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Blower door questions

Erik Olofsson | Posted in General Questions on

We are about to perform a blower door test on our small 650 sq. ft. new construction house. I have four quick questions:

1. We used T&G OSB and Siga tape for our air barrier, and i think we did an extremely thorough job detailing it. the house is not quite complete, however. for example, the clothes dryer is not installed, some of the joints in the hrv ducting in the crawlspace have not been sealed, the drywalled window returns have not been latexed to the window frames, etc… will these items have a significant effect on the test results?

2. Can you share any tricks of the trade to ensure the best results?

3. Is it possible to put a % number on the disadvantage of our house being so small compared to a more typicall house size of, say, 1800-2200sq’? this question is akwardly phrased but you guys know what i mean…

4. Does performing a blower door test on a fully furnished, lived-in-for-several-years house make a huge mess? i mean, mess up papers on a desk, kick up dust everywhere… stuff like that

Thanks again.

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  1. Expert Member
    Dana Dorsett | | #1

    1> Yes, as yet unimplemented air sealing details can have a significant effect on the measured result.

    2> Air sealing isn't rocket science, it's goop science. The most important thing is to have a defined "all sides the cube" primary air barrier layer in the plan, and to implement the plan, the key factor being the air sealing details at the transitions.

    3> There is NO disadvantage in being small, if your metric is in CFM per square foot of exterior building surface @ 50 pascals (a metric Building Science Corp and others are pushing as a more appropriate standard), as opposed to the traditional air changes per hour @ 50 pascals number, now enshrined in the IRC.

    4>Blower door tests don't make a huge mess unless you have huge air leakage. While they are capable of moving a lot of air, that air doesn't move unless there's a path for it to move. A "kinda-tight" 650 square foot house will be leaking less than 500cfm, and while you'll feel the breeze directly in front of the blower, 500cfm is not too dramatic. A "really tight" house that size could be under 100cfm @ 50pa. PassiveHouse tight would be on the order of 50cfm. As long as your desk isn't right in front of the blower, your papers will still be there, unless you left the window open. In older not-so-tight homes it's possible to suck in some dust at high infiltration points, which is why depressurization tests really shouldn't be done on houses with asbestos insulated pipes in walls, or asbestos siding, etc.

  2. Erik Olofsson | | #2

    thanks for your info, dana. as i said, my AB is complete and i am confident it is better than average. my main concern is my ignorance. obviously i will seal the dryer vent but what about the hrv? will the depressurized house suck outside air in through the hrv? should i tape it shut? and the plumbing vent? do i need to make sure all the p-traps have water in them?
    unfortunately, canada's r2000 program requires [email protected] they do not use any other metric. i was just wondering, then, what sort of handicap i could use for my personal edification. for example, if the result is 1.6ach, would the same house at 2200sq', ceteris paribus, test <1.5ach?

  3. GBA Editor
    Martin Holladay | | #3

    You may be interested in reading Blower Door Basics. The article answers many of your questions.

  4. Expert Member
    Dana Dorsett | | #4

    Erik: It really depends on the shape of the house, and the ratio of the exterior surface area to the contained volume. There isn't really a generic 2200' house shape, and a 2-story cube would have a lower surface area to volume than a 2200' 1-story rectangle, and the 1-story rectangle would have a lower surface area to volume ratio than a complex footprint 2200' 1-story with bump-outs, etc. If your 650' house has a simple shape with clean simple lines it's surface/volume ratio may be lower than a complex shaped house with 3x the floor area. But simple clean surfaces are also comparatively easier to air seal, in which case the simple 650 has an advantage over the complex 2200.

    HRVs and other active ventilation schemes are normally capped off for purposes of blower door testing. And yes, drain traps have to have water in them or it'll test 100s of CFM leakier than wet-drain reality.

  5. Mike LaCrosse | | #5


    While you wont get an accurate finished CFM50 reading when certain details have yet to be sealed, doing a blower door test while under construction can still prove very useful. For instance if your finish materials are not yet installed, you may still have an opportunity to seal leaks within the structure that might have otherwise gone unnoticed without the blower door. If you still have this opportunity I would recommend it in addition to testing for a final leakage number.

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