Two R15 Batts in a 7.25” Space vs Compressing an 8.25” inch R30 Cathedral Batt?
What are your thoughts about using two R15 batts totaling 7 inches depth and R30 in a 7.25 inch cavity vs an R30Cathedral batt which is 8.25 inches that gets compressed an inch down to approximately R27?
The two R15 batts would come out to about $.40 cents cheaper per sq ft and has a better overall Rvalue (R30) than the compressed R30 Cathedral (R27).
This is DIY so don’t factor any labor difference. There will be no exterior insulation. Blown insulation options are not being considered. I am not going to spring for the extra cost of R30 mineral wool.
I believe the two R15 batts is the overall best option, but is the possible .25 inch gap on the warm side of the house big enough of a deal to go with the R30 Cathedral due to potential convective looping?
Thanks!
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Replies
jwolfe1,
The depth of fibreglass batts varies quite a bit when installed. Differences under say 1/2" are in the weeds - and 1/4" doesn't exist in the world of even rock wool batts. You can install a 5 1/2" fibreglass batt in a 2"x6" cavity so that it slightly overfills it, or leaves a void.
I would have no problem using two R15 batts, it's probably just a bit more labor. Maybe that extra labor to install two layers will be offset by you having to do less valiant battle stuffing a too-fat single bat into the available space though. Probably not a huge difference either way.
Note that R values don't change by the ratio of the thickness when you compress a batt into a smaller space. Whan you compress a batt like that, the total R value decreases, but the R per inch increases a bit, so you're probably more around R28 or a bit better when squishing that R30 batt down. It's similar to how R11, R13, and R15 fiberglass batts all fit in a 3.5" cavity, but the density of the fiberglass making up the material of the batt gets progressively denser from the R11 batt up to the R15 batt. There is a limit to that too, above which you start seeing R per inch drop down again, but you're not going to hit that limit doing any reasonable amount of insulation stuffing.
Bill
Malcom, Bill, I really appreciated your thoughts. Thank you GBA!
For your reference. https://www.energyvanguard.com/blog/compressed-fiberglass-insulation-really-so-bad/