I sometimes do Blower Door testing in Michigan where the code says a new home must achieve a level of air tightness < or = 4 ACH50.  We use this number together with the house’s volume to get a target number in CFM50 that we hope to beat with the blower door.

I was doing some reading and came across this article by Allison Bailes:

“How Much Air Leakage in Your Home Is Too Much?”

In the article he says, “I prefer cfm50 per square foot of building envelope, or better, cfm50 per hundred square feet of building envelope (sfbe). (A cfm50 is a cubic foot per minute at 50 Pascals.) 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 a Blower Door is really good at – measuring the amount of air moving across the building envelope at elevated pressure.”

That sounds very reasonable to me.  He then went on to mention that, “ 3 ACH50 — translates to about 0.25 cfm50 per square foot of envelope, or 25 cfm50 per hundred square feet of envelope. ”And, “ The house I built ten years ago came in at 14 cfm50 per square (1.7 ACH50).

Can anyone tell me how he calculated the 0.25 CFM50 from 3 ACH50?, and the 0.14 CFM50 from the 1.7 ACH50?

I’m just wondering because we ran into another energy rater that was using CFM50/sq ft to justify an apartment passing code for air tightness.  Is this even legal?  Because the code seems to just refer to ACH50 which references just the volume of the space.

Thanks in advance for anyone’s insight.