Assistance with new residential insulation
Hi there,
I am currently under construction on my new home in southern california (climate zone 3), and I am reviewing details with insulation subcontractors, and seeking some advice as I am receiving differing recommendations. I live near the coast so it rarely gets under 40 or above 85.
1) I have a few rooms with unvented cathedral ceilings with 2×8 rafters. My plans call for a flash and batt with 1.5″ of closed cell foam and R21 batts. One contractor had no comment. Another mentioned that I should just price 2″ of foam to ensure there is no underspray. And a third thought that I should just do 3″ of foam with no batts, as I need atleast 2.5″ of foam to create a real moisture/vapor barrier, and that will create a u-value of pretty much zero, thus rendering batts useless (also mentioning that it is cheaper). I am inclined to just proceed with the flash and batt as designed, but would love to hear any thoughts from folks here that have no incentive to steer me in any certain direction
2) I also have unconditioned attic space with low profile roof vents (o’hagins). It is a low sloped roof (2:12), so not a ton of space, and the attic floor/ceiling joists are only 2×6. The plans call for R-38 batts between the roof rafters, and R-19 between the ceiling joists (attic floor)–I’m guessing to meet Title 24 via a performance method. While most contractors priced per plans, the contractor that recommended 3″ of foam also pointed out that insulating at the roof rafters will create moisture issues, particularly because of the low slope roof which will limit airflow between vents. That contractor recommends I only insulate at the attic flow (between ceiling joists) and that the inspector will never check (he is correct). Is the contractor correct on that point, and if so, should I plan to either (a) install R-38 batts in between the ceiling joists, which will extend above the joists, or (b) install R-19 in the joists and layer R-19 above?
Appreciate any help or insight you can provide. I’ve attached a snip of the plan section
Thanks
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Replies
What is your local code requirement for minimum R value in that unvented ceiling assembly? I'd be surprised if it's anything less than R38. You need to hit the code minimum, and none of your proposed insulating plans is going to do that. Closed cell spray foam is going to be around R6/inch at best long term, regardless of what the installers are likely to claim. Thinner layers (like 1.5") are more prone to thin spots and other issues, so I would go with a thicker layer. 3" would be about R18, and that plus R21 batts would get you up to around R39, so you'd be a bit better than R38 if that's your minimum allowable, and you'd be safe in terms of foam/batt mix for vapor control too. I'd be reluctant to try for anything less than 3" of closed cell spray foam myself, but 2" can probably be applied evenly enough by a competant installer to work too. That "competant installer" part is key here. It can be tricky to get an even layer of foam, especially when working overhead like this.
U value doesn't matter if it's below code minimum insulation levels. 2.5" of a perfectly applied, even layer, is about R15, which is a U value of about 0.067. For another perspective, that's about what you'd get with a 2x4 wall insulated with mineral wool batts. That's not very much insulation.
I don't know anything about title 24 since I don't work in your region, but in normal projects insulating at the roof AND attic floor is a no-no. The R values are not additive when done in this way, and you do introduce new problems. I would insulate entirely at the roof line unless you have some local requirement to insulate additionally the attic floor.
Bill