Radiant Barrier for Cathedral Ceiling / Metal Roof
Mecheng313
| Posted in General Questions on
Hello,
I’m in northern California. I’m installing a standing seam metal roof on my house with cathedral ceilings. I’m wanting un vented due to potential critter intrusion.
Ceiling is 2×8 construction. I’m replacing the plywood and wondering if I should use radiant barrier or not.
Current assembly I’m considering from inside to outside is:
Interior 5/8″ drywall
2×8 rafters with R19 no backing
5/8 osb(radiant or not?)
2 x layers of 1.5″ mule hide #1 polyiso
2 x layers of 1.5″ mule hide #1 polyiso
Polystick xfr peel and stick
Then install metal roof.
This should get me R38 and class A fire rating due to the polystick xfr.
Question is, does the radiant barrier do anything, or is it negligible?
Any suggestions? Thanks
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Replies
If the radiant barrier isn't facing an air gap, it won't really be doing anything. I don't think it's worth spending extra for a radiant barrier here, because it's in the middle of the assembly sandwiched in tight where there is no air gap.
I would put a smart vapor retarder on the interior side behind the drywall for some extra insurance against moisture getting into the roof assembly. Even using kraft faced fiberglass batts would help here, but I'd use something like MemBrain if it were my project.
Bill
The long-term r-value of the polyiso is probably closer to r-5 (based on what I've read on GBA). So you might want to install two layers of 2 inch instead. Just something to consider since going up on thickness shouldn't have much of an affect on cost.
So I would have an unvented air gap between the radiant and the r19 because I have 2x8 rafters. Does that help?
Also forgot to mention I'm doing a re-roof from the outside in so membrain isn't really feasible.
The batt insulation should be placed at the top of the rafter bays in direct contact with the underside of your sheathing, so there wouldn't be an air gap for the radiant barrier. Ideally, the full depth of the rafter bay would be filled with insulation. Rockwool makes R-30 batts for 2x8 bays. There are other fiberglass batt/roll products that could potentially be used, though they may need to be compressed to fit the bay depth, which would reduce their value but still be more than R-19.