Will rigid foam on the ceiling of a screen porch help the room above?
We are building a screen porch with a bonus room above to an architect’s specs. Ceiling joists/Floor joists are 11 7/8″ TJI with R-19 fiberglass batts. The batts will be in contact with the subfloor of the bonus room. There is a 1/2″ rigid foam sheathing called for on the bottom of the TJI. From what I have read on other posts – seems like this will serve as the air barrier, reduce thermal bridging and add a little (not much) R-Value to the floor assembly. Also read alot that the air barrier must be in contact with the insulation in order for it to be effective – so am wondering if the air gap between the rigid foam and the bottom of the insulation is going to lessen the benefit. Other than that – the floor system will be sealed pretty well at the band joist and there is not any knee wall attic space – exterior walls of the bonus room align with the exterior walls of the screen porch. The screen porch ceiling is to be T&G beadboard. As an added note – we are in NC. Thanks.
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The only reason the air barrier should be contiguous with the thermal barrier is to avoid the potential for convective currents. With sealed joist bays and a foam air barrier below, there should be no air movement and the gap will add r-value to the assembly. Make sure the foam edges are taped so that the air barrier has integrity.
Thanks Robert
Danny,
This question has been a contentious one for Energy Star builders, since the Energy Star program was considering (and almost implemented) a requirement for insulation separating a bonus room floor from a garage ceiling to be in direct contact with the ceiling drywall.
The building-science argument centered around the following question: how common is it for builders to successfully air seal the perimeter of their joist bays? Many building scientists answered, "Not very common" — unless, of course, the perimeter of the joist bays was sealed with spray foam.
If there is a little air leakage, your proposed insulating method isn't very good.
Fiberglass batts are relatively cheap. If I had to insulate those joist bays, I'd want to cram in some batts with a total thickness of at least 12 inches, so that there was no air gap between the batts and the subfloor above, or between the batts and the rigid foam insulation below.