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Community and Q&A

Applying Roof spray foam from the exterior

Rooke | Posted in Energy Efficiency and Durability on

I have vaulted ceilings in my house.  The roof joists are 2×10.  The interior ceilings are 1×6 t&g pine.  There are 9 inch R30 insulation batts between the joists. For a couple of reasons the current roof venting is not so good.  I will be putting a new roof on in the near future.  I will be removing the roof sheathing and plan to upgrade the insulation.  I would prefer an unvented solution.  I would like to fill the ceiling void with closed cell spray foam but everything I have read says it should be sprayed on the underside of the roof deck.  This seems is might be hard to do from the exterior.  Is any one aware of this being possible.  If not would it be possible to use about 8 inches of spray foam leaving the last 1 1/2 inches of the joist space open and then vent that cavity.  This would give me about R40-50 depending on the spray foam.  So basically a closed spray foam vented ceiling.  

Any thoughts or comments are appreciated.

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  1. freyr_design | | #1

    you should just use polyiso boards on the exterior. They are cheaper, come in flat sheets, have compressive strength so that you can just add a new roof deck over with long screws/pan head lags. If you are replacing your roof anyways I see no reason to use spray foam on exterior to create and unvented assembly.

    If you need an air barrier place it at your current roof deck by using either tape or a SA membrane. then use standard underlayment on new roof deck. Idk your climate zone but just use enough to keep your sheathing warm. There are multiple articles discussing foam ratio

    1. Expert Member
      MALCOLM TAYLOR | | #2


      I agree, but I wonder how to deal with the ventilation gap under the existing sheathing?My understanding is that when there is roof top foam, the IRC wants the permeable insulation to be in contact with the sheathing.

      1. freyr_design | | #5

        Blow in cellulose from exterior? I’m not sure how feasible that is, but if you could just dense pack on top fiberglass?

        I also wonder how important the existing 1-2” air gap is in terms of keeping the sheathing above condensation temp….

    2. Rooke | | #3

      Thanks for y0ur reply. I have considered this option but could only use it on 2 of my 3 roof levels. The 3rd level has clerestory windows mounted right above the roof deck so I cannot add any thickness to the roof. Although feasible on the other 2 levels (and probably the best solution) it would require modification to existing trim and fascia, so just seeing if there may be another option that would be consistent on all 3 levels

  2. Expert Member
    BILL WICHERS | | #4

    You can't install spray foam from the exterior the way you describe. The reason is that you won't be able to get a fully adhered layer of sprayfoam on the underside of the exterior sheathing that way. The reason that closed cell spray foam is safe in these assemblies is that the fully adhered layer of spray foam leaves NO GAPS for moisture to accumulate in up under the cold sheathing. The vapor permeance of the spray foam is secondary to that fully adherered part. If you apply spray foam from above, it will adhere to the back of the finished ceiling, but will leave an air gap under the sheathing. If you can arrange things so that that air gap is a vent channel (i.e. vents at the top and bottom of every rafter bay), then you should be OK, otherwise it's not going to be a safe assembly.

    I like the polyiso over the top idea, but as Malcolm mentioned, you'd need to fill the rafter bays with insulation -- you can't have an air gap between layers of insulation. The places where you can't make the roof thicker get tricky though -- can you maybe vent those sections seperately from the rest of the roof? Maybe open up the interior in those sections to apply spray foam from below?


  3. Rooke | | #6


    Thanks for your reply. I understand that the spray foam needs to adhere to the bottom side of the roof deck. I was wondering if when adding the last level of spray foam just below the top of the roof joist and then applying the roof sheathing immediately the foam would expand and adhere to the underside of the sheathing. Seems like it might be possible but difficult and perhaps not feasible due to the imprecise nature of the foam expansion. Short of the option, I could vent all three roofs at the top and bottom and then spay foam to a depth of 7 to 8 inches thus venting the space between the top of the foam and the underside of the roof deck. This would give me an improved insulation value and significantly reduce air flow. I just have not heard of this approach and was wondering if there is something technically wrong with doing it this way. Also my interior ceiling is 1x6 T&G pine. Is it OK to apply spray foam directly to this or is there some type of additional fire barrier required.

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