Rigid foam insulation on roof deck, under membrane?–seeking advice
I have a 150 year old home which needs a new roof. I’d like to take advantage of the disruption to ensure we have decent (or better yet, great) insulation of the roof. The house has a very low slope (practically flat) roof.
About 15 years ago, the previous owners had insulation added to the joist space. the mentioned that they had some initial condensation problems, but that they subsequently had additional venting put in, and the problems ceased. A recent leak gave me the opportunity to open up a ceiling, and find that the joist spaces have fiberglass bats, some still half in their bags, no vapor barrier (I’m assuming that whoever did the insulation thought th ebags would help in that regard!) Obviously, it doesn’t seem ideal.
The roofer I’d like to work with has suggested stripping the roof back to the wood, adding 4 inches of rigid insulation, a layer of sheathing, and then applying the membrane over that. While I like the idea, I have a couple of concerns:
1. The fiberglass insulation and random bags will still be in the joist spaces.
2. The new insulation will be sitting on the roof deck, while the fiberglass will be sitting somewhere betwen the top of the cieling and the bottom of the roof deck–I’m assuming that cold air will be penetrating the perimeter of the joist space and stiing above the fiberglass, and below the roof deck/rigid foam/roofing assembly.
In this scenario, how likely is the new isulation to be of benefit?
Best case to me sounds like spray foam between the joists plus a layer of rigid foam on the roof deck to reduce thermal bridging. The obvious downside is the cost of spray foam, compounded by needing to either lift the full roof deck, or pull down all m ceilings to get the foam in there.
Has anyone used the slow expanding foam? Do you think it could be sprayed into the joist spaces through a serise of holes, as one might do with blown-in insulation?
I need to commit to the roofer very soon in order to ensure I get a roof before November–thanks to all for any advice you can offer!
Ben
Replies
Insulation is not a layered defense strategy - you choose the plane where the hot/cold SHALL NOT PASS! (hopefully you don't then fall off a bridge like Gandalf did) After the deck is insulated, the attic insulation becomes sound insulation - nothing more. It would actually be better to recycle it by picking it up and stuffing it between the rafters to help out that foam.
150 year old homes are also a different beast than the ones built today - namely they leak air like a sieve. That is actually a feature that helped them last 150 years! Where you home is located is also important - are you heating or cooling your home most of the time?