive House standard was originated there, by building scientists, with strong support from the German government.

Quantum Builders already had strong ties with Germany, and has chosen to work with factories that achieve precision and who are committed to using high-quality, non-toxic products. These producers offer custom details as well as a wide range of standard products to satisfy design-oriented architects.

Jonah Stanford confirms that Passive House standard does not mandate a particular type of material, only a specified performance threshold. “The German assemblies that Quantum uses are actually quite reasonable in terms of cost. We’ve also price-compared both the German imported assemblies versus site-built or prefab assemblies made locally, and it came out about 20% less than importing – basically the cost of shipping.” 

When building manually, you have to pay a lot of attention to thoroughly sealing all electrical and plumbing penetrations, to keep the vapor-lock tightness. “You have to be obsessed with it, and even so, the seals might not last as long as the building,” says Stanford.

Stanford is working on his own assembly, a double stud framed wall. What distinguishes this wall from a “normal” wall is the layering. “The interior wall is load-bearing. Then, I use oriented strand board – NOT particleboard, followed by another layer of studs that are not vertically bearing.” This idea was, he says, inspired by the Ukranian wood frame Passive Houses, which are literally built from the inside out. 

There can be conflicts with local codes or incentives. In NM the incentives are generous, but require adherence to ASHRAE Standard 62 which requires outside venting for appliances like dryers. In a Passive House, however, the heat from that dryer should really be kept inside the house, at least in the wintertime.

What special skills are needed to design systems for, and actually build, a Passive House? 

Bronwyn had a couple of thoughts on this. The first was to have an integrated team from the start. “Architect, owner, builder, energy analyst, structural engineer, mechanical – they all have to review the early drawings together,” she emphasized. The second was to produce good construction drawings and details. “If the details are clear, any builder should be able to build to them – as long as they understand the why.”

Can an existing home be remodeled to meet the Passive House standard?

Yes, although some details such as under-floor insulation, strongly encouraged in Passive House construction, can be difficult to retrofit in existing slabs. Bronwyn showed me some details for Quantum’s remodel project in Larkspur. 

Although the added thickness does increase the home’s footprint very slightly, this in and of itself is not a problem unless the home is on an urban lot right up to the property line. In that case, the retrofit might have to concede a little space on the interior.

For the roof, you might have to actually raise the roof in some cases, in order to create additional room to fit the necessary amount of insulation. In jurisdictions where they might be picky about adding 6 inches to the building height, you might have to build down or lower an interior ceiling.

Isn’t rigid foam toxic, though?

During the roof discussion, Bronwyn and I got into a side discussion of different insulation types and R values, which is a measure of the resistance to heat transfer (higher is better). Typical batt insulation has an R value of around 3.7 per inch, meaning you can fit up to around R13 into a typical 2×4 framed wall. However, some types of rigid foam insulation can do better. Polyisocyanurate, for example, has been claimed as being R8 or even R11 per inch. I’d been wondering about the toxicity of this – “polyisocyanurate” just SOUNDS toxic!

Bronwyn pointed out that in the case of the roof assembly, the foam is on the outside of the air barrier, and the polyiso isn’t the worst thing out there. Formaldehyde from conventionally made engineered lumber products is a LOT worse, lasts a lot longer after installation, and it’s ubiquitous in buildings already.

What happens as part of the Passive House design and certification process?

The steps to Passive House certification are as follows:

1. During the schematic design phase, use the software to determine which wall assemblies (R-value) will meet the Passive House standard for the particular project based on climate. From there, you can proceed to create a detailed energy model of the project including surface areas, ventilation, windows, shading, even prevailing wind speed. It is essential to have input from your design and build team during this refinement. When design is complete, the drawings and the project modeling file are sent to the Passive House Institute US for pre-certification prior to construction. This takes around 4-6 weeks and costs around $800.
2. During construction, a third-party inspector comes out to verify that the house is actually built to the drawings.
3. After construction is completed, a third-party inspector conducts an official blower door test, to verify that the home is airtight. This test can be done by a HERS rater, as long as that person knows how to test specifically to the Passive House standard. This includes verifying a neutral air pressure balance inside and outside the home.
4. The final step in certification is to re-verify the home against the as-built drawings. This takes another 4-6 weeks and costs an additional $300 depending on complexity and size of the project.

As Bronwyn noted earlier, special attention should be paid to construction detail drawings. Construction Documents are one phase that sometimes gets short-changed, because clients mistakenly believe that it will “save them money” – and then those details get worked out in the field by the builder. With a Passive House, you can’t do this because those detail drawings will be required for verification during construction.

What do architects need to know in order to design a Passive House? Would they work with a special builder or consultant to do the modeling?

