Cathedral ceiling insulation
I’ve got a cathedral ceiling built of 2×12’s. Currently insulated with R-38 fiberglass batts with no regard to air sealing. I’ve gotten quotes to spray foam the ceiling to R-38 with closed cell foam for $4,400. I also got quotes to re-drywall the ceiling after it is all done for $2k. I could be into this project for nearly $6500… yikes!!!
Do I have any other less costly options? Yes, I’ve read the article “How to build a cathedral ceiling.” I think that I need to be a unvented since the geometry does not lend itself to a unvented.
I have included a picture of my roof. The wall that is facing you is the north wall to the great room with the cathedral ceiling. You can see that the roof from the second story comes down to the cathedral part. The cathedral ceiling goes all the way to the blue line wall. The red circle shows were I had nasty ice dams last winter.
There is a stairway that goes to the second story behind those two windows. This gives me about half the area that has a straight shot to the soffit. Above the ceiling (on the part where the roof comes down from the second story) is open to the vented attic space.
Can I build a vented assembly on the one side and rely on the attic to vent the other side? I assume that stairway cutout has some issues which result in my ice dams. Maybe I just cut out the stairway part and spray foam it? As I type this, I’m beginning to think that my best option is to spray foam the entire ceiling.
Thoughts/comments?
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Replies
Kacey,
Spray foam is probably the best way to go. It can be installed from below -- a job that will require new ceiling drywall -- or it can done from above -- a job that will require new roofing.
If your current bids seem high to you, you can always contact other contractors for more bids. But I wouldn't be surprised if the job ended up costing what you were told.
I've received a few bids. The guy that I plan on going with will be spraying Corbond closed cell. I think the R38 thickness will be around 6-7 inches. Will there be any problems if I leave the drywall off this winter (in order to spread the costs a bit)? I suppose those 2x12's could see some condensation? I also feel that it would be a waste not to fill the cavity completely so i will fill the rest of the 2x12 cavity with fiberglass, probably a R19 batt. Will i need an unfaced batt in this application?
Kacey,
Q. "Will there be any problems if I leave the drywall off this winter (in order to spread the costs a bit)?"
A. Exposed foam is a fire hazard. You need to cover the cured foam with gypsum drywall to meet local code requirements.
Q. "I suppose those 2x12's could see some condensation?"
A. No, your 2x12s will not cause problems. There will be some thermal bridging through your rafters, but not enough to cause any problems.
Q. "I also feel that it would be a waste not to fill the cavity completely so I will fill the rest of the 2x12 cavity with fiberglass, probably a R-19 batt. Will I need an unfaced batt in this application?"
A. Unfaced is better than faced in this application.
I am new to this forum and I look forward to learning as much as possible. I could have asked an new question, but thought we could expand on this question! I am crrently completeing a Canadian Home Inspectors Course and we were discussing the issue of insulating a cathedral ceiling with sprayed closed cell foam. The 2 different theorys are to leave an air space above the foam, below the roofs sheathing to allow air to vent from soffit areas to a ridge vent which would keep the roof deck cooler and avoid condensation to build. The second theory is to fill the entire cavity tight with foam making contact with the bottom side of the roof sheathing. Although this all helps with preventing condensation problems the question that is being asked in the class is, will this effect the life expectancy of the Asphalt Shingles covering the roof area? Will manufacters of the shingles warrent the product considering there is no air flow below the sheathing, thus creating the possibility of extreme heat of the shingles. Thanks for any imput you can provide, respectfuly, Dave Sheard, Ottawa Canada.
Kacey: Rather than an ungodly R38 of closed cell foam, it's fully code-legal, moisture-safe, and cheaper to go with R20 in closed cell foam against the roof and fill the rest of the cavity with fiber insulation to bring it up to code-R:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
Almost all closed cell polyurethane is blow with HFC245fa, which has a very heavy global warming footprint (~1000x CO2). Using any more than the minimum required for meeting the other goals is the way to go. This is getting there with half the foam in a safe & responsible manner that can't be argued by inspectors.
If you have an installer of Icynene's water-blown 2lb semi-open cell product (MD-R-200) at a bit more than R5 inch you'd still be able to get to R38 even in 2x10 without burning up the planet, and it would be sufficiently vapor retardent to protect the roof deck. Aloha Energy (a smaller manufacturer not too far from you in upstate NY) also has water-blown closed cell 1.8lb goods at ~R6/inch, if you can find an installer in your area.
http://www.aloha-energy.com/installers.html
http://www.icynene.com/builders/products/product-portfolio/md-r-200-spray-foam-insulation
This is going to be more expensive than an R20 foam + fiber solution. At 4"/R20 even the semi-open cell Icynene is a minimal class-II vapor retarder, which is sufficiently protective of the roof deck. The 1.8lb Aloha Energy goods are even more vapor-tight- comparable to other ~2lb closed cell foams (which is both good and bad.)
Not code-legal (but still not a problem) you could get by with as little as 2" of HFC-blown closed cell foam (or 2" of 1.8lb Aloha Energy), as carefully simulated by the Building Science Corp folks:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems (See the 2" ccSPF column in Table 3.)
David Sheard,
Q. "We were discussing the issue of insulating a cathedral ceiling with sprayed closed-cell foam. The 2 different theories are to leave an air space above the foam, below the roofs sheathing to allow air to vent from soffit areas to a ridge vent which would keep the roof deck cooler and avoid condensation to build. The second theory is to fill the entire cavity tight with foam making contact with the bottom side of the roof sheathing."
Q. Either way will work. If you want to have a ventilation gap between the top of the cured spray foam and the underside of the roof sheathