# Converting ACH50 to cfm – Basement

| Posted in General Questions on

When converting ACH50 results to cfm, is there a factor that’s used to allow for the air volume in the basement (to account for zero leakage through the concrete walls)? Or is the basement volume subtracted when they do the blower tests?

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### Replies

1. Expert Member
| | #1

"To calculate ach50, multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet."

Don't assume that basements are leak-free- it's quite the contrary. Even the cracks in the slab and the seam between the slab and wall can leak substantially under pressure (ask any radon abatment contractor!).

But one of the biggest air leaks in homes with sloppy air sealing detailing is the basement band joist and the seam between the foundation sill and the top of the foundation, even if it has a cheap foamy sill gasket stuffed in there!

2. GBA Editor
| | #2

T. Barker,
The answer can be found in my article, "Blower Door Basics," where I wrote:

"ACH50 can only be calculated once the building’s volume has been determined. To calculate ach50, multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet."

[P.S. Evidently Dana was two minutes faster than me.]

3. | | #3

Thanks Dana. I had the calculation figured out, but I assumed the basement shouldn't be treated like the other floors so I wondered if there was a fudge factor that's commonly used for concrete basements.

So you treat the basement volume essentially the same as the other floors usually?

1. Expert Member
| | #4

Yes- the basement is inside the pressure boundary of the house, and considered part of the total volume, even if it doesn't count as living space in a real-estate sales listing or tax assessment.

And all details aside, basements often leak BIG time!

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