Usually, around October when I turn on the heat, I also start noticing cold drafts coming in through the cracks in the Cold House’s century-old windows. I then run around like a madman with rolls of caulking putty, sealing up all the little gaps. I always blame the sudden draftiness on the cold outside. Until this year, I never thought that turning on the furnace might be half of the cause. But it was, and in fact an unanticipated beneficial side-effect of keeping the house cold has been a huge reduction in draftiness. To the point that I actually forgot, until last night’s very cold temps, to seal up a couple of the windows. The reduced draftiness has, in turn, made the lower house temperatures much more easily tolerable.
Many house drafts are caused by convection, — warm air rising, cold air sinking, producing air currents. The bigger the temperature differential between a cold spot and a warm spot, the faster a draft will develop between them. So, for example, if you have a little gap under a door to the outside, the the warmer you keep the room the more draft will come in under the door. Other convection drafts are intentional: a hot radiator produces a massive upward draft, pulling cold air along the floor and jetting hot air up at the ceiling (when we were kids, brother and I used to make parachutes out of paper napkins, string, and a paperclip, then hold them over the radiators to be carried skyward.)
A cold window convects in the reverse way, drawing warm air at the top, cooling it, and pouring it down the bottom. The warmer the room is relative to the window, the faster the air current becomes. So, the draft is caused as much by the warm room as it is by the cold outdoors. This year, I’ve stopped passively blaming these drafts on “the cold”, and started thinking of them a a result of a temperature difference— one side of which is completely under my control.
Another contribution to drafts is actual air pressure. Some is unintentional and unavoidable– a stiff wind on one side of the house will blow cold air through the cracks there, sucking air out on the opposite side of the house. Aside from sealing up cracks, there isn’t much to be done about that one. But quite a few pressure-induced drafts come from within the house itself. Ceiling fans, for example– they have the benefit of moving warm air from the tops of high rooms down to human level. But at the same time, they produce a breeze. Similarly, my neighbor’s forced-hot-air heating system produces a prodigious draft, constantly moving air all around her house. Just the act of combusting fuel in a furnace or woodstove will induce a draft– a lot of air is sucked in to burn the fuel. It goes up the chimney. It has to come from somewhere.
You might wonder how much all these little drafts matter. Answer: potentially quite a bit. In very still conditions, the human body (like all surfaces) maintains an “air film” that traps a tiny bit of air in the couple millimeters immediately around it. If undisturbed, this film provides a significant layer of insulation (very roughly equal to 2mm of neoprene). But the film is very tenuously held in place by the surface friction of the skin, and is easily blown away. Here’s a fabulous graph from an experiment that illustrates the fragility of air film:
The x-axis shows ambient temperature at which people felt comfortable. The y-axis shows increasing drafts that were directed over the subjects. (The two different line represent different levels of activity– 1 met is lying down at complete rest, 2 mets is doing light deskwork.) As the caption notes, comfort at a given temperature holds up until the draft reaches 0.3ft/sec– beyond that, the air film breaks down, and considerably higher temperatures are needed to effect the same level of comfort.
0.3f/sec is not much of a breeze. It’s 1/4 mph, or 0.4 kmh. You would never detect this outdoors, and it wouldn’t be much use for sailing. When you get up to a slight draft of 1ft/sec (2/3 mph, 1 kmh)– which is much less than you would experience sitting under a slow-moving ceiling fan– you need to have the temperature turned up 4˚F just to keep the same level of comfort.
Moral of story: heating air starts to move the air. The warmer you make the house, compared to outdoors, the more draftiness you’ll have. And the more draftiness you have, the warmer you will need the house, in order to feel comfortable.
Reverse moral: At lower house temperatures, you’ll be more comfortable than you expect.
Corollary: Don’t get me started on the idea that less breeze = less skin drying at any given humidity level…