Occupant Thermal Comfort in Energy Trade-Offs

Occupant Thermal Comfort in Energy Trade-Offs

Posted on 16. Mar, 2010 by in Thermal Comfort, Windows and Glass

Green living is sometimes viewed as a sacrificial process whereby one by one, all our pleasures and comforts must be set aside in the name of saving the planet: walking instead of driving, sweeping instead of vacuuming, home cooking instead of take-out, turning the thermostat down in the winter while our hands and feet turn into blocks of ice, low-flow showerheads designed by bald men that take forever to rinse the shampoo out of a long-haired-girl’s mane, limiting one’s diet to only locally available seasonal produce (which could be nothing but cabbages if you live in Chicago), calling three hardware stores to find one that carries low-VOC paint, giving up meat because it takes too much grain to feed a cow, trudging everywhere with a backpack filled with stuff that otherwise we could just keep in the car. In essence, the increased physical hardship comes from asking our own bodies to start doing more of the work. And what’s our reward? A nice warm feeling of altruistic glow, and maybe a slimmer figure.

Efficiency is often seen as achievable only at the cost of comfort – some of us East Coasters remember shivering through the 1970s oil crisis as our dads re-defined 58 degrees during the day as “normal” and turned the thermostat down at night till the pipes froze, and our mothers finally complained. Well, so what? What’s the big deal? We all have to give up something. Well, the problem is that this “fix” didn’t really fix anything. Reducing consumption is not the same thing as having an efficient building, and neither approach presents qualitative factors like comfort or contentment as worthy of consideration.

It’s time we started measuring the benefits, started quantifying the value of comfort in both human and performance terms. After all, a bottom line applies both to corporate and to individual operations, even if the qualitative benefits of “improved morale” and “comfort” aren’t seen as “adding value”. People with severe allergies and asthma can already tell you exactly how much more productive they are in a mold- and chemical- free environment when they’re not gasping their life away, but what about the rest of us who can breathe diesel fumes all day with nothing worse than a little nausea?

Current Focus Is On Efficiency Alone

California’s energy code requires efficient buildings in order to reduce fuel consumption, reduce peak demand, and – yes it’s a stated goal – reduce fossil fuel consumption as well. Title 24 energy compliance trade-offs in residential designs are usually a matter of running the proposed design through a software model with different options, with an assumed average air temperature that is not configurable, to simulate the building’s performance throughout the seasons for a specified climate zone.

Title 24 seeks to make homes more efficient so that they leak less heat in the winter. Infrared photography shows that this home is losing heat through the windows and the attic.

If the proposed design does not meet compliance standards, one can weigh various building improvements against the costs and time needed to achieve each measure. Is it cheaper to replace all the windows or to upgrade the furnace and add some field verifications? Does it make sense to add a radiant barrier to the roof, or would it be better to spend that money on better wall insulation?

On paper, it’s the same either way. In practice, however, which option you choose can have a big impact on the comfort of the occupants – who may be your own design clients. Every time they sit in their patio, or in their kitchen, or in a reading nook looking out the window, it’s an opportunity for them to remember their architect with fondness. However, if every time they sit near their picture window they catch a chill, they’ll remember that, too – especially if they’re me. I’ve spent far too much of my life in buildings that were too cold for me, and most of it was bad design – needless discomfort. (In office buildings, the chill was exacerbated by inappropriate professional dress codes that required suits, pumps and nylons when a wool hat, a big bathrobe and slippers would’ve saved me countless weeks of colds and sinus trouble.)

Contemporary lifestyles presume that people can comfortably wear light clothing or go barefoot indoors, even in winter. However if the walls or the floor are cold, this home will feel much colder even when the air temperature is the same. That wall of glass just visible on the left could make it chilly just to sit on the sofa, if it were cold enough outside.

The thing is, indoor air temperature is easier to measure, and that’s what Title 24 takes into account. However, the indoor air temperature alone may not be enough to ensure comfort. Other factors, such as humidity, air movement, temperature of surfaces in direct physical contact with the user, the occupant’s level of activity, and radiant heat transfer from windows can make a huge difference.

So how the heck do you measure something as individual and subjective as “comfort”? How are these “findings” actually validated in practice? And how can the proven findings from this type of research be useful for residential designs to go beyond energy compliance? For this article, I visited the offices of Loisos + Ubbelohde in Alameda, California.

Advanced Modeling and Thermal Controls – Is It Really Any Better?

