Installing 2″ rigid insulation under OSB for exposed post addition
Greetings,
I have a home in West Virginia that is in Climate Zone 5, and the home has two additions, each built on either end of the home, both elevated over the earth on posts…open to the ambient air. The temperatures in those rooms are about 10 degrees cooler in the winter than the main house. I pulled down one sheet of the OSB plywood under the bedroom addition and there is R30 insulation in the cavity. The OSB plywood is nailed with twisted nails to the 2×10 floor joints and the seams are covered with 1″ x 1/4 strips, nailed and calked so that the OSB sheathing under the addition is air tight. I really had a tough time getting one sheet down, and there are 13 of them.. so pulling them all down to do spray insulation is not something I want to do…would end up being destructive and needing to replace all the sheathing.
Instead, I purchased sheets of R-13 rigid insulation board and am ready to screw them to the exposed side of the OSB sheathing. My question is that if I do that as is, then the edges of the 2″ rigid insulation board will be flush with the underside of the 1:x 1/4″strips covering the plywood gaps… and much of the insulation board will have a 1/4″ or less air gap between it and the underside of the plywood sheathing.
My preference is to not pull the 1″ strips down, as its sealing the plywood sheathing and it’s more work.. but that’s the only way it will lie flush up against the plywood.
What issues would I have if I didn’t pull the 1/4″ thick strips down that cover the plywood gaps, and just screw the 2″ rigid foam board to the underside of the sheathing…the edges will be tight to the strips, and the middle might be tight to the plywood as the 2″ foam board may flex when I screw it in… but there will be areas that will have that small air gap.
Any thoughts and assistance is appreciated.
Thanks
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Replies
I doubt the small air gap would make much difference as long as there is no ventilation through the space (the rigid foam needs to be air sealed). You could use the 1/4” fanfold XPS to fill the gap too, although it’s actually a bit less than 1/4” thick in my experience.
The other option I could see would be to use a router to notch out the edges of the 2” board so that it would fit flush to the OSB while still clearing the 1/4” strips. That would make a lot of foam dust though.
Bill
Mike,
I'm doubtful that your floor assembly is air sealed nearly as well as it needs to be for you to have a comfortable space above with OSB (Or plywood. You seem to use the terms interchangeably in your post, but they are very different materials) sealed with caulked battens as the air barrier. And, if the existing cavity insulation is not held up to the subfloor and/or the rim area isn't very well air sealed, you won't be doing much by adding insulation below. This may be a case where the hard work of pulling down those panels is necessary to do the job right. Before you do anything, check out these articles from GBA and FHB for more information on insulating floors like yours:
https://www.greenbuildingadvisor.com/article/the-best-way-to-insulate-a-floor
https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor