Cape Cod 2nd floor insulation with unvented roof
I live in a 1.5 story Cape Cod house in climate zone 4A that has very inadequate 2nd floor insulation, and I’m looking for some input on how best to remedy this. The sloped ceiling is currently insulated with R-19 fiberglass between 2×6” rafters, and the ceiling is finished up to the roof peak. The roof is also unvented (the original gable vents were sealed up by a previous owner), and there are no overhangs, so no place to add soffit vents. The roof was redone 2-3 years ago, so adding exterior rigid foam doesn’t seem practical in the near future.
From what I’ve read here, flash and batt might be a good solution for a situation like this with an unvented roof and minimal depth available for insulation. Does that seem like a good approach here?
Also, when doing ccSPF, is the portion of the roof behind the kneewalls typically sprayed as well (this area would be very tight to work in)? Is there an issue with doing this in stages so that only half of the upstairs would be sprayed for some longish period of time (e.g. 6-12 months)? The roof sheathing looks to be original 1950s-vintage 1×6” tongue and groove; is that anything to worry about spray foaming? And would spray foaming preclude the use of rigid foam on the interior ceiling to mitigate thermal bridging?
Thanks,
Dave
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Replies
Dave,
Flash 'n batt is a good idea if venting is not viable. Retrofits are the best use for spray foam; it's expensive, but probably the most cost effective option available.
It's going to be easier to spray the whole roof deck rather than bring the air/thermal barrier down the back side of the kneewall and over the attic floor to the wall. If you have the work done in two stages, you will have an incomplete air barrier until the second part of the work is finished. You will also have to leave your house twice while the work is performed.
If you want a thermal break over the rafters, it would less risky and less expensive to strap the ceiling with 2x4's on the flat and blow extra fiber, or to smoosh batts into place. If you have the head room, you can install the strapping on edge and have room for 3.5" batts/blown fiber.
I had a very similar project on my house completed recently; same climate zone, same rafter dimensions. My advice would be to determine who is using HFO and/or water blown closed cell products in your area prior to looking for bids. It may narrow your search pretty quickly. I also found these articles to be helpful:
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
https://www.greenbuildingadvisor.com/article/getting-spray-foam-right
Hi Dave,
In addition to Bryan's excellent response and reading suggestions, this article describes the difference between insulating the roof line all the way to the eave and insulating the kneewall: Insulating Behind Kneewalls.
Thanks very much to both of you for the links and advice. It does seem to make more sense to spray the roof deck behind the kneewalls as suggested; I suppose anyone in that line of work is probably used to working in tight spaces.
Other than any spray foaming, I'm hoping to do most of the work myself (that's one reason for trying to stage things over time). I was thinking batt insulation would be easier for me to install than any type of blown fiber. There's not a ton of head room, so the 1.5" of 2x lumber is probably about as thick as I could go for strapping.
Bryan, do you mind sharing the specifics of what your insulation stackup looked like since the scenario is so similar? I was thinking about 2" of HFO + slightly compressed 5.5" mineral wool with Bonfiglioli strapping as a thermal break. My math puts that at roughly R-30 for the whole assembly.