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Blower door test results

airfix | Posted in General Questions on

Climate Zone 6b – New Construction. Just going for a “pretty good” house concept.

The test results are in for the blower door test.  The blower door test was a sheathing sealed and complete – pre-insulation and pre-sheetrock test to check my primary air barrier.

As with everything in this build I was disappointed in how the test was conducted.  Despite me asking to be present at the test the general scheduled it when I was out of town.  As a result they just did the test and did not utilize the test to find leaks and patch up any leak paths.  They did the test and just accepted the numbers.  Like I told my builder, I don’t care specifically about the numbers I just want to make sure we seal up holes we can find or know about and the numbers will come in at what they are.

Anyway here are the numbers:
2400CFM at 50 pascals
Volume 69,335 cubic feet
(4000sqft home with oversized attached garage & workshop)

I know we could have been much better had we used the air test to find leaks and patch them. 

The blower door test included an over sized (almost 4 car garage) with one double deep bay having a 14ft ceiling.  The garage doors were sealed.  I think the garage will skew the results to look better than they are due to the garage volume compared to the garage exterior perimeter.

The reason they included the garage is that there is no sheetrock up and there was no easy way to seal the partition between the house and the garage.  The garage will be heated in the winter anyway so it’s probably legit to include it.

It’s a big flow rate escaping from my house every minute that I have to heat.

My target at this stage was about 2ACH50 because I know it will get better when we add our secondary air barrier (the sheet rock).  The target for the whole house was less than 1.0ACH50 however I’d probably be happy enough with anything under 1.5ACH50.

The guy doing the blower door test thinks we will definitely be under 1.0ACH50 when we add sheetrock.

With respect to the volume, what is industry standard for calculating the actual house volume?.  When testing at the sheathing stage I assume the volume used should include everything from the sheathing in.  However when you do a blower door test on a finished house do you use the same volume or do you only look at the volume contained within the sheetrock, ignoring the stud and rafter bay cavities?


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Steve,

    I'm not sure of the answer to your question, which I have never seen come up before. Maybe someone else can jump in and answer that.

    As far as improving the numbers goes, detailing the drywall as an air barrier will definitely help tighten things up. Window and door installation usually does the opposite. It's common that builders get there best numbers on that first test.

    Depending on your budget and desire to get the house as tight as possible, you could bring the blower door folks back and do some Blower-Door-Directed Air Sealing. And/or you could consider AeroBarrier, which people are having success with.

  2. airfix | | #2

    Yeah We're kind of past the point of blower door directed sealing now. All the interior BIBS insulation has gone in and the exterior foam is being put on at the moment. The next step for targeted blower door test sealing would be when we do our final blower test after the sheetrock.

    I did look at aerobarrier but opted for a more traditional tape, caulk and tactical spray foam approach.

    The air volume in the home makes a huge difference to the ACH number that is calculated. I'm surprised nobody here knows what volume should be used. I'd assume after you do sheetrock the volume to be used is the volume contained on the living side of the exterior wall and ceiling sheet rock and not the volume of the home inside of the exterior sheathing.


  3. airfix | | #3

    I'm surprised nobody here knows what volume should be used for calculating the leakage rate in a blower door test?

  4. Trevor_Lambert | | #4

    There are different standards. I know for passive house, they use interior dimensions, and subtract even interior walls and non-living space. I suspect that if the person doing the test is in any way connected to the party responsible to meeting an air tightness target, they use whatever tricks they can think of to make the numbers look better. I think a pretty reasonable metric is area inside drywall of the exterior walls. 2ACH seems pretty high for a fully taped and caulked house. That represents a pretty big equivalent hole area, about 120 square inches if my math and memory are not faulty.

    1. airfix | | #5


      I would have thought that was high too but a close friend built a house to a much higher standard than I did with double stud walls and near net zero. His first blower door test came in at 4ACH50. Then with some effort he got it to 2.6ACH50 at the same stage I'm at. Once he put up sheet rock he got his leakage to less than 1ACH50.

      I've been around the house and I can't see too many places that we've missed. There are a few gaps here and there that I'm going back and filling with a spray can of foam.

      I wish I'd thought about the importance of how the volume is measured before hand. I'd just assumed all guys would use the same standard. For my next test (after drywall) I'll make sure I specify the volume I want tested. It will be drywall in.


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