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Airtightness Goals

homedesign | Posted in General Questions on

I think that we should raise or SET the bar for air tightness goals.

What is the bar? Is there an Energy Star standard? A LEED standard?

Should it be the Building America Westford Metric?
What is the Westford metric? is it 1.5 Air changes per Hour at 50p or is it 3 ACH @ 50?

How about Passivhaus Standards? 0.6 ACH50

How many out there are surpassing the Westford Metric?

Is any one besides Passive House USA meeting Passive house airtightness standards?

What is your best blower door result and how did you achieve your air barrier?

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    Our last house tested at 0.7 ACH50. We had 8" of open cell in the roof, 6" JM Spider in the walls, moderately well detailed Tyvek (missed the FEMA header-wrap detail on this one) and caulked bottom plate and routine E-star thermal bypass checklist details.

    Our 110 cfm bath fans on time delay motion sensors were only pulling 70 CFM even with well detailed metal duct. We had a sealed crawl with a bath fan pulling another 70 CFM 24/7 through the chimney to the roof.

    I'd vote for setting the bar at 1.5 ACH50 provided it is accompanied with a 7.5 CFM/occupant fresh air strategy.

  2. homedesign | | #2

    That is an impressive ACH (almost Passivhaus)
    I think that ALL new homes(not just extra tight) should have a calculated fresh air strategy.
    Leakage is not a reliable Fresh Air Strategy.
    That also means that we MUST measure the ventilation airflow as you have done.

    I agree that 1.5 ACH50 is a respectable and achievable goal for all houses.
    I also think that it will be very difficult for most builders to achieve "1.5" with air tight drywall strategy alone. Just consider the Building America Westford House.

    Air tight Drywall seems like a good idea .. but it is aparently Not So User Friendly.

    I think the airtightness of my house(1.4 ACH50) is mostly due to the combination of 100% structural sheathing, open cell spray foam and careful sealing between framing to framing and framing to concrete.

    If I had been more aware of Passivhaus at the time..than I would have put more effort into Airtightness and set my goal to be 0.6 ACH or better.

    Besides Passive House USA and Thorsten Chlupp in Alaska .. I have not seen any published USA Airchange results that would meet minimum PHouse standards.(you got close)
    Thorsten's results are likely due to the fact that he does predrywall blower door testing and corrections.
    according to Thorsten's recent JLC article
    "If you hold off doing the blower-door test until the insulation and drywall are installed, it’ll be too late."
    This strategy means that Air tight drywall aproach is not an option.

    Air leakage is not just an Energy factor ... it is a Significant durabilty Issue.

    Michael.. I understand your air barrier at the roof... but I am not clear about your walls.
    Did you use 100% structural sheathing?
    Do you test before drywall?

    Can you 'splain Fema Header wrap? I am not familiar

  3. homedesign | | #3

    Well, this is strange... I didn't edit this old post...
    but somebody did...and the time stamp says 1/04/2013 -14:21

  4. Expert Member
    Dana Dorsett | | #4

    Some people prefer to specify the standard in cfm/50 per square foot of exterior surface area, rather than ACH/50 or cfm/50 per square foot of conditioned floor area. There's some rationale to that, since leakage is a function of surface area, not volume, and houses of equal surface area can have dramatically differing volumes or floor area.

    But none of it adequately expresses the true infiltration levels, and despite some complicated models for ACHnatural based on raw blower door test numbers, they're complete fiction: The LOCATION of the leakage matters (a lot!). If the leakage is concentrated as a single hole at the stack-effect neutral plane there is almost zero infiltration. If it's equal sized holes at the foundation & attic the natural infiltration will be several orders of magnitude greater than the 1-hole @ neutral plane model. Real houses are all over the place, but the important leaks are still the ones at the bottom & top of the house.

    From a practical point of view, it can sometimes be pretty labor intensive to build to 1ACH/50 or even 1.5ACH/50, whereas it's pretty cheap & easy to build to 3 ACH50 just by avoiding all of the stupid-attack leaks- it's an easy standard to beat. But the thermal performance boost of reducing it to 1.5ACH/50 will often be negligible, as long as the stack-effect leakage has been dealt with. (But it's a bit hard to isolate the ceiling or foundation for single-surface leakage test, eh? ;-) )

    Not my project, but a Deep Energy Retrofit was helping a friend manage came in at 464cfm/50 on a 3-story + full basement on a century old balloon framed house that had been divided into 3 apartments (by floor), which was roughly 1.2-1.4 ACH/50. Most of the identifiable leakage was at framing of the basement door at the bottom of the bulkhead steps, which he fixed with low-expansion gun foam post-test, without re-testing. A lot of spray foam (more than I was recommending) went into stud & rafter bays on plank-sheathing, but both the drain-plane wrap and exterior layer of iso were detailed as air barriers.

  5. GBA Editor
    Martin Holladay | | #5

    I'm the culprit. I was doing a search for "FEMA header wrap detail" and came across this old thread (because of Michael's old post).

    Then I noticed that you had misspelled "airtightness" in the title (you wrote "Airtighness") and it bugged me enough to correct the spelling mistake. (I figure that correcting misspellings makes searches work better.)

  6. homedesign | | #6

    thanks Martin...that was back before we had the handy editing feature

    check out post 24 at this Thread for more comments from Chandler

  7. GBA Editor
    Martin Holladay | | #7

    Thanks for the link! Very helpful.

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