Insulation
pffowler
| Posted in Energy Efficiency and Durability on
Would R 32 closed cell foam of 5 in. thick preform as good as R 40 in a fiberglass bat at 11in. thick.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Paul,
Assuming that the fiberglass batt is properly installed in a cavity that has been sealed against air leakage, the fiberglass batt will perform better. R-40 beats R-32.
For more information on this topic, see It’s OK to Skimp On Insulation, Icynene Says.
-- Martin Holladay
If it's thermally bridged by framing such as rafters are joists, low density R32 fiberglass at 11" with air barriers on both sides will outperform R32 closed cell foam at 5", since the framing fraction has more than twice the thickness, and thus more than twice the R value , which is half the thermal bridging.
"R-5 - 80% uvalaue heat blockage. r-10 - 90%, r-20 - 95%, r-40 97.5%"
How accurate are these numbers?
Real world never use fiberglass insulation unless ultra dense. in which case It's the same price as rockwool which is a better product.
Rvalue has a steep diminishing returns. R-5 - 80% uvalaue heat blockage. r-10 - 90%, r-20 - 95%, r-40 97.5%. So the difference between both insulation rvalues are tinier than their rvalues suggest. Always focus on air sealing first which has a bigger impact on heat retention. Going beyond r10 true wall rvalue is mostly icing on the cake.
based on uvalue. Same you see in Europe and on windows. It's where rvalue number comes from. RValue is better for marketing, whereas Uvalue isn't. One of my pet peeves is the general public is mostly in the dark about this topic. So the argument for r32 verses r40 isn't r8 difference, but around 1% difference.
I wasn't suggesting that ultra-low density fiberglass like that even exists as a product, but was making the case for the performance difference between R32 at 5" vs. R32 at 11".
R40 @ 11" would be a mid-density batt, and would clearly outperform the R32 foam both at center-cavity and the lower thermal bridging. The U-factor of the assembly as a whole is quite a bit more than a 1" difference due to that thermal bridging. At a typical US framing fraction of 7% using typical framing species and typical roofing, deck & materials the 5" /R32 closed cell comes in at a whole-assembly U-factor of about U0.038 BTU/hr per square foot per degree F.
The 11" /R40 fiber with the same roofing stackup comes in at U0.027. That's about 40% more heat transfer through the roof assembly for the R32 foam vs. R40 fiber not 1%.
Okay, thanks for your input yesterday, great help. I am working on a cathedral celing with 2x8 rafters with 2x3 attached underneat. Have a 2 inch air space at top under roof sheeting so that leaves me with 7 1/4 of space. Foam guys wont fill the whole space but i should be able to squeeze in R 38 so i can live with that. Next question. I want to address the thermal bridging. Can attaching 1 inch of ridgid foam in 1 1/2 inch strips to the underside of the rafter before adding the drywall strapping achieve this? Thanks.
Edge strips on rafters work well if the cavity insulation extends to the same depth. There is still a thermal bridge where the 2x3s cross
R38 in 7.25 inches is over R5/inch- are you still planning to use closed cell foam? The performance improvement if the high-R cavity fill while better than walls with 25% framing fractions, really isn't all that great, since it's still high-R foam bridged by low-R wood.
You'd be better off filling the 2 inch air gap under the roof deck with closed cell foam, adding 1.5" thick polyiso edge strips (same depth as the 2x3s) and blowing the 8.75" of depth from the cc foam to the gypsum with cellulose or fiberglass. The cc foam is R12-ish, 8.75" of fluff R31 (if cellulose) - R35 (if 1.8lb fiberglass) for R43+ at center-cavity, but it would still make it to code-min performance on a U-factor basis due to the lower thermal bridging provided by the edge strips and extra 1.5" of fluff between the 2x3s.
Dana
Yes i am still planning on using closed cell foam. I have already installed the 2 inch air gap and the 2x3 have been glued and screwed to the 2x8. I know that i can go without the vent but i have a hard time getting my head around this, bit like jumping out of a plane with a parachute, should be okay but the lingering doubt remains. Cannot add much more depth to the rafters and getting cellulose or anything of that nature blown into this type of ceiling in my area might be nil with the exception of Spider. Is it a waste of time to add the 1 inch of foam to the underside of the rafters. I should end up with R 39 with the foam, not high numbers i know but much better than the R 7.5 that i just took out. Thanks for your help.
Thermally bridged by a 7% framing fraction the additional performance of 5" of closed cell vs 5" of open cell foam is quite small. Going with the 1.5" thick edge strips and 8.75" of open cell foam in the cavities, with a 1" continuous layer of polyiso on the interior next to the gypsum will outperform it.
With open cell in the cavities the assembly can dry into the vent space, and the low vapor retardency of the polyiso layer is inconsequential. But if you have the 5" of closed cell you'd have ~0.2 perms of foam to get through to dry into the cavity vs 10+ perms with the open cell foam. With the R6 polyiso and R32-ish open cell foam you would still have about R39 at center-cavity, but you would have R15 of thermal break on most of the framing fraction, and at least R6 of thermal break where the 2x3s cross. That's both a lot cheaper than the 5" of closed cell, and a lot greener too.
Spider would be a good option for cavity fill too, but in my area it's usually more expensive than open cell foam.
Your climate zone relative to what stackups work (or don't) hasn't come up yet, but with foil-faced polyiso (seams taped) on the interior side with a vented roof deck works in all climate zones.
Dana.
Friday i had 3 1/2 in. of closed cell installed in my cathedral ceiling. I wanted to go with open cell as i mentioned earlier but the foam guy said that they hadn't had much success getting it to bond to the closed cell with lots of wastage in this type of ceiling. Anyway i will go with R 20 batt. Will add 2 in. Thick strips of ridgid foam to the underside of the rafter to help with the thermal bridging as . I will get R 42 as it is now which is okay for i don't have any more space left to work with. I really do appreciate you taking the time to answer my questions, thanks
Paul