Insulating 2×8 Cathedral Ceiling
R40 in 2×8 Cathedral Ceiling
Hello GBA Community,
I’m finishing a garage loft/bonus room that is supported by attic trusses. Unfortunately I only have 2×8 rafters to work with. My thought was to insulate the roof as best as possible to handle most of the insulation for both the loft and the below garage space. I’ve read several articles and questions on this topic but want to get clarity on a few questions.
Location: Climate Zone 5 – Minnesota
1. Any concerns with XPS being under roof? I’ve read it’s not the best with high heat.
2. If anyone has any recommendations on how to create the air gap, i’m all ears. My plan was to fasten 1″ firing strips to 2×8 as a place to screw into.
3. Any concerns on trapping moisture between intello plus barrier and the XPS? Perhaps I don’t seal around the XPS edges with spray foam/caulk?
Any other call outs or simple suggestions are welcome.
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Replies
A few thoughts:
You can make the insulation thicker than the joists, either by using board foam over the joists or by furring the joists out.
You generally want to insulate the shortest path. Looking at your roof profile, the shortest line is along the roofline from the eave to the ceiling, then across the ceiling. That path also only has two changes of plane instead of four, which simplifies things.
A common way to do a vent channel is to use 1" faced polyiso insulation. Cut it 2" wider than the rafter spacing, then score the facing on one face 1" in from each edge. Snap it and fold over the intact facing. If there are roofing nails protruding through the sheathing you can stick it into them to get it to hold while you do the rest of the insulation.
Over the ceiling you can pile batt or loose insulation pretty much as thick as you want, that will be cheaper and easier than foam board.
That method of creating the channels is genius. I unfortunately already installed 1"x1" strips in all of the bays to use as a fastening point for the foamboard.
Are the 3 locations I highlighted with red inside or outside the thermal envelope?
If they are inside then you will provide them with supply and return registers keeping them more or less the same temp and humidity as the rest of the home and what is the point of spending money on the If fiberglass.
If the spaces are outside the envelope don’t insulate the rafters their and show the soffit and ridge vents.
Will the first floor be heated and where will you draw the thermal bounty.
Did you happen to see this article?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
I have seen that. The issue with my situation is that the rafters are not big enough to do any of those very well. I have 2x8's. Also, the roof is only 5-7 years old so it doesn't make sense to add exterior insulation for another 15-20 years.
Right now it feels like those two red sections would be within the envelope. My plan is to seal around the 2" of XPS in the rafter bays and thus the thermal and vapor barrier will be just inside the 1" air vent (see blue line in updated picture). For heating and cooling I was going to put one mini-split heat pump in the bed room and one in the garage.
The only other alternative I was thinking about is NOT sealing the edges of the XPS allowing it to dry to the outside and put air/vapor barrier on interior of both spaces (See blue lines in 2nd picture).