Don’t you just love to strip down to your socks and jump on the bed? As a kid, you may not get away with it, but you’re a grown man or woman now, so who’s to stop you? And then, when you’ve exhausted yourself, you can walk naked to the kitchen to get a glass of water. You can sit down au naturel and check your email or update your Facebook page, maybe even write a blog article.
Later, after you’ve cooled down from your exercise on this cold December day, you start feeling the chill. Hmmmmm. The thermostat says it’s 70° F in the house, so why are you cold?
The answer to your discomfort lies in building science. Comfort depends, of course, on internal factors—metabolism, activity, the hot flashes of menopause—but four external factors affect your comfort as well. (I wrote about these four factors of comfort a while back.)
The one that gets all the attention is air temperature, but that’s not the operative factor for unclad folks in homes with weak building envelopes. Convection could be one, as drafts cause air to move over bare skin, cooling it down. Even without drafts, though, the disrobed can be cold. It has to do with this thing called mean radiant temperature. Here’s how it works.
Every object radiates heat. The amount of radiant heat it gives off depends on its temperature (to the 4th power!), surface area, and emissivity. So our naked man jumping on the bed in front of the single pane window is giving off not only more views than he’s getting back but also more heat. The surface of the window is much colder and gives off far less heat, so the net flow of radiant heat is away from the man in his birthday suit. He’s cold! (And none of us needs reminding what George Costanza said about cold men. Oh, wait, I guess I just did remind you.)
In your home, there are surfaces all around you, and they have a big effect on your comfort, whether you’re in the raw or not. You’re giving off heat, and so are they. If they’re cooler than you, you lose heat. If they’re warmer (think bonus room in summer), you gain heat. If you keep the air temperature at 70° F in winter, the closer your walls and windows are to that temperature, the more comfortable you’ll be.
Obviously, a home with single pane windows and uninsulated walls, like the old house above where I used to live, will have colder surfaces. On cold days, you feel the chill almost as soon as the heat goes off because those surfaces quickly suck the heat right out of your body. That’s why insulation and air sealing is so important. A good building envelope is critical to comfort, especially for the threadbare, but even when you’re dressed, you don’t want to lose excess heat to a bad building enclosure.
For those of you who want a more rigorous, mathematical look at this topic, check out the Healthy Heating page on mean radiant temperature. The Healthy Heating website and blog, written by engineer Robert Bean, is a great resource for technical information. Since he’s in Calgary, he knows a thing or two about cold surfaces.
So, the takeaway is that you can enjoy leafless comfort in your home without cranking up the heat to 90° F. You just need to make sure your building envelope has a high enough mean radiant temperature by having good insulation and air sealing. Whether the neighbors will be happy with your level of comfort is another matter, though.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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Photo of naked man by Pixel Addict from flickr.com, used under a Creative Commons license.