Insulating Garage with Conditioned Space Above
I’m wanting to redo the insulation in the ceiling of my tuck under garage. The living room is above the garage. There is fiberglass insulation there now that is in rough shape in spots and I want to add more lights. So I’ll tear down the fiberglass.
The general recommendation for the rim joist area is adding foam board or spray foam to a thickness great enough to stop the condensing problems. This thickness is usually determined using the tables they have for insulation thickness and exterior foam thickness. This takes into account climate zone (I’m in zone 6 or 7) fiber insulation thickness and then gives you a minimum thickness of foam. For the case of the garage ceiling the joist bays are going to be filled all the way with fiberglass insulation, it isn’t going to be partially filled like you would normally insulate a rim joist in a basement or a wall. Now regarding the tables the foam thickness increases as you increase the thickness of the fiber insulation. How is this going to work with the cavity being completed filled with fiberglass? How thick of foam will I need? Do the rules change because this is a cavity sealed from the top by subfloor and will have drywall on the ceiling of the garage?
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We just went through this. There is a recommendation by zone somewhere on this site for insulation thickness. We are in zone 4a and installed 3 1/2" of closed-cell foam in the ceiling of our garage with a kitchen over it. Due to all of the soffits and beams, we skipped rigid foam under the joists. That is recommended to decrease thermal bridging. You could also dense-pack cellulose and skip the spray foam.
I believe this article will be helpful (it includes information on and links to other articles about determining thickness of rigid foam): Combining Exterior Rigid Foam with Fluffy Insulation.
The rigid foam thickness table states that 36% of the R-value needs to be in rigid foam on the exterior in zone 6. With the cavity filled with R-30 you will have several feet of fiberglass across the entire roof of the garage. How do you use that chart when the assembly is nothing like a wall?