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Passive House Standard for Max Air Leakage

Patrick Mccombe | Posted in General Questions on

PH airtightness in CFM per square foot

Hello GBA,
I figured this was the best place to ask. Is 0.06 CFM50 per square foot the Passive House standard for maximum allowable air leakage? I appreciate your help.

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Replies

  1. GBA Editor
    Kiley Jacques | | #1

    Giving you a bump by noting the PH 2021+ prescriptive path puts maximum leakage at 0.04 CFM50 per sq. ft. of envelope area according to Carl Seville's quick-and-dirty outline of alternative measures for reaching PH standards.

  2. benwolk | | #2

    Patrick,

    The PHIUS 2021 guidebook has two airtightness requirements for single family construction. 0.06 CFM50/sf of envelope area is the requirement for the performance path (ie full WUFI modeling) where the prescriptive path (no WUFI modeling needed) requires a tighter envelope metric of 0.04 CFM50/sf as Kiley noted above. The more strict standard for the prescriptive path is to account for the lack of modeling required by the prescriptive path to help ensure an efficient building.

    See the link for the latest guidebook: https://www.phius.org/phius-certification-guidebook

  3. user-5946022 | | #3

    Seems like the standard of measure has changed, which could be confusing.

    I see the reference in Carl Seville's article to "maximum leakage of 0.04CFM50 per sq. ft. of envelope area." Seems to imply measuring the envelope - walls & roof make sense, but does it include the sf of the floor system? Does is include crawl space or basement walls below grade?

    Wasn't the previous standard 0.6 ACH 50, where ACH = Air Changes per Hour. In that case, the CFM quantity would be based on the volume of the house.

    With this new method of measurement, are the homes tighter than before?

    1. matthew25 | | #5

      You are thinking of the original PassivHaus standard which is 0.6 ACH50.

      The OP is referring to the watered-down American version which is PHIUS or Passive House Institute U.S which is 0.06 (or 0.04) CFM50/sq. ft. envelope area. They define envelope area as: "Gross envelope is measured at the exterior of the thermal boundary, the same as for the energy model, and includes surfaces in contact with the ground." So it includes floor area as well (https://www.phius.org/sites/default/files/2022-03/Phius%20Certification%20Guidebook%20v3.02.pdf)

      "With this new method of measurement, are the homes tighter than before?" No, unfortunately it is the exact opposite since the PHIUS standard is so much easier to achieve.

      Take a 40' x 60', 2400 sq. ft. home for example. The worst-case (i.e. lowest allowed leakage) for PHIUS would be a simple rectangle form with a flat roof. This would minimize the surface area of the house and have the lowest allowed leakage. If we assume 10' ceilings, gross envelope area would be 6800 sq. ft., which means you can have up to 408 CFM50 of leakage using PHIUS methods. The same home has a volume of 24,000 ft^3 which means according to PassivHaus you can only have up to 240 CFM50 of leakage. Big difference.

      Also, keep in mind that very few houses today are simple rectangles with flat roofs. A more complex shape with jut-outs and a complex footprint would have far more envelope area, allowing EVEN MORE leakage under the PHIUS method. And don't get me started on the climate zone-specific energy usage requirements under PHIUS where mild climates literally have to do nothing to reach certification (I say this as someone who lives in CZ2). It's just another example of watering down standards so mediocre builders can tout their projects as being energy efficient. It detracts from the diligent work the good ones are doing out there, some of whom frequent this forum.

  4. GBA Editor
    Patrick Mccombe | | #4

    Thanks Kiley and others, Y'all have a great community here. I appreciate your help.

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