Some notes on heat and cold and moisture in your home
Updated: Nov 25, 2020
Woke up to -28 this morning in Wakefield. A bit of a shocker considering the weather's been pretty mild this winter. Compared to last year we've had it easy! I think we've only had a few really cold days so far. And it's worth noting that this is normal for mid-February in these parts, or, at least, it used to be normal. People in other parts of the world can't believe we go from +30 in summer to -30 in winter. That's a 60 degree spread, in a region that's also very humid. That combination - the temperature spread and the humidity - is really hard on our homes for a number of reasons.
In deep cold or high heat a well-designed and built structure will manage heat, cold and rain and humidity to keep you comfortable. As a rule (a) heat goes to cold, (b) moisture goes to dry (air or material). Also keep in mind that insulation and air barriers work together with other materials (especially windows) to control heat and moisture exchange. Both can impact on your comfort. Here are a couple of simple ways to illustrate: first put your hand on a window. It's cold, right? But it's also stopping warm air from escaping because it's an air barrier. Glass doesn't insulate. Even gas-filled double pane windows are poor thermal barriers so some heat is escaping. Trying to control heat loss or gain with just an air barrier (e.g. the window) is not going to do the job. Conversely, if you try to stop heat from escaping with just insulation, that won't work either because warm air will travel through Fiberglass or rock wool insulation. Without something blocking the air, heat goes to cold, remember? Except that sometimes cold will actually stop heat from escaping. Try holding a burning match up to your upstairs bathroom fan. Although it's a hole to the outside and 'heat rises' so they say, cold air may actually be sinking into your bathroom because a fan in another part of the house is creating negative pressure in your house. Then there's the "stack effect" where air escaping an upper floor draws cold air in through holes lower in the building envelope.
Let's complicate this a little more: that window you just put your hand on? If sunshine is coming through it, even on a day like this, it's heating materials inside the house including you, an exchange called solar or thermal gain. In winter this is a good thing but in summer solar gain can cause a house and people to overheat. Summers are getting warmer here. Was your house built with this in mind? And then there's the moisture issue.The inside of your home is typically drier than outside in summer - especially with air conditioning. Moisture goes to dry air, remember? And now building scientists have discovered that the sun can drive humid air into a wall from the outside. If you don't have perfect air and moisture barriers inside and out, humid air can get trapped and turn into water at the dew point around 3 or 4 inches into a conventional wall assembly, where it can ruin insulation, rot framing and create mold.... which is why there's a huge debate about vapour barriers. And keep in mind I'm oversimplifying the physics for a short blog post! For a great discussion of walls see Fine Homebuilding's article at this link. See also this link by everyone's favorite building science guy Joe Lstiburek. My point is this: stopping heat and moisture from getting out of or into your house is complicated because our climate has such extremes. Weather patterns are changing here: we're getting more dramatic rain events in summer and freeze-thaw cycles in the winter - weather which was rare in the past.
This is already a tough climate on houses and people. All of these dynamics are changing as our weather changes. Except that most or our homes aren't built to adjust to these changes. Are you ok with that? I'm not! We can do better. Let's build for today's climate, not yesterday.