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We have a small rear entry porch - about 6' x 6' -- it is partially enclosed, but not conditioned. There is a room which has a portion that extends over this unconditioned space.

Not surprisingly, like a room over an unconditioned garage, this room tends to have a cold spot on the floor.

I have the opportunity to re-do the ceiling over the porch and am planning on adding some insulation as there is almost certainly none currently.

I'm assuming a 2x8 joist cavity and was thinking 1" unfaced XPS + spray foam the perimeters + 2x6 batts of Roxul unfaced insulation (labeled rating is R23). And then will close up the ceiling over that.

If the 2x6 batt + 1" xps doesn't actually fill the bay, what should I do? More xps, overstuff a 2x8 batt in?

Am I missing anything that could improve this?

Thanks!
 

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The number one improvement is air sealing. In a cold climate, the cold air pushes in the lower leaks and forces the warm air up and out the upper leaks. The result is your floors above will always have a supply of cold air leaking in if not sealed. Note, air seal with caulking or can foam, but not a layer of plastic.

Then use some combination of rigid and Roxul for a complete fill. 2x8 would be 7.5" so 1" of rigid plus 2 layers of 3.5" Roxul would be a nice tight fit. Roxul will compress a little, but not a lot. Neatness counts so be sure all gaps are filled.

Then cover the bottom with plywood or osb. The plywood becomes your rigid air barrier to the outside keeping cold air out of the cavities. Seal all seams as you install it.

Bud
 
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