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Community and Q&A

Air barrier using closed-cell spray foam

IrishGuyJohn | Posted in Energy Efficiency and Durability on


I am building a house, Zone 4A, with a Cathedral ceiling in the south half of the roof and a loft using trusses in the north half.  Exterior insulation on the roof is not possible.  The roof will be vented.  One builder I interviewed did say he’d apply 1” sheet insulation on the exterior walls.  I’d like to use closed cell foam for an inch throughout the interior space to create an air barrier and to begin the insulation process.

If the closed cell is continuously sprayed on the walls, top plate and up the roof rafters [Cathedral portion, across the attic floor in the truss portion]  will that be an adequate air barrier?   Will doing that tie the walls to the roof as one continuous air barrier?

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  1. Peter Yost | | #1

    Hi John -

    Spray foam is not a magic bullet; it's Achilles Heel is formulation/installation. You essentially need some level of chemist on the job site to make sure that the spray foam is "manufactured" properly on the job site.

    The spray foam is going to be continuous from wall to roof around a complex geometry of your roof trusses. And they move over time. Will the result be airtight? Prove it with a blower door test when the spray foam contractor is done but not yet been paid.

    I sprayed closed-cell spray foam on the exterior of my deep energy retrofit project (See this GBA case study: I installed the clapboards with stainless steel trim screws (tedious and expensive, yes). I remove random clapboards each year to check the adhesion of the closed-cell spray foam to the 2 by 2s, window jamb extensions, to the original CMU. So far over 18 going on 19 years, still great adhesion. But I really held the spray foam installer to the chemical company's manufacturing/installation requirements.

    1. Aedi | | #2

      Hi Peter,

      I am a bit surprised by your level of diligence in regard to your spray foam. Making sure the install was done right is good sense, but the yearly inspections seem a little extra. Is this an approach you would recommend others undertake, or do they simply represent an abundance of caution on your part?

      I've considered an unusual approach of using closed-cell spray foam to take the place of Ice and Water Shield on a PERSIST-style build (surprisingly similar costs, and the spray foam adds more insulation). I figured the spray foam would seal more completely and be more durable if done right. Is this wrongheaded of me?

  2. Expert Member
    Dana Dorsett | | #3

    Most of those issues were addressed on your related thread:

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