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What is N-factor?

14K5 | Posted in GBA Pro Help on

I was asked by a client — I’m an Energy Advisor — what the N factor was for his town requirements, and I didn’t know the answer.

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  1. homedesign | | #1

    N factor is a correlation factor used to estimate "Natural" air changes per hour when the Blower door ACH(50) is known

    ACH(Natural) = ACH(50) / N

  2. GBA Editor
    Martin Holladay | | #2

    The N-factor is the number by which you divide your ach50 number in order to estimate so-called "natural airflow," or ACH(nat).

    It varies from 9.8 to 29.4. It cannot be determined by climate zone alone; it also depends on the number of stories in the building under consideration. (See the image below, which was taken from

    For more information on the N-factor, see What is the N-Factor?

  3. GBA Editor
    Martin Holladay | | #3

    It looks like we were posting almost simultaneously ... you beat me to it. Thanks.

  4. Expert Member
    Dana Dorsett | | #4

    That's the definition, but it's a VERY squishy number, difficult to impossible to measure in-situ. An ACH/50 test number measures the combined size of the leak holes, but tells nothing about the leakage location or distribution, and those factors matter (a lot!) when it comes to the real infiltration rates.

  5. homedesign | | #5

    I agree ACH-50 (and N-factor) tell us nothing about how the "openings" relate to or interact with the Neutral Pressure Plane, the prevailing winds,human intervention and or mechanical effects.

  6. jwolfe1 | | #6

    Almost decade after the last post and I found this to be very helpful. Thanks!

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