When most people hear words like “daylighting” or “integrated facade systems” they think of the elaborate sensors and controls that are increasingly employed for commercial high-rise buildings to reduce heating and cooling loads and ensure optimal light levels, mostly without human intervention. Of course, the public then hears of supposedly cutting-edge “sustainable” buildings that are no more efficient or comfortable than the old, wasteful kind – at least from the point of view of the occupants themselves.

An "old-school" skyscraper on the left by Louis Sullivan is a precursor to the modern curtain wall, but at least the windows still open. The all-glass facade of the Prudential building in Boston - famous for occasionally shedding windowpanes onto nearby sidewalks - incurs high heating and cooling loads.

The usual complaints are the lack of manual overrides and the general inaccuracy or insensitivity of these control systems to what is actually happening at different points in the building. People can’t open the windows for air or control for glare on their own, and they don’t like it. I personally spent years in offices where we taped the vents closed when they blew frigid air down our necks in the summertime – and then the management would come around at night and open them up – then the next day the war would continue, along with incessant bouts of colds and flu.

(Apparently these systems are a lot smarter now, and they actually DO know when the sun is shining in each window of a 15,000-window high-rise. At every minute, for every single day of the year. Just like Stonehenge! But that’s a topic for another article. Meantime, back to comfort.)

The New York Times Building by Renzo Piano incorporates a state-of-the-art multi-layered skin, as well as advanced daylighting controls that know exactly where and when the sun shines into each window, all year round. Loisos + Ubbelohde worked on the daylighting.

Who’s Using Comfort Research Now?

The firm of Loisos + Ubbelohde takes these daylighting criticisms seriously enough to address user comfort – and user behavior – using fairly sophisticated measures developed in conjunction with ASHRAE and the Daylighting folks at the Lawrence Berkeley National Laboratories. Apparently it *is* possible to measure something as variable and subjective as “comfort” with enough precision to make design decisions based on it.

My first questions to the folks at Loisos + Ubbelohde were:

  • How do you know that these comfort predictions are valid?
  • How do you explain your findings to your clients, who may not have the scientific background to understand all the reasoning behind it?
  • How can you clearly and convincingly demonstrate to your client the VALUE of occupant thermal comfort?

San Francisco Office Building Example

They say a picture is worth a thousand words. So I’ll start with three pictures from a recent analysis done by Loisos + Ubbelohde for an office building in San Francisco. These three color-coded images show how much usable floor space you lose by having clear glass windows in the wintertime that draw radiant heat out of the room, and also from anyone who’s sitting too close to the glass. Basically, if you sit too close to a cold surface, you’ll feel colder – even if you’re not actually touching it, and even if the actual air temperature is the same. A consistently cold surface can, through radiant heat transfer alone, literally suck the heat out of your body.

Each image shows the same office space with a desk, with three different curtain-wall options. The color-coding indicates the Mean Radiant Temperature (MRT) which is a combination of air and radiant temperatures. The yellowish zones are comfort zones with an MRT of 71-72 degrees – neither too hot, nor too cold, for the majority of individuals. Each dot represents one square foot of vertical space as measured on the wall behind the desk, which moves closer to or farther from the window.

Thermal comfort zone on a San Francisco winter's day next to a clear plate glass window requires almost a six-foot setback.

In the first image, you see the “baseline” – clear glass. Right next to the window (indicated in cross section on the right), are almost 6 linear feet of unusable wall space. The blue and green color indicates “too cold” and “much too cold”. In this particular building, that’s a total of 26,000 SF going to waste!

A medium performing curtain wall reduces the arctic zone by half. Now only three feet of wall space is wasted.

In the second image, the desk has moved three feet to the right; and by the last image, showing the best curtain-wall option, total space victory is achieved.

With a better-performing curtain wall, the desk can go flush against the window, with no wasted floor space.

The most interesting thing about this graphic was that it wasn’t self-evident to present the data in this manner – but, once created, these illustrations are immediately convincing, even for non-technical people, of the value of thermal comfort. The study examined a number of other factors, including the feasibility using a perimeter heating system in the winter – but the impact of radiant temperature on thermal comfort remains the same.

Why Not Just Freeze Your Workforce in the Winter?

