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Renovating kitchen: Interior rigid foam an option?

Eric Nash | Posted in Energy Efficiency and Durability on

Hi Gang,

I’m renovating a kitchen and trying to present some insulation options to the homeowners. The walls are getting stripped from the inside down to the studs, but the exterior is not being altered.

I would like to use some rigid foamboard to crank up the R value and to provide airsealing, but I’m worried about not having a place to secure the cabinets. Someone suggested installing a layer of plywood under the drywall and screwing things to that.

How about this:

From the inside to the outside…


1/2″ drywall fastened to ply with 1″ drywall screws
1/2″ plywood fastened to studs with 3″ screws
1 1/2″ rigid foam [tape seams and foam penetrations]
4″ old school studs insulated with cellulose
7/8″ old school sheathing


Thanks in advance!

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  1. Riversong | | #1

    If it's only the cabinets you're concerned about, I would forgo the plywood, install appropriate blocking and use 3" cabinet screws. I would rather have cabinets secured to framing than to ½" plywood.

    You'll have to move the electrical boxes inward or use deep box extensions. For that, I've used plywood pieces attached to sides of studs to attach the boxes.

  2. David Meiland | | #2

    Once you get everything stripped I would check carefully for any signs of leaking around windows, etc. That wall without the foam probably dries pretty well if it gets wet. Once you add the foam you reduce the amount of air and heat going out and it may not dry as well.

    I probably wouldn't install the ply just for hanging cabinets either, but I might run a 2x4 across the wall at the top of the uppers, interrupting the foam. Tape the 2x4 to the foam.

  3. Eric Nash | | #3

    Hey thanks for the ideas! Does anyone have a problem with the admittedly sub-optimal "vapor profile" of this proposed wall system, eg that it has the foam on the inside, with the dew point happening out in the cellulose and the first condensing surface is the sheathing? I guess the saving grace would be that the foam would reduce the air moving through the wall. Any thoughts?

  4. Andy Ault, CLC | | #4

    Given the possibility of reduced drying potential, I would probably go with something like Roxul vs. cellulose. That way it's just one less food source if moisture does end up in the cavity for longer periods. As for the plywood, the system you described it what we do as SOP for our wine cellars. The only difference being that we step up to 3/4 ply to increase our holding power. Granted those racking systems have many more points of attachment, but it's still super helpful to know that no matter where the layout lands and no matter where we may need to shim that we will have a solid material to bite into.

    The problem with adding blocking is that you are just adding gaps in the insulation and your are continuing the thermal bridge. Without the blocking you have a complete thermal break. The other possible issue with the 3" screws is what if your layout just barely catches the edge of a stud, Then your in a bind trying to get through 1.5" material and still get enough bite to hold. Same goes for needing to shim, then your adding even more thickness until you reach a stud. We've fought these battles to many times in cellars which is why we just default to the ply.

    As for electrical, Robert is correct that you have to plan for this in advance. In your scenario, you could probably use old-work boxes with the flip-out ears and have them grab onto the back of the 1.5" board. Between that on the back side and 3/4" ply on the face, that box should be fully stable. This would also help with not compromising the thermal break since they wouldn't be attached to framing.

    The last thought might be that while you have the back side of the original sheathing fully exposed, it might make sense to tack some tar paper to that before putting in the new insulation. This way you still have a fairly high perm-rate, but you might be doing yourself a favor in the event of a bulk-water event from the exterior. And depending on what region you're in and what direction the wall faces, this may also help to give you a little resistance to inward solar vapor drive.

  5. Thomas Jefferson | | #5

    It could be helpful to place a relatively thin batt of mineral wool (say 2") behind the sheathing in this situation, to increase the separation between potentially wet outer layers and the cellulose insulation. This would remain vapor open but would tend to repel any wetness from the exterior.

    To avoid the possibility of trapping moisture in the wall (as with plastic foam covering the studs), one approach would be adding horizontal furring across the studs to increase the cavity depth, and filling that space with cellulose. This could be 2x2 or even 2x3 furring on edge.

  6. Riversong | | #6


    You don't indicate what climate you're in, so it's a little difficult to make appropriate recommendations. But, if you're in a cold climate, I would go with the interior foam (XPS, not foil-faced polyiso, for some vapor permeance), no plywood, and blocking between the studs for cabinets.

