Decision time
I previously asked about 2×8 walls and a cathedral ceiling in this post https://www.greenbuildingadvisor.com/question/whats-the-good-enough-wall-assembly-under-cedar-shingles
Context is: DC area of Maryland (Climate Zone 4). We are using cedar shingles (WRC) for siding, the shingles will be installed over classic slicker by B. Obodyke, applied over Henry Blueskin VP100 applied to the 1/2″ 5 ply plywood sheathing.
I proceeded with 2×8 walls and 2×10 cathedral rafters. I will have an unvented roof assembly with closed cell spray foam exceeding R-49 (following Martin H’s advice in the 5 cathedral ceilings article and followup questions).
I had planned to use R-30 Rockwool for the stud cavities, floowing Akos’ advice in the Q&A linked above.
I got quotes and all the places have Rockwool around $5-7k more than putting in open cell foam in the walls.
So, in my wall assembly described above, in my climate zone, is there any concern about using open cell foam filling the 7.5″ wall cavity vs. using R-30 of mineral wool?
Thanks everyone.
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Replies
Consider keeping the 2x8 top and bottom plates but use staggered 2x4s for the studs and fill the wall with damp sprayed cellulose.
https://www.greenbuildingadvisor.com/question/blowing-cellulose-in-staggered-stud-walls
Walta
I went back to the original post & there were lots of thermal bridge mitigation scenarios in the walls that he was considering including staggered stud 2X4s - so my guess is that he selected this option ...
It then comes down to how/what he wants to fill the wall cavity with ...
Lots of options: cellulose, rockwool, FG, open cell foam - flash w/closed cell then batt or blown (wet or dry), etc
Open cell will work just as well as any insulation in that wall. About the only issue is that it is spray foam.
There is an R30C fiberglass insulation that could work or two layers of the newer R15 high density fiberglass batts. The price on either option is pretty close to R30 MW so might not save that much.
How many square feet? $5-7k more sounds like a lot if a smaller job. Are you comfortable installing the Rockwool yourself?
I'm pretty familiar with your area, since I used to do a lot of work out there. You mostly just have a LOT of humidity to deal with, and hot summers. I don't think you're going to have issues with open cell spray foam in this application though, but I would ABSOLUTELY avoid using open cell in any roof assembly -- you want closed cell in the roof.
You may want to consider high density fiberglass batts as a more economical alternative to mineral wool if you want to avoid spray foam. These are available, but usually have to be special ordered.
Bill
Thanks. That’s doubly reassuring. The humidity is so thick today my piece of paper with stair layout just fell apart in my hands. Outside 20 minutes.
Definitely closed for the roof. Eliminated two firms immediately who said “you can save some money by doing open cell in the roof. We do it all the time with no problems”. (!!!)
Ha. I usually stayed in Federal Hill in Baltimore, plenty of humidity there! Usually a breeze off the harbor though, which was nice. Friend of mine who lives there has said he can leave his house, walk next door (14 feet away since they're row houses), and be sweaty by the time he gets there...
Anyway, open cell spray foam has shown a tendency to hang on to moisture when used in attics, and in areas which very high average humidity levels, you're likely to have problems over time. Maybe someone has been OK, or maybe they just don't look closely, but I think it's a huge risk and I would NOT try it myself.
I should clarify that the high density batts I mentioned would be OK in the WALLS. In the roof, I would use closed cell spray foam since you said you can't do a vented assembly.
I'd wait to see if DC (a member here) comments too, as he is also in your area and may have some ideas that could help you.
Bill