This morning, we got our blower door test to find out for real how tight our envelope is. Turns out its pretty good, our infiltration rate is 0.16 air changes per hour. Infiltration is an important factor in controlling heat loss, the number 0.16 air changes means that enough air seeps into our house through windows and vent openings and other seams in the house every hour to replace 16% of the inside air with outside air. For many houses infiltration is one of the primary sources of heat loss, and one of the main strategies of passive house construction is to keep infiltration extremely low. For us, because we have such a tight envelope, infiltration makes up only about 3% of our heat loss, our big windows are where most of the heat loss happens. The down side to keeping infiltration so low, is that it limits the amount of fresh air that gets inside, we leave windows open all summer but in the winter the house will be mostly closed up… and thats where an HRV comes in, allowing you to bring fresh air into the house without the associated heat loss because it pre-warms the incoming air with heat taken from outgoing stale air.
To start with, all the windows in the house need to be closed, then a tarp is fitted on a frame that covers up one opening in the house… For us, it was the main entry. Inside that tarp are two openings, a large one that a fan sits inside of, and a small one that a thin tube slips through. The thin tube attaches to a manometer and allows it to measure the pressure difference between the inside and outside.
The fan is then turned on sucking air out of the house until the pressure inside is 50 Pascals lower inside than out. At that point you know how much air the fan is moving in cubic feet per minute, and you know the volume of your house, so its just a simple calculation to find your air changes per hour at a 50 pascal pressure difference (usually noted as something like ACH50). Then once you have that number, divide it by 20. That gives you the air change rate when pressure is pretty equal inside and out, and that is the number that is the most useful in calculating heat loss.