I recently just got my house air sealed. Before air sealed the Building Air Standard (BAS) was 1.4 now it is 1.2. Based off a 1300 sq ft house. What should the BAS be? Is there a calculator to determine if I have the correct air flow per hour?



1 Answer 1


Building Air Standard (BAS) is sometimes referred to as BAS - ACH This appears to be a (I begin to think deliberately, after quite a few vague web results) slippery number to track down, but it appears to be essentially an estimation of "natural air changes per hour" based on the blower door test. Simply calling it "Estimated ACH" would apparently be too straightforward for whomever came up with "BAS"...

0.35 or below is "tight" (requires mechanical ventilation below that to maintain indoor air quality.) It might be a good target to shoot towards or near, though that may be difficult on an older home. Some newer builds are evidently shooting for 0.06 and then mechanically ventilating, but that's a new build where you can take steps at the design and build phase to make things tight. Retrofitting is more difficult.

As such, 1.2 is better than 1.4, but not all that great, or all that much improvement. i.e. you still have significant air leakage.

  • Thank you for your feed back. Besides air sealing the joists in the attic and having the correct insulation R-value. Installing insulation in the rim joist of the basement I do not know anything else to do to improve it. My house is build in 1960 and my basement is unfinished, so it could be due to lack of insulation on exterior walls... I was hoping when they were done it go from 1.4 to .9 I would be happy with that but as you mentioned and what I figured going from 1.4 to 1.2 is not much at all.
    – MOE12
    Mar 10, 2015 at 17:47
  • 1
    One point of doing a blower door test is supposed to be to actually go around the house while the blower door is running and look for air leaks, so you (or the competent contractor) knows where the air leakage is coming from, so it can be fixed. Insulation as such has no effect - this is purely an air leakage number. Typical areas of issue are leaks around windows and doors, at wall-to floor joints, around electrical outlets, etc. Cracks or gaps in the basement walls would be more likely impact the number than insulation, unless the insulation was also an air barrier.
    – Ecnerwal
    Mar 10, 2015 at 17:53
  • From what I've read, 1.4 to 1.2 seems to be proportionate with what's typically reasonably attainable with basic air sealing. Doing better usually requires taking the siding off to fix issues with the walls.
    – Zhentar
    Mar 10, 2015 at 18:38
  • Also, the BAS number is supposed to be cfm50 value. I suspect that "1.4 BAS" means "140% of target air exchange" and 1.2 means 120%, so they aren't actually trying to get below 1 without makeup ventilation. That's much better performance than I would expect from a 1960s house, though.
    – Zhentar
    Mar 10, 2015 at 18:40
  • This seemed to be the least weaselly of many documents I looked at, and does not imply that "1 is the target" but rather that any number above 0.35 (without mechanical ventilation) was cause for tightening up. nuwnotes1.nu.com/apps/clm/eventcalendar.nsf/…
    – Ecnerwal
    Mar 10, 2015 at 19:19

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