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Community and Q&A

Spray Foam insulation and foam board

kevinkeegan | Posted in GBA Pro Help on

Climate zone 4, Does it matter  in terms of what will bond properly to the foam board, what type or brand name of foam board is used, if my intention is to use spray foam over the foam board? Also does it matter if it is closed cell or open cell spray foam?  Also unrelated to the above question. I have gotten several quotes from spray foam installers and they do not quote by the square foot and it seems like they do not accurately subtract the framing members in the measuring process. In new construction I assume they can estimate that a framing member will occur every 16 inches, yet in old house construction it can be substantial different especially if reframing is performed to plumb and square walls and ceilings. I wonder if there is a way to sort this issue out? I’m located in Westchester County NY. 
Thank you

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  1. Expert Member
    Akos | | #1

    Spray foam bonds to pretty much anything, so you can use any rigid foam as backer. This only works for walls and vented roofs, if you want to create an unvented roof with cut and cobble rigid plus SPF for air seal, it will likely fail. The important part for an unvented roof is the SPF to be adhered directly to the underside of the roof deck to prevent any air movement into that space.

    For walls, provided your rigid sheet is thick enough for condensation control in your climate, you can use either closed cell or open cell foam afterwards. Unless you need an air gap between the insulation and sheathing, the simplest is skip the rigid foam and fill the whole cavity with open cell foam.

    Most foam contractors quote based on board feet and job complexity, I've not seen framing factor is subtracted from that.

    1. kevinkeegan | | #4

      I'm new to your blog so I'm not sure how to continue my conversation with you. I believe I replied to my own question, where I left more information and details about the project. if you could reply to the post I left that would be great.
      Thank you

  2. Expert Member
    BILL WICHERS | | #2

    I've used canned foam from both Dow (Great Stuff), and Loctite (Tite Foam) with EPS, Polyiso, and XPS. They all stick just fine, none are better than the others. Closed cell spray foam sticks to all of those too. Spray foam is probably one of the only normal materials that reliably sticks to XPS in fact.

    I've never used open cell spray foam in combination with rigid foam sheets, but I have no reason to think it wouldn't stick too. I'd be careful with either closed or open cell spray foam and EPS though just to make sure the spray foam doesn't cause any melting issues with the EPS in the moments before it cures.


    1. kevinkeegan | | #5

      I posted a question yesterday about insulating a roof yet I do not see my posting. It did show up after I first posted it but I was trying to find it today to see if I received a reply. I posted it on expert help, . I would appreciate any advice to locate replies to my missing question.
      Thank you

    2. kevinkeegan | | #6

      Climate zone 4. So in this construction. It is a heated room and a portion of this room floor is exposed to the exterior, AKA breezeway, how should I best insulate it I only have access from the exterior underside. What should the R value be in this floor assembly. I assume air sealing should be at the underside of the subfloor. Should this just be a sealant like OSI or a couple of inches of closed cell spray foam and the balance of the framing cavity filled with what open cell? I assume you can not have two vapor barriers in the assembly. The wind and cold air passing under this breezeway is my big concern. If I were to use open cell would I have to fill the whole framing cavity, which is 8 inches?

  3. kevinkeegan | | #3

    Thank you for your fast reply, I should have included in my post the location I intend to install the foam board and type of foam board. The foam board will most likely be Owens Corning Foamular NGX F-250 2 inches in thickness SSE R-10 XPS rigid. The location is the underside of a floor and that exterior side of the floor framing bays spans two exterior foundation walls, which I guess would classify as a breezeway, so no enclosure on the two open sides of the breezeway. The exterior finished side is covered with plywood. I was considering covering the underside with 1/2 inch foam board to help with thermal bridging. I would install that under the finished plywood. the framing bays are 7 1/2 inches in depth. The room above this floor is heated space.
    Thank you

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