# Understanding blower door numbers. Air seal to 62.5 cfm 50

| Posted in General Questions on

I’m trying to understand a number that I was given by mass save. I convinced them to reconsider air sealing and they’ve agreed. They’ve proposed to air-seal to “62.5 cfm50.” How do I convert that to ach50? I had a blower door test done already which found “20,980 cubic ft.; 2600 cu ft/min = 156,000 cu ft/hr. hence 7.4 air changes/hour.” which included our basement. 5.7 ach50 with basement door closed. Zone 5, outside of Boston, 1915, 1.5 level bungalow, 1620sqft which does not include the basement.

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### Replies

1. | | #1

You've got the formula right there. 62.5*60/20980= 0.18 ACH50.

2. | | #2

Thank you. I didn’t think this was correct though. Mass Save is not air sealing to less passive house standards.

3. | | #3

Maybe they've got an errant decimal point.

4. | | #4

I suspect they meant 62.5 cfm ACHnat. Which is around 3 ACH50.

5. | | #5

Jon, I believe that makes the most sense. Not a decimal error because I’ve seen others with similar quotes. Thanks!

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