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Considering Foam sheets outside sheathing + inside sheetrock

robbmc's picture

Renovating a largish split level ranch. The house needs new siding and the existing 2 x 4 frame walls are filled with fiberglass batts. there are 2 issues I am facing ->

1 - The windows have all been replaced with Andersen 400 series but the house was forclosed (2 years ago) before th house wrap was repaired and siding replaced. I'm planning new Boral plank siding butted to the windows with no casing and mitered joints at the corners of the walls. The plywood has not responded well to the exposure and the sheathing ends at the top of the subfloor. This creates minor surface irregularities that could be evened out by adding 1/2" foam insulation board on top of the sheating. Where would you put the house wrap in this case? I assume new flashing tape at all doors and windows and caulk the new siding butts at the sides and tops of all openings. I like the look of the narrow jambs without casings. 

2 - In the interest of more insulation I am considering 2" of foam board on the interior of the house. Currently there is oil fired radiant baseboard at all exterior wall and I really dislike the look of those metal covers intruding into the rooms. My design would be to add the insulation on top of the existing sheetrock down to the top of the BB heaters with new 1/2" rock and appropriate closure over the BB to have a flush wall surface. The BB heaters would remain in place. Any thoughts?

Thanks in advance for all input.


I think you'd be better off (post #216367, reply #1 of 5)

I think you'd be better off replacing the sheathing with ZipWall which will give you a flat surface and the vapor barrier you need.  To go a step further I'd pull the windows, install the foam on the outside under the sheathing and reinstall them correctly with proper flashing tape. You'd be keeping the heat and cold outside the envelope completely and wouldn't lose floor space. 

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

That seems like a LOT more work (post #216367, reply #2 of 5)

Given the complexity of the house and the number of windows and doors... Pulling windows, doors, and re-sheathing seems crazy to me. I appreciate that you are describing the ideal solution but reality confines. The rooms are all plenty large and I am not concerned about losing floor space. I am concerned about the most reasonable approach to managing moisture and thermal loads.

What do you see as the problem of adding the foam boards inside the house?


Thanks for your input,


hmmmm... (post #216367, reply #3 of 5)

One problem  with foam on the inside of a house is that if it ignites you die pretty quick from the combustion gases. Like way quicker than most combusting fumes. I wouldn't feel all that comfy with just 1/2" drywall over foam.  As for the other parts of building science your climate zone matters a lot also.


Foam in stud cavity usually (post #216367, reply #4 of 5)

It would seem that burning foam would be a problem where ever it is. The house is in New Yorks Hudson Valley - Hyde Park to be specific.Climate zone 5A I believe.

In digging deeper it seems this could be a good approach but I am looking for more direct knowledge. Where might the moisture problem be?

My proposed wall assembly inside to outside ->

Latex paint - 1/2" gyp bd - 2" XPS foam panel w/ taped seams - latex paint - 1/2" gyp bd - 2 x 4 wood studs w/ R-13 fiberflass batts - 5/8" plywood sheating - 1/2"  foam insulation - house wrap w/ taped seams and flashings - Boral Plank 11/16" siding - Latex paint.


Thanks again for any and all input,


Unless the foam has a (post #216367, reply #5 of 5)

Unless the foam has a fireproof rating, it does require a fireproof dryall covering.  So long as fire-rated drywall is used, though, and all the seams are carefully taped, there's no significant danger to having foam inside.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville