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IBC Airtightness conversion

eyremountllc | Posted in Building Code Questions on

How do you convert cfm/sq. ft. @ 75 Pa to ACH 50?


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  1. GBA Editor
    Martin Holladay | | #1

    I don't know of any way to convert cfm per square foot @ 75 Pascals pressure difference to ach50.

    Do you know cfm/sq.ft. @ 50 Pascals -- otherwise known as cfm50?

    To calculate air changes per hour at 50 Pa (ACH @ 50 Pa or ach50), multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet.

  2. user-1061844 | | #2

    you can't...... if you don't know the envelope area of the building and the volume of the building.....

    if you do, you can figure it out - although you would need to use an approximate conversion factor to account for the pressure difference. Infiltec list this as a 1.30 factor to convert the flow rate (cfm)

    Keep it tight!

  3. eyremountllc | | #3

    Thanks Martin and Floris! That's what I thought after doing some research too.


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