About two weeks ago, I posted a survey to see what people thought was the most important part of the building envelope when it comes to comfort (it is important to remember that all these are needed for an effective building envelope, and without any one-part failure will occur). The responses were as follow:
- Weather Barrier 41%
- Thermal Barrier 25%
- Vapour Barrier 18%
- Air Barrier 15%
These are interesting results. The weather barrier is indeed the first priority in designing an envelope. As noted in the comments of the survey, “No one calls in the middle of the night to complain of air leaks”.
“No one calls in the middle of the night to complain of air leaks.”
Personally the analogy I like, is looking at it like clothes. The first thing you want is to stay dry. Being wet, makes everything worse.
Second, in my opinion, is the air barrier. If you have ever been out on a cold windy day with only a nice wool sweater, you’ll know the wind goes straight through the sweater and will leave you chilled. This effect is called wind washing (I have some great infrared images of this).
Next is the thermal barrier. Once you’ve stopped the rain from getting you wet, and then stopped the wind blowing away your heat, insulation becomes easy and effective.
Finally, is vapour. Vapour is last because vapour drive moves moisture in a two-dimensional flow only (high to low), and the volume of water moved is orders of magnitude smaller than that moved in an air leak. Also if you are cold and wet, who cares about a little condensation!
If you want to figure out the priority of building envelope systems on your own, there is a fairly simple test (for cold climates) that sticks to my analogy above. You will need: a sweater, a shell jacket (like a rain coat), and a cold windy day. On the cold windy day, first wear your sweater by itself and stand in the wind. Then wear the shell on it’s own and stand in the wind. Finally wear both. What had the biggest impact on your immediate comfort? You could even play around, and try switching the order of the jackets to see if there is further impact on your comfort!
Now when you look at architectural details, ask yourself if all these parts are easily identifiable. Are they in the right spot and connected properly? And, is it clear who is building each part?
If you have a building envelope project that you need some advise on, or notice condensation or drafts in your building, feel free to reach out to me with your questions or contact one of our other Building Science & Sustainability experts at one of our 40 Pinchin offices across Canada.