A residential blower door test result is usually performed at a pressure difference of 50 Pascals, and the results are reported as an airflow rate in cfm. A report might read, for example, that the result is 320 cfm @50 Pascals.
If you have this result, and you know that your home has a volume of 19,200 cubic feet, you can calculate the air changes per hour (ACH) @50 Pascals. Here’s how: You multiply 320 by 60 (to convert cfm to cubic feet per hour), and then you divide the product by the home’s volume. The result: Your home is rated at 1.0 ACH @ 50 Pascals (or 1.0 ach50). Most blower door contractors include ach50 in their reports.
Compact homes with few bump-outs are easier to air seal than convoluted or stretched-out homes with bay windows and dormers. Given the same air sealing approach, a compact home is likely to have a lower ach50 result than a stretched-out, convoluted home. It’s also much easier to hit a certain ach50 target with a larger home than a smaller home. Because of these facts, some blower-door specialists note that it makes sense to focus less on ACH @ 50 Pascals, and to adopt a different metric — namely cfm50 per square foot of building envelope (that is, cfm50 per square foot of shell). This latter approach provides a better snapshot of the tightness of a building’s shell than ach50.
For example, in his 2012 blog titled “How Much Air Leakage In Your Home is Too Much?”, Allison Bailes wrote, “The most common unit used by blower door operators is ACH50… I prefer cfm50 per square foot of building envelope … The two reasons for that choice are that (i) air leakage happens at the surface, not in the volume, and (ii) it’s the best unit, in my opinion, to express what…