Rigid foam board under rafters
If my existing ceiling has sheetrock with fiberglass insulation between the joists, is there any benefit to attaching insulated foam board with a thermal barrier to the underside of the rafters? Between the rafters would be open, so there would be air space from the soffit vents all the way up to the ridge vents. I live in south Louisiana, so I’m in a hot, humid climate. Would this help stop the heat coming down from the roof? The roof decking is 1x, as it is an old house.
OR, since I’m getting close to time to replace the shingles, would it be better to wait and install foam board over my existing shingles, put my stripping over the foam board, screwed through the foam board and roof decking, and into the rafters, then a metal roof screwed down onto the stripping?
Do either of these cause any moisture issues, and if I do either one, is it ok the keep the existing fiberglass insulation in place over the ceiling?
Thanks in advance!
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Replies
In your climate zone if there is continuous soffit to ridge venting there are no moisture problems created by attaching continuous layer of foam board on the interior side of the rafters.
If there is enough depth to install some high density batts between the rafter and still maintain 1" (1.5-2" is better) clearance between the batts and roof deck, that in combination with continuous foam under the rafters would be even better.
If its gable vented it may be better to wait until it's possible to install exterior foam board, or apply a flash-inch of closed cell spray foam to the underside of the roof deck and fill between the rafters with high density fiber.
If there are ducts & air handlers in the attic, air sealing the attic and insulating at the roof assembly is going to make a measurable improvement in energy efficiency.
> Would this help stop the heat coming down from the roof?
Yes. But so would spraying cellulose over your existing fiberglass.
Thanks for the answers! If I used 3/4" or 1" foam board, with or without some thin batts over it to still allow airflow, should I remove the ceiling attic insulation? My AC plenum and ducts are all in the attic, and the R value of foam board wouldn't be very high.
I would say to leave it there. No benefit to removing it.
In any climate, pulling HVAC equipment and ducting into conditioned space makes a huge difference in energy efficiency and thermal comfort. You won't regret recladding your roof a bit before you need to to get this benefit.
In terms of your existing fiberglass batt ceiling insulation: no harm; think of it as acoustical insulation from now on...
Peter