Jonah Stanford mentions that you have to design to the Passive House standard from the beginning of the project, which would seem obvious but it’s worth pointing out that if you start out designing a standard home (or standard remodel) and you’ve already gotten as far as construction drawings, and THEN you decide to meet the Passive House standard, you will have to re-do all the wall assemblies. Expensive. “You can’t change horses in the middle of a stream,” I said, to which Stanford responded, “No, it’s more like switching from horseback riding to driving a herd of pigs through the water.”

How did you choose Lundberg Design for a passive house project? 

There was a long and careful selection process, where we interviewed several architects.

This straw-bale house, also built by Quantum Builders, isn't a certified Passive House, but it uses passive solar design principles, and it's quite a nifty shape.

What’s the response from Planning to your project?

They’re fully behind it – as long as it still complies with the building code.

After January 1, 2010, all new homes in CA must include whole-house ventilation systems. Yes… we’ve made building envelopes so efficient, that now we have to in essence introduce highly controlled leakage. There are two mandatory ventilation features in the new Title 24: 

A good ventilation system will filter out indoor air pollutants (VOCs like formaldehyde from particleboard or acetone nail polish remover) as well as filtering outdoor air on the intake side; however, not all whole-house ventilation systems include outdoor air filtering. Here’s a summary of the ventilation course module from last week’s Title 24 update class. 

Moisture Control

Exhaust fans, which were common enough before, now required in all kitchens and bathrooms. Really, any “wet” room like a laundry room that has plumbing, should have an exhaust fan. One of the side effects of tighter building envelopes has been increased concentrations of mold in the indoor air, and keeping moisture down helps prevent the growth of mold. 

Single-room exhaust fans can be intermittent (operating only when when occupied) or continuous with an override switch. They must meet minimum rated capacities for cubic feet per minute (cfm) or air changes per hour (AC/h) and be ducted to the outdoors. Look for exhaust fans with Energy Star ratings for low noise level and high efficacy.

You may need to educate the owner on how to use these fans, but unfortunately my notes trail off at this point. Required sizing for exhaust  fans was roughly 1 cfm/SF, in 50-cfm increments, so a typical range might be 50-300 cfm based on the number of showers, hot tubs, toilets, etc.

Whole House Ventilation

The motto is “build tight, ventilate right.” The reason is that a tight home costs less for conditioning and ventilation combined than a leaky home does for conditioning alone. 

Ideally, the indoor air pressure should remain balanced. A really effective whole-house ventilation system includes both intake and exhaust, in equal amounts, running all the time. Intake is filtered, from a controllable location. They can include heat-exchange features too. However, these balanced ventilation systems are harder to install, and are, in the words of the presenter, “susceptible to insulation neglect.” 

A balanced ventilation system includes supply and exhaust. Images from www.energysavers.gov.

In California’ relatively dry Mediterranean climate, supply-only ventilation is OK. In very cold climates, a positive indoor pressure can lead to moisture buildup inside the walls as moisture-laden warm air is forced outward through the walls until it meets with cold air and condenses. Advantages include filtering of outdoor air, and better delivery of air where it’s needed – including bedrooms behind closed doors. The presenter noted that they can range widely in energy-efficiency, but didn’t explain why.

Supply ventilation creates a positive indoor air pressure.

When using forced-air fans for continuous ventilation in a supply-only system, it was recommended NOT to use a typical forced-air fan, but instead to use either a variable-speed forced-air fan or a supply fan that is separate from the forced-air system itself. 

An exhaust-only ventilation system is cheaper, but brings in unfiltered air, draws in other outdoor pollutants, and air distribution is easily disrupted by closed doors. Exhaust-only systems are not good with tight homes and open fireplaces, either – the negative indoor pressure can actually draws smoke from the fireplace or carbon monoxide from the furnace back into the house.

Exhaust-only ventilation creates negative indoor air pressure that relies on infiltration rather than an intake fan.

To size the system, multiply the cubic volume of the conditioned space by the number of air changes per hour. Although the code minimum for air changes is 0.35 an hour, the presenter noted that most international standards are higher: 0.5 or even 1 AC/h, and some exhaust systems seem to specify a much higher number. 

Whole-house ventilation system sizing formula

For example, a 2,000 SF house with 10 foot ceilings would have a cubic volume of 20,000 ft3. (Note the conversion of hours to minutes.)

The recommended best practice is to take the cubic feet of conditioned space times the number of air changes per hour, and then convert that to minutes in order to get the cubic feet per minute (cfm) of the house ventilation system.

Ventilation Energy Efficacy

Although there was a lot of discussion on this topic, the summary slide says to pay attention to the cfm/W – that’s the cubic feet per minute of air moved, per watt consumed. A lot of the efficiencies seem to be dependent upon how well the system is sized, designed and installed, and how well it’s suited for the particular home and climate. 

A well-designed continuous mechanical ventilation system should consume the power equivalent of a single 20W bulb running in the background 24-7-365.