I have to ask this question, because it’s been the de facto answer for all the years that I was actually an office worker. What’s wrong with treating office workers like the commodities that they are, and letting them suck it up and deal? Why coddle them with expensive thermally insulated windows when cheaper glass will do? Well, aside from the fact that it sounds really bad to say it this way, it’s a poor idea to ignore conditions that promote ill-health. Even mild illnesses can cause absenteeism, or worse. People who can’t afford to stay home will continue to come to work – and less efficiently, too – while sick, thus spreading illness, reducing their own productivity, and prolonging their own recovery time.

A 2003 report on sustainable building for one particular facility cites productivity increases of 5 percent and absentee decreases of 40 percent after moving into a renovated facility. [1] Other site-specific studies also cite increased productivity, reduced absenteeism, improved recruitment, and reduced turnover resulting from improved workplace environments. One might add that this would be particularly important for public agencies that don’t always pay competitive salaries, and who can no longer offset lower salaries by guarantees of job security.

How Do you “Measure” Comfort, Anyway?

I’m going to shamelessly plagiarize a summary from an article titled “Window Comfort and Energy Codes” [2] because I can’t say it any better than the writer himself – Jim Larsen of Cardinal Glass. He’s citing ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy:

“Comfort can be evaluated with a statistical index called predicted percent dissatisfied (PPD). The calculation of PPD requires a knowledge of room conditions (air temperature, air velocity, humidity, and mean radiant temperature), and the occupant conditions (clothing level and metabolic rate). When comparing two conditions, a lower PPD is desirable as this reduces the risk of occupant discomfort.

Some common examples where cold weather PPD will be improved (lower):

  • Increase thermostat setting;
  • Adding layers of clothing; and
  • Increase level of physical activity.

During hot weather the converse of these will improve comfort as well as increasing air movement and/or reducing humidity.”

(Note: ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, is not a free download. Here’s one place that you can purchase it.)

How Hot Is Really Too Hot?

So far we’ve discussed thermal comfort and warmth, but of course in some climates cooling is a much greater concern, and even modern urbanites are questioning whether we really need it to be 65 degrees inside in the summertime, when that means a 40-degree differential with the outdoors. After all, humans have survived for millions of years without air conditioning, and complaining about the weather at least gave people something safe to talk about.

It turns out that yes, humidity and air movement have a lot to do with how overheated we feel, and a study I can’t locate at the moment was cited showing that subjects in a blind trial were able to perceive 92 degrees as comfortable – as long as there was sufficient air movement.

Stone lattices were a pre-industrial solution for desert climates that filtered incoming light while allowing air movement - although, making it dark enough to be comfortable might be too dark for most types of "productive" office work.

Maybe We’re Just Spoiled Here In America

This is another topic unto itself, but yes, cultural expectations can affect perceptions of “comfort”. However, those same cultural expectations that might induce one to tolerate greater extremes might also be more forgiving of seasonal and even diurnal fluctuations in productivity – AKA the afternoon siesta. Could siestas save the planet? Well… it’s probably an easier sell than telling everyone to just tough it out.

[1] Kats, G., Alevantis, L, Berman, A. et al. “The Costs and Financial Benefits of Green Buildings. A Report to California’s Sustainable Building Task Force”, October 2003, cited in “Occupant Thermal Comfort and Curtain Wall Selection” by Susan Ubbelohde, in the Journal of Building Enclosure Design, Summer 2006, pp 32-34.

[2] “Window Comfort & Energy Codes”, by Jim Larsen,  in the Journal of Building Enclosure Design, Summer 2006, pp 37-38.

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2 Responses to “Occupant Thermal Comfort in Energy Trade-Offs”

  1. Yee

    28. Aug, 2010

    Interesting concept. Sometimes it is not necessary to have a trade-off to be environmental friendly. A simple way to look at it also is in a house design. Make sure every corner we have windows to save the electricity and increase brightness. Solar panels that warm up the water heater. These are the basic and common sense.
    .-= Yee´s last blog ..Home Aquarium Guide =-.

  2. Alex B Gardiner

    22. Jul, 2011

    Hi Rebecca

    Have a look at my website – http://www.eco-wan.com. We are endevouring to measure comfort and advise people in their offices and homes how they can reduce their enenry costs and still remain comfortable. We do this by measuring the temperature and humidity of the workspace and asking what they are wearing and what the airflow is like. The computer program works out the ideal temperature for the situation and, taking the above factors into account, advises on what can be changed to reduce costs without being uncomfortable. The system can also act as a data logger so that you can see the effect of changes.



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