Plywood sheathing over rigid foam
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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