I've just been reading about this. Indoor air sealing is measured with a Blower Door Test, which applies a very slight pressure (50 Pascals, less than one psi) and determines airflow. Results are typically provided in ACH50, Air Changes per Hour at 50 Pascals.
But what most people really want to know is how many air changes there are under natural conditions, without a blower door applying air pressure. ACH by definition cannot be measured (it's "natural") but can be estimated applying factors related to building height, climate, and building shielding from plants and other structures.
Suppose I get an ACH50 of 3, a very good number. A correction factor for a single story unshielded home in Austin is 16.7. I'm figuring a good number for me would be about 15, since there is some shielding from other nearby homes and my fence. So than the natural ACH would be 3 / 15 or 0.2. 0.2 would mean about 5 natural air changes per day. That does seem a lot higher than I would expect for a well sealed building.
But what most people really want to know is how many air changes there are under natural conditions, without a blower door applying air pressure. ACH by definition cannot be measured (it's "natural") but can be estimated applying factors related to building height, climate, and building shielding from plants and other structures.
Suppose I get an ACH50 of 3, a very good number. A correction factor for a single story unshielded home in Austin is 16.7. I'm figuring a good number for me would be about 15, since there is some shielding from other nearby homes and my fence. So than the natural ACH would be 3 / 15 or 0.2. 0.2 would mean about 5 natural air changes per day. That does seem a lot higher than I would expect for a well sealed building.
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