How to insulate a cathedral ceiling under an exposed metal roof?
I am in the process of building out a small log building on my property into a livable space for my older kids. It is 24×24 and the downstairs hasn’t been a problem so far. What I am unsure about is how to insulate the ceiling in the attic into a small “A” roof bedroom space. The joists are 24″ OC and I have ordered ProVent batts to create an air space between the metal roof and the insulation. I live in rural Maine, so the winters are LONG and COLD. Once the batts are in place I will only have about 8-9 inches of space for insulation. The downstairs will be heated with a Pellet stove and there is also Gas heat available in the carriage house if needed (which I don’t anticipate needing).
I will be finishing the entire attic space with Shiplap (ceiling and the knee walls on each side). Is there anything I would need to put between the purlins, batts, insulation and the shiplap to properly insulate the room and protect the Joists from damage?
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First of all, you need to create a ventilation gap and an air barrier at the top of your insulation assembly. If your metal roofing is installed on purlins, the easiest way to create an air barrier would be to install a 1-inch layer of rigid foam (EPS will work) tight to the underside of the purlins in each rafter bay. The perimeter of each piece of rigid foam should be air sealed with caulk, canned spray foam, or high quality tape. Seams between adjacent pieces of rigid foam should also be sealed.
If you install 1 inch of rigid foam in this manner, you will only have 8.25 inches left in your rafter bays (assuming that your rafters are 2x10s). To install the minimum needed R-value of R-49 -- or R-45 if you count R-4 for the rigid foam -- you could install about 7 inches of closed-cell spray foam. If you want to install 13 inches of fiberglass batts instead, you will need to scab on additional framing to make your rafters deeper.
For more information on these issues, see How to Build an Insulated Cathedral Ceiling.
One more point: if you choose fiberglass batt insulation, you will also need an air barrier on the interior side of your fiberglass batts. Shiplap boards are not an air barrier, so you will first need to install a layer of taped gypsum drywall on the interior; after the drywall is taped, you can install shiplap boards if you want.
A couple of thoughts:
Aside from drywall as an interior air barrier, Jen could use rigid foam of 1 or 2 inches, well sealed with tape/caulk/foam. This would have the benefit of adding R value (8 -12 with 2" of foam), valuable here due to the limited rafter bay depth. Furring strips would then be needed to hold the foam and attach the boards, so more expensive than drywall.
And I was wondering about putting rigid foam tight to the purlins beneath the roofing metal. Specifically, I'm thinking the purlins would be horizontal (across the rafters), so there would be an air space but not the typical type with soffit to ridge vent air movement. This would appear to be a cold but dead air space. Does this work okay in this situation? I have a similar air space on my house, but I have a cold attic (gable roof) and that above deck air space was really just for attaching the roofing metal.
In my experience, the air space between purlins adequately ventilates the underside of metal roofing. Although rake-to-rake venting is unusual, it works. (Verify that your local building inspector accepts this approach before proceeding, however.)
The main purpose of the air space is to encourage evaporation of any condensation that drips off the underside of the metal roofing. Ideally, the roof assembly includes a layer of asphalt felt under the purlins; this asphalt felt is best installed over a continuous layer of OSB or plywood sheathing.
Jennifer lacks the continuous layer of roof sheathing, unfortunately. Well-sealed rigid foam is not as good as asphalt felt -- but it should work well enough to hold the condensation in place until it can evaporate.