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Open-cell spray foam vs. Roxul

Again I am asking insulation questions. I have a bid for a full foam compliment in my new construction home. They want to do 4" of open cell in the exterior walls trouble is my walls are 2x6 and it just doesn't make sense to leave that area open to me plus the foam performs best at 4" thick so I understand the cost difference for payback on no the extra inch is tough to justify.

R-14 in the stud bays with the r-9 outside with polyiso just seems a little short on r value to me??? Seems like I can do better.

I'm in 47460 with 1.5 inches of polyiso foam on the outside. I am really struggling with this decision because I simply am running out of options.

Dense pack cellulose is not an option in my area, wet sprayed cellulose contractor gets horrible reviews and I don't think I want the wet inside the wall cavity.

Some of you have recommended getting a power caulk gun, sealing the stud voids and installing the batts on my own....I think I would have a better r value in the end with the batt but honestly it's turning out to be a budget thing. Cost wise am I getting more bang for my buck with the foam?

This is killing me with all the variables and I just want to make sure I am getting the most r value for my dollar. If that is batts then so be it. Please help

Asked by Anthony Hughes
Posted Sep 4, 2014 5:33 PM ET
Edited Sep 5, 2014 6:22 AM ET


3 Answers

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Many GBA readers have been helping you by providing information on wall insulation. I think that at this point, you just need to make a decision and move on. We can't make the decision for you.

Only you know what your budget is, and only you know whether or not you want to do the air sealing work yourself or hire a contractor to do it.

Answered by Martin Holladay
Posted Sep 5, 2014 6:20 AM ET


What about loose cellulose? Also, I'm surprised foam is cheaper than batts. What are the costs?

Answered by Charlie Sullivan
Posted Sep 6, 2014 12:54 PM ET


I'm dubious that you'd be getting more bang/buck out of 4" foam than with R21 fiberglass or R23 rock wool batts, since the thermal bridging is higher with only 4" of fill in a 5.5" deep cavity. Don't go with any lower density batts though (Most fiberglass manufacturers' R22s are actually crummier than R21s when compressed into a 5.5" cavity, according to their own compression charts.)

From an average winter time performance point of view the 1.5" polyiso is probably going to come in closer to R8 than R9 in your location, no matter what the label reads.

Leaving 1.5" vertical channels inside a wall assembly is a potential fire-spread issue. If you go with the open cell foam it's worth buying a contractor-rolls of R13 fiberglass and splitting it into ~1.75" thick batts to fill those voids with something insulating and air/fire retardent.

Answered by Dana Dorsett
Posted Sep 8, 2014 2:15 PM ET

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