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Community and Q&A

Spray Foam on Exterior Walls, Cellulose to Interior

Michael_Kalman | Posted in General Questions on

Hey guys!

I am wondering if having spray foam applied to my exterior walls in about 1” thickness will cause any problems if I place cellulose in the interior side of the spray foam in my 2×6 cavity.

on the exterior I have plywood, tyvek, furring strips, and pine siding. My plywood seams are all taped but I fear I have not taped the where the plywood meets the top plate and also where the bottom sheets meet the sill.

Would this create a vapor sandwich if I used open cell instead of closed cell and would this result in a more airtight structure? Is it worth it?

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  1. Expert Member
    RICHARD EVANS | | #1


    There is a lot to address here. What climate zone are you in? This will determine how thick your spray foam should be.

    I believe closed cell foam is only reliably considered an an air barrier when it's thickness exceeds 2". You'll need about 5" of open cell foam for it to be considered an air barrier. Neither foam will create a "vapor sandwich" in your assembly but condensation may occur if the foams isn't sufficiently thick for your climate.

    If you've already taped the seams, and your goal is increase air tightness, then adding spray foam to the cavities isn't going to help much. Unfortunately, you missed an opportunity to air-seal top and bottom plates. These are key areas to make air tight due to the stack effect. Spray foaming the perimeter of each stud cavity might help but I wouldn't rely upon it personally. Perhaps moving your air barrier to the interior would help. (Air tight drywall, interior smart vapor retarder tied into ceiling, etc)

    There are minimal benefits to adding an inch of spray foam in terms of added r value. This might get you an extra r-2 or so. The payback period is likely decades.

    Hope this helps!

  2. JC72 | | #2

    "I am wondering if having spray foam applied to my exterior walls in about 1” thickness will cause any problems if I place cellulose in the interior side of the spray foam in my 2×6 cavity."

    - No problem because the interior walls can dry out towards the interior. As for the thickness of exterior form (spray or rigid) that depends on your climate zone.

  3. Michael_Kalman | | #3

    Okay. So I passed on the 1” in the walls. Now my question is, if I have my 2x6 joists on metal trailer and I may not have a skirt installed this winter, would I benefit from insulating the last 1.5” of my subfloor cavity with cellulose, could this cause condensation problems/rot from condensation in the winter, if so, should I put down a floor underpayment? I only have 4-4.5” of sprayfoam in my joists and I worry that It might not be enough and I don’t want to suffer with extremely cold floors, zone 5b

    1. Patrick_OSullivan | | #4

      Could you describe the assembly a bit more and how it's laid out? I'm envisioning a (not readily moveable) trailer, elevated off the ground, with joists in the bottom that have spray foam between them and against the subfloor. But I suspect I might be wrong based on what you're asking.

      1. Michael_Kalman | | #5

        Hey Patrick,

        I have a trailer with 4 axles, on top of the trailer is a subfloor built out of 2x6 joists. On the bottom of those joists is 3/8 ply, and spray foam is sprayed on top of the ply in between the joists. 3/4” ply will go on top of the joists.

        1. Patrick_OSullivan | | #6

          A ha! Now I get it. Cool structure.

          I suspect any comfort issues on the floor are going to come from lack of a thermal break between the outside and inside. Some cellulose on top of the spray foam might help, but probably less than you'd like. The joists coupled to the subfloor will act as heat sinks for the floor overall.

          I don't know what the interior looks like, or if you have the headroom, but one option would be to put a layer of plywood down over the joists, then as thick a layer of rigid foam as you can afford above it, then another layer of plywood.

          1. Michael_Kalman | | #7

            Hey Patrick, would there be any disadvantages to topping it up with cellulose? I can’t afford that much height in the tiny house lost with the rigid foam. I guess I’ll be cranking the wood stove haha

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