Insulating a cathedral ceilig
RFlanagan777
| Posted in General Questions on
I’m in Delaware which is zone 4. I am finishing an attic room with cathedral ceilings. I need to allow air flow from the soffits to the ridge vent. My plan is to use 2 inch faced closed cell rigid foam cut to fit in between the rafters, with 2 in spacers to hold the insulation away from the roof. This creates the needed air channel. I’ll seal all joints with spray foam.
In doing so, there is a gap of about 3-4 inches from the foam board to the drywall. I plan to fill this gap with unfaced R-13 insulation so as not to create a second moisture barrier but to add more insulation to the ceiling.
Does the above design sound ok?
Bob
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Replies
You're on the right track.
The venting is good. The insulation is a little light. Delaware seems to require R49 for ceilings. Two inches of polyiso would give you R13, plus R13 for the batting gives you R26. It sounds like you have 2x8 rafters, which is not going to be enough space to get R49.
To get to R49 you need the R13 from the foam plus another R36. You can get high-density R-38 fiberglass (see https://www.homedepot.com/p/Knauf-Insulation-R-38-EcoBatt-Kraft-Faced-High-Density-Fiberglass-Insulation-Batt-10-1-4-in-x-23-in-x-48-in-8-Bags-691003/313646775 ) It's R-38 at 10.25 inches and R-35 at 9.25 inches. You could cut the vent space down to one inch, so one inch vent, two inches foam, ten inches batts equals thirteen inches or so. The existing 2x8's are 7.25 inches, so you need to drop the ceiling about six inches, maybe a little less.
I think the consensus is that the vents themselves don't need to be sealed, if moisture does get under them it's better to allow them to dry upward. The ceiling does need to be sealed.
thanks for the response. I did talk to a building inspector who initially said as you did that the requirements are R49. But he then agreed that applied to new construction. As my home was built in 1987 or so, the only requirement would be to fill the rafters with insulation. If I built out the rafters, the room would be un-usable. So, maxing the insulation in the available space (7" rafters) meets code. The rigid foam + insulation gives the max possible R value in the rafters.
I have the exact same situation as you in one small area of my home. What I did (well, what I'm DOING actually since I have the materials but not yet the time to install them :-D) is to use some foil faced polyiso as the top layer, spaced about 1.25" below the roof sheathing with some furring strips, then I use cheaper roofing polyiso (the fiberglass mat faced stuff) to fill up the remaining space above the drywall. With my 2x8 rafters here, that gets me about R39 worth of polyiso in the remaining 6" of space not used by the vent channel. I'm putting a 1/2" layer of foil faced polyiso under the rafters with seams taped for a bit shy of R3 worth of continuous insulation and a vapor barrier.
Here is the reason for this assembly (this is for a 2x8 with about 7.25" actual depth):
The foil faced polyiso on the top, right under the vent space, to get the free gain of a radiant barrier, but also to provide a slick surface (the foil facer) which I hope might discourage insects from trying to build anything in the vent space. These pieces will be held in place with canned foam around the edges.
The rest of the space is filled with kraft/fiberglass mat faced "roofing" polyiso simply because it's cheaper than foil faced.
The 1/2" polyiso under the rafters helps with thermal bridging, but also provides a vapor barrier behind the drywall.
The top layer can "leak" moisture around the edges and through the wood of the rafters into the vent channel. This works as long as you can ensure there is very little moisture getting up there that needs to get out. The 1/2" polyiso on the "bottom" of the assembly with taped seams creates a near perfect vapor barrier, preventing moisture from getting up there from the room below the drywall. This makes sure there is very little moisture that can get in, so very little that needs to be able to get out -- the vent channel can easily deal with anything that might need to "get out".
You don't have double vapor barriers when the materials are right against each other, so that's not really a concern here. The only issue is to make sure that not much moisture gets into the assembly in the first place, which, in my case, that lowest layer of taped 1/2" foil faced polyiso ensures.
Bill