A-frame roof insulation
Hello all,
We recently purchased a small a-frame in zone 4a as a vacation home.
The construction of the cabin is fairly rudamentary. The roof is composed of asphalt shingle on top of plywood sheathing, which is supported by sistered 2x8s rafters, with a four foot span between rafters. There is currently no insulation or ceiling. Several of the rafter bays are interrupted by skylights.
From reading the articles and comments on this site, I understand the best practice would have been to insulate using rigid foam on top of the roof sheathing. The roof has been recently replaced, so that is not an option at this time.
I understand the next best option would be to do an unvented roof assembly with closed cell spray foam. We are concerned about potential health effects and enviromental impact, and have largely (and reluctantly) ruled out that option.
That would seem to leave us with only vented options. My contractor has proposed either: (1) adding new rafters between the existing rafters to support r-19 batt fiberglass insulation, with baffles to vent; or (2) cut and cobble with rigid foam, with vented airspace behind the foam. I’m not sure at this time how we will vent around the skylights.
Neither these options would get us to code, so we may add rigid foam on top of the rafters to add r value and prevent thermal bridging from the rafters. Our preference would be to use EPS foam.
Having taken the best options off the table, are there any other options we are failing to consider?
Thanks for any help you all can provide!
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Replies
If you look at the cost of new framing and interior drywall work, you are probably looking at similar cost as a re-roof. I know it sounds wrong to toss away a new roof, but it might be the simplest. This would let you clad the interior with t&g, which would look much nicer than drywall.
If you are fine with the interior work, I would not do cut and cobble. Because of the thermal bridging of the rafters, the extra R value from the foam doesn't do anything to increase of the R value of the whole assembly. Lot of extra money and work for minimal gain. Stick to batts.
If you want slightly higher R value assembly, go with 6" R24 mineral wool batts (can also go with 2x4 batt+ layer of safeNsound ~R26).
You can vent around skylight by drilling holes into the rafters and creating an air path to the bay beside it. Also since the building is not occupied full time, a couple of unvented cavities won't matter much, they will be able to dry through the rafters while you are not there.
Akos- Thanks for your response.
I am wondering if the rigid foam might still be a preferable option, since we could put it in the existing four foot span without doubling up on the rafters, which I would think would have a pretty significant impact on the amount of thermal bridging. Skipping on the new rafters also reduces the cost differential somewhat, though the foam is still $1-2k more expensive, as quoted. Does that change the analysis at all?
Thanks again.
It could work. The challenge is the existing sistered 2x8. Even a small gap such as 1/32" between them times 12' length is a big hole. By drywalling over the whole thing, you can ensure a continuous air barrier. In taller structures such as an A frame, air leaks could be a much larger source of heat loss than insulation.
It is possible to do the foam between the rafters. You still have the option to crate a vent channel but I think this might be worse because it brings more outside air into a structure with a lot of 3d air leak paths that are hard to seal.
One option could be is to go with unvented roof with the foam between the existing rafters. In this case make sure to use a permeable foam (unfaced EPS or permeable/paper faced Polyiso) as the roof can now only dry towards the interior. Provided you are not there most of the time in the winter, the assembly should have plenty of time to dry towards the inside. You need to do a decent job air sealing (tape the seams of the foam, fill the gaps around the edges with canned foam and tape the edges against the rafter), it should work well. The good thing with this assembly is the foam is the air barrier, you can clad it with T&G if you don't want drywall.