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Spray foam over foam board

kevinkeegan | Posted in GBA Pro Help on

Climate zone 4, If there is no problem installing spray foam insulation open or closed cell spray over foam board can the foam board go up against the underside of the heated space plywood sub floor? Or is it better to install the spray foam up against the underside of the heated room subfloor, and fill the balanceĀ  of the framing cavity with an air permeable insulation towards the direction of the cold side of the cavity? Also would a vapor barrier be necessary? If so where should it be placed? No mech. in the insulated space.
Thank you

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you can use spray foam over rigid foam. The only potential issue you may have is that you could potentially have accumulation of "stuff" (moisture, crud, etc.) in any air space between the rigid foam and the subfloor. Basically you lose the benefit of the "fully adhered" part of spray foam doing what you're planning. I think in your particular application there is minimal risk though.

    If this is just a typical heated floor over a basement, standard practice is to just use "regular" insulation underneath, often rigid foam but sometimes batts. You don't really need spray foam here.


    1. kevinkeegan | | #2

      Even though the exposed side of this floor is not fully enclosed as it would be with a conventional basement? I understand what you are saying regarding the potential air space as the spray foam will adhere uniformly without gaps across the subfloor. I think I'm going to have a spray foam Contractor come to perform other work, so for this location should I have him install open cell or closed cell and how many inches? The framing cavity is 8 inches. Thank you, Kevin

      1. Expert Member
        BILL WICHERS | | #4

        If this is something like a crawlspace or a floor built over piers that is open to the outdoors underneath, then I agree with Akos suggestion about rigid foam under the joists to fully enclose the floor structure in insulation. You could then use open cell in the joist cavities, but I'd probably just use batts (cheaper). Use thick rigid foam, at least 2", and ideally more. If critters can access the foam, then you want some kind of critter barrier. The cheapest is probably 1/4" hardboard, but plytwood sheathing would be better.


  2. Expert Member
    Akos | | #3

    Here is the drawing detail for a floor cantilever:

    You generally want a layer of rigid under the joists to reduce thermal bridging (1.5" to 2"), this makes a big difference for comfort as the floor joists are much warmer. The rest can be pretty much any type of insulation, for floors a vapor barrier is not needed in any climate.

    If you are getting SPF already installed, the best would be open cell. Closed cell between floor joists doesn't buy you much extra assembly R value because of the thermal bridging of the joists, more cost for almost no benefit.

    Make sure they carry the spray foam up around the rim joists and the underside of the wall plates around the perimeter and across the foundation. The key to comfortable overhangs is to air seal as much as possible, you want to make sure all possible leakage paths are sealed.

    In cold weather, a cheap IR camera will show you where the are leaks, you should even be able to feel air leaks with your hand, so check for leaks before closing up the floors and touch up as needed with SPF.

    1. kevinkeegan | | #5

      Wow, You guys are great!! Where can I get a reasonably priced IR camera? I don't know if you fellows get in to sound transmission issues, but I'll ask anyway I have a first and second floor ceiling areas that I would like to reduce sound transmission and I have been told by an acoustical Engineer that it does not matter what insulation I use, fiberglass , rockwool, or cellulose in the joist framing cavities they will all work the same as long as the cavity is full. I have access to the floor joist cavities for the first floor from the second floor and access to the third floor floor joist cavities from the second floor. I do not want to remove the sheetrock on the first floor ceiling and I can not lower the second floor ceiling. The Acoustical Engineer also recommended that I use a product under finished floor and on top of the subfloor called Regupol which he said will help with impact sound. I intend to have the band joist area on all floors sprayed with closed cell foam and for sound between floors use fiber glass. I original thought of just dumping cellulose into the open joist bays on the second floor, but I have read that it needs to be fluffed up to provide the acoustical benefit. I would really like to get your advice on this information.
      Thank you, Kevin

      1. Expert Member
        BILL WICHERS | | #6

        Your sound engineer is correct about the type of batt-style insulation not making a lot of difference. Density does matter though, so you want to use high-density fiberglass or mineral wool for best results. Mineral wool is slightly better than fiberglass due to the higher density of the material, but it's not enough of a difference in terms of sound reduction for you to notice in most cases.

        The best way to block noise would be to hang the lower level ceiling from hat channel for decoupling from the structure, but that would lower your ceiling and you said you want to avoid doing that. A simpler option that doesn't lower the ceiling very much is to add a second layer of drywall, ideally 5/8" type X, with green glue between it and the original ceiling drywall. The extra mass of the second layer helps to deaden sound, and green glue helps to absorb some too. That and a layer of sound dampening material like Homasote (I'm not familiar with Regupol), along with stuffing batts between the joists, is about as good as you can do without getting into the fancy stuff.


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