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Community and Q&A

airsealing deatail – flash and batt?

harrison55 | Posted in GBA Pro Help on
We are building a “pretty good” house in Virginia.  We will do a blower door test, and we shooting for low air infiltration (1 -2 ACH50).
We have a cathedral ceiling, and the air seal on the ceiling will be a continuous layer of closed-cell foam on the inside of the roof, burying the top chord of the trusses. We are using Zip R6 wall sheathing, and relying on the Zip for our main airbarrier on the wall.  

We originally planned to use Zip tape to carry the air barrier from the surface of the Zip across the top of the stud wall to the interior, so that our interior foam air seal could join to it and give us a continuous air seal.  But that didn’t happen.  The Zip tape wasn’t placed correctly on the gable wall and now our interior spray foam is looking to join the polyiso side of the Zip R.

If we run the spray foam to the polyiso side of the Zip R – basically, flash and batt our airseal to the Zip polyiso –  will that provide an adequate air barrier?  

Another way to ask this question: What sort of air infiltration numbers do you get with flash and batt construction?  (If flash and batt will give respectable air sealing, I may not need to worry about this detail.)

Any advice or experience is appreciated.



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  1. Expert Member
    Peter Yost | | #1

    Hi Mark -

    Here is my cut: the geometry of a continuous rigid air barrier such as ZIP is simple: flat most places and right angles on corners. The geometry of cavity fill is complex and not continuous.

    I find the former to give better inherent air tightness than the latter. Can either approach be done poorly? You bet. So just as important as your approach is the quality of the work. In either case, QC your air control layer with a blower door/IR performance test.


  2. jollygreenshortguy | | #2

    Mark - How did your project turn out? Did you achieve your desired blower door results? These are details I'm wrestling with right now.

  3. harrison55 | | #3

    It turned out well. We cut away the foam on the top 6' of Zip_R. Then we foamed up to the inside (OSB) of the ZIP-R, and got a pretty good seal. The blower door was 0.54 ACH@50PA.

    But this is just a workaround. If I were doing it again, I would make sure to wrap the top of the sheathing at the gable wall in Zip tape, so that the foam on the underside of the roof has something better to seal to.

  4. harrison55 | | #4

    That should have been * top 6 " *

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