Open Cell on top of Closed Cell foam
robbmc
| Posted in Energy, Heating & Insulation on
My renovation has finally started and I am looking at replacing a LOT of rodent infested batts on the attic ceilings with spray foam to the underside of the roof deck. R 49 is current code here in the Hudson Valley and I have 2×4 truss roofs for the most part. One contractor suggested 2″ closed cell to the underside of the roof deck then an additional 6″ of open cell to get to R38 that is probably required for renovations. This is an attempt to be economical for the 4,600 sq ft of roof requiring insulation.
Any thoughts?
Replies
Have you considered using 2" rigid foam first pushed up against the underside of the roof sheathing and closed cell spray foam over that? This does add more work to the job but in case the sheathing ever needs to be replaced it won't be adhered to the spray foam. You may have to increase the depth of the spray foam in order to attain the required R value.
By batts on the attic ceiling, do you mean the attic floor (the ceiling above the living space)? And is the roof vented?
Batts on the attic floor / ceilings of the spaces below. We are replacing the roof shingles and putting Ice & Watershield on 100% of the roof decking. Don't plan to vent the roof with the new foam insulation in place.
HI there, I am in a similar climate zone (North of Boston). Your contractor's suggestion is very common around here. We usually go a little thicker on the closed cell to be safe, at least 3 inches, then 6 or 7 of open cell. We are typically filling 2"x10" rafter bays that also have strapping so we have a total available depth of only 10". R- Value wise I think it comes up just short of stretch code, but it is such a superior insulation material to fiberglass that inspectors are happy to approve it. The first roof slope we insulated like this unvented assembly is probably 20 years old and performing extremely well. Good luck.
Building Science Corp has a good article on this: https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
Make sure to check out the table specifying the percentage of closed-cell to open-cell foam. https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies#t01
Thanks for this Matt. Exactly what I was looking for.
Putting rigid against underside of roof deck...ie cut-n-cobble, has a not so great history. walls its FINE,roof you really need continuous air barrier
what your talking about we call "hybrid" spray foam I just used 3" closed with 7" open , 2x10,s with strapping open trimmed flush to acheive the new R-49 for 2019 stretch in massachusetts
Hi, wondering how this went and if would recommend your contractor (if you used one)? I’m in Rosendale and looking to do pretty much the exact same thing. Get rid of mouse pee batting and go with closed/open cell combination on the roof to bring my attic into my conditioned envelope. Would love to hear your feedback. Thanks!
I live in Northern VA. Zone 4a. I would like to do R-49 in my attic to make a conditioned attic.
I plan on doing 4" of closed cell foam on the roof deck followed by 10" of open cell foam on top. What is the groups thought on this?
My HVAC unit is in the attic and I will also being installing a venting dehumidifier to help with moisture control.
I also have to spray the exposed foam with an ignition barrier.