Old house challenge with unvented cathedral ceiling (rigid foam on roof, batt insulation interior)
I’m working on an unvented cathedral ceiling with 4” of polyiso insulation on top of the roof and fiberglass batts on the interior in the rafter bays (one of the recommended “cathedral ceilings that work”: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work)
There’s a catch: the sheathing sits on top of some old 1” furring strips that run horizontally (left behind after some cedar shingles from ages ago, I imagine). See the attached images.
If I put batts in that attempt to fully fill the cavity depth-wise, they’ll rest against the furring strips and may or may not fill the gaps between the furring strips. If they don’t, there will be a 1” gap in much of the surface area between the sheathing.
a) Do you believe that the batt insulation, upon compression, would fill the gaps between the furring strips and touch the sheathing?
b) If they don’t, will this materially undermine my ceiling performance (knowing that I have polyiso on the roof)?
c) Is there a creative solution you’d recommend? The polyiso on the roof is already in place and fixed.
Thank you,
Luke
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Replies
The main concern is with air leaks. Air leaking in anywhere through the ceiling (ex. at those light fixtures) will be able to easily travel up and down the rafter bays and horizontally along the skip sheathing gaps. If there is any air leakage path to the exterior (how are those gaps sealed at the ends?), there will be an uninterrupted path from inside to out. You could spray foam the gaps at the exterior walls and then make sure your ceiling is air tight. A blower door test before drywall might be a good idea.
Thanks, I plan to spray foam the ends at the exterior to prevent exactly what you're describing. A blower door test is a good idea.
The most important part here is a solid air barrier under the polyiso. As long as you have that, I think your roof will work.
I would get some thicker batts to get a bit of compression to fill the gaps as much as possible though.
We followed the specs in the attached image from gba. We had plywood, grip rite underlayment, then 4 inch of Polyiso using two layers of two inch with staggered seams, plywood on that, weatherwatch membrane on top of that as an air barrier, another grip rite on that and then asphalt shingles.
I plan to seal above the top plates as in the diagram.
I believe this assembly follows the gba specs, but may depart, Akos, from your point of having an air barrier UNDER the polyiso, right?
Given the specs we followed (my other comment) , are there any major risks I should keep an eye on?