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Community and Q&A

Plywood sheathing over rigid foam

rg14 | Posted in Plans Review on

THIS IS LONG BUT PLEASE BEAR WITH ME. Any advice on this project would be greatly appreciated! My home is located in the 6A zone. To start I purchased a home 3 years ago and decided I wanted to replace the windows and doors. The house was built in 1978. Upon further inspection of the home (which my home buyer inspector did not catch) I discovered that my house does not have any wood sheathing. It was built with 1in blue tounge and groove styrofoam board and had aluminum siding attached to it nailed to the wall studs. The rigid foam was never taped at the joints either. Im assuming that there is metal strapping or wood let ins at the corners. The walls are 2×4 construction with fiberglass insulation. I live in a rural area and have found that mice have chewed the foam sheathing as well as what appears to possibly be ants in some areas.

My plan is as follows. Rather than remove the foam and resheathe with osb or plywood and apply a weather barrier such as tyvek while replacing the doors and windows with new construction replacements-and then having to reinstall foam to the exterior again (where the mice and ants are going to chew it!) Could I: 1. simply remove the siding exposing the rigid foam. 2. Tape the joints of the foam with an approved tapeand fill any bad spots with spray foam. 3. Sheathe with osb or plywood over the foam. 4. install tyvek. 5. Replace the windows and doors (correctly with flashing tape etc). And 6. Install vinyl siding to the outside.

Essentially I would be creating a SIP panel. My plan is to also cut the bottom 3-4in of foam at the sill plate and replace it with (2) 3/4in thick strips of pressure treated plywood to create a 1.5in ledge for the osb or plywood to sit on before securing it over the rigid foam. I also read on here and plan to use headlok fasteners to secure the wood sheathing. According to their website it can be done with 4in fasteners 12in o.c. The foam is attached vertically and I would install wood sheathing the same. I see it may be a problem with the headlock screws at the sheet edges. I may stagger them a bit at the edges or was considering nailing with 3in nails at the sheet edges sharing a stud and using the 4in headlok screws in the center of the sheet. The studs are 16in on center. Ive even found a site that said its acceptable to hang apa rated wood cladding (t1-11) over ridged foam. Would this be any different?

My thoughts are this would eliminate a cold exterior wall for condensation and would reduce the amount of waste of having to remove and replace the existing foam sheathing. It would greatly reduce my drafts also. It would also give the plywood or osb a shelf to sit on. Ive spoken to the local building inspectors for the counties in the area and they dont seem to think this would be a problem at all. It seems to me it may make the walls more air tight not allowing any escape to the outside but at the same time its keeping the damp air thats been blowing in out. I do run a dehumidifier in the summers and with the 7 months a year that my funace runs the air is very dry in the house during the heating season. IS THERE ANY REASON I SHOULD NOT DO THIS?? AND IF I CAN IS THERE ANYTHING ABOUT MY PLAN THAT I SHOULD CHANGE? I have been up for weeks looking for answers and this is the only site that i would even attempt to ask this to. Thank you for reading this and any advise you may have.

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  1. GBA Editor
    Martin Holladay | | #1

    Briefly, you can probably proceed with your plan.

    If you are worried about your wall's bracing details -- it sounds like you assume that your house has let-in bracing at the corners -- you might want to talk to an engineer. Your new sheathing will help a little, but the new plywood sheathing won't provide the same bracing capabilities of sheathing installed directly against the studs, because the intervening foam reduces the effectiveness of the bracing. That may not matter in your case, especially if the original builder provided good alternate bracing details -- but if you are worried about that issue, you might want to consult an engineer.

    If you proceed with your plan, pay attention to air sealing. If I were you, I would tape the seams of the old rigid foam, and I would also tape the seams of the new plywood. In addition, do your best to seal air leaks at penetrations. For more information on tapes, see Return to the Backyard Tape Test.

  2. STEPHEN SHEEHY | | #2

    RG14: Since you are replacing the siding and installing new windows anyway, I'd take off the foam and resheath with plywood or Advantech, taping the joints. Then reapply the foam (if it is in good shape) and add another couple of inches of additional foam, using reclaimed if you can find it. This is your only real chance to augment your insulation and do good job of air sealing.
    One inch of foam and 3 1/2" of (badly installed?) fiberglass gives you pretty marginal insulation. Adding another 2" of foam and good air sealing should significantly help. And don't assume there is already let in bracing in place.

  3. rg14 | | #3

    My main reasons for sheathing over the foam would be to provide a solid rodent barrier. Any exposed foam where I live is a mouse nest. Next the ease of installing would be much greater. I'll basically be doing this with limited help. And lastly the cost of the new foam and time to replace it. My main thought is this takes the osb out of the equation as a condensing plane. The Advantech panel seems like a great idea. I'm looking to see if this is feasible done in this way without causing a new problem. The Advantech seems it would be a better option than the plywood or the osb. I would be removing foam at the corners to check for horizontal bracing but I'd like to just reinstall it and have a continuos layer around the house.

  4. TP | | #4

    I am researching this in 2023 and basically have the same situation as you (without the mice chewing the foam). I just replaced my sliding door and will be redoing the siding next Spring 2024. There is batt insulation in the wall cavities except where I removed it in this picture to access a the wires that I want to fix. I am trying to figure out if I should remove the foam and replace with OSB or put OSB over the exciting foam (assuming it’s in good condition when I remove the old siding next Spring), then Tyvek over the OSB since there is no house wrap and the gaps between the foam are not taped. I used AtticCat in the attic a few years ago but the house is still always so cold. I feel like anything I do will be better than what’s there now.
    Also, from what I can tell for structural purposes, there is one sheet of plywood on each corner.
    Any suggestions from anyone who is reading this is 2023 or 2024 would be so greatly appreciated!! Thanks. 😊

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