After a couple more spray foam cans, I thought I had taken care of unwanted air infiltration around our 1st floor replacement windows. Not so! Take the bathroom, for example. We began to notice unpleasant cold drafts!
After some more probing, I traced the drafts back to the window. Not from around the window, where we sealed with foam, but from the operable casement. It appears that the gaskets in the corners don’t properly seal between the casement and window frame.
On a day with temperatures well below freezing and the wind blowing, we can be three feet from the window and feel the uncomfortable draft. This represents a major breach in what should be an almost air tight building envelope and almost air tight windows.
Air leakage spec recap
We bought these rather nice casement windows from Newtec because they came with an excellent National Fenestration Rating Council (NFRC) air leakage rating of 0.02 cfm/sf (cubic feet per minute per square foot).
The bathroom window has a surface of almost 10 square feet. That translates into an air leakage of no more than 0.2 cfm – or 1.5 gallons per minute. This air leakage rate is tied to a pressure differential of 75 pascal between the inside and outside. (see also NFRC-400 test protocol)
Think back to our blower door test, where the building was depressurized by 50 pascal. The window testing protocol kicks it up a small notch with 75 pascal.
In other words, on relatively calm days, the pressure differential between the inside and outside of the window would be lower than 75 pascal. That would also mean that the air leakage for the bathroom window should be below 0.2 cfm. A document published by the Canada Mortgage and Housing Corporation states that only up to 25% of the seasonal air exchange rate is caused by wind.
It appears that the bathroom window does not meet the promised air leakage rate. And its not just a problem in the bathroom. It appears that all five casement windows on the 1st floor have gaskets that don’t seal in the corners and thus cause unwanted drafts.