    Between-the studs blocking will not interrupt the foam thermal break, will allow a good air barrier (also foam around and behind electrical boxes), will allow minimal drying to the interior, but will not disturb the excellent drying to the exterior of your "old school" sheathing (which is far superior in terms of moisture management than most of what we do now).

    I would not install any felt or other membrane inside of the sheathing, as that will do little to prevent water intrusion, which will still soak into sheathing and framing members, making them more vulnerable. And I would not hesitate to use dense-pack cellulose, since it can tolerate cyclical wetting and redistributes moisture so well as to reduce the vulnerability of the framing in the event of leakage.

    In cold country, the dominant moisture drive is from inside to out. The foam/air barrier will eliminate almost all interior moisture penetration and the "old school" sheathing will expedite drying and hygric redistribution.

    With continuous horizontal flat blocking at cabinet top and bottom levels, there should be no problem getting good attachment with long cabinet screws. And old-work electrical boxes won't work in foam-backed drywall, since the total thickness will not allow the wings to catch. Pre-attaching plastic boxes to plywood extensions with screws, as I suggested (and have done), allows the back of the boxes (with all those cable holes) to be sealed before the rigid foam is installed. This is critical to creating a proper air barrier.

  7. Eric Nash | | #7

    Many many thanks to everyone for the ideas, I am greatly in your debt.

    I forgot to include my climate, as I somehow thought this was a North-East regional website. I'm in Western Mass. Great ideas.

    Here's two more questions for everyone.

    1] Re: XPS vs. Foil Faced polyiso: If a goal is to reduce the moist air moving into the wall in winter, why not go with the foil faced? Is it to have some potential to dry to the inside when needed?

    2] I'm a little nervous about having to put the drywall on with 2 1/2" screws as I haven't done this before and am concerned about some unforeseen consequence. How has the long-screw-through-foam thing worked out for everyone in terms of drywall problems like nailpops?

    Thanks again!

  8. GBA Editor
    Martin Holladay | | #8

    1. I don't believe the difference in vapor permeability between EPS and foil-faced polyiso is significant enough to matter in this application.

    2. I've done the "attach drywall with long screws through 1-inch foam" routine several times without any significant nail-pop problems.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Eric... if your wall is loaded with upper cabinets and you apply tile Etc. there will be few if any drywall fasteners to see possibly... pop. Lots of ideas posted, post some updates ....

  10. Riversong | | #10

    1] Re: XPS vs. Foil Faced polyiso: If a goal is to reduce the moist air moving into the wall in winter, why not go with the foil faced? Is it to have some potential to dry to the inside when needed?

    Any unperforated material will prevent air movement, and vapor diffusion into the wall assembly is more easily controlled by ventilation. XPS is as perfect an air barrier as polyiso, if all joints, edges and penetrations are sealed.

    But 1½" XPS has a perm of .75 while foil has a perm close to zero. So XPS will allow some inward drying potential while foil will trap moisture.

    2] I'm a little nervous about having to put the drywall on with 2 1/2" screws as I haven't done this before and am concerned about some unforeseen consequence. How has the long-screw-through-foam thing worked out for everyone in terms of drywall problems like nailpops?

    I use long screws to apply horizontal strapping over the foam board and screw the drywall to that. But I don't foresee any problems using 2½" screws, particularly if you use XPS, which has more compressive strength than polyiso. Nail popping is caused by framing shrinkage and exacerbated by excessive fastener penetration into the wood. With ½" or so penetration, this shouldn't be an issue.

    But you have to consider trim attachment as well, which would require even longer fasteners unless you use some strapping.

  11. Ed Voytovich | | #11

    I'm late to the game, but I've taken the time here in my cold climate to cut and fie XPS in the stud pays, caulking them in place and then adding another 2" horizontally with taped seams and caulk or foam at the top, bottom, and corners. In for a penny, in for a pound.

    As Dr. Joe says, spend your time and money on stuff you can only do once.

    If I couldn't afford to do that I'd cancel the cabinets and put up open shelves. They can always buy cabinets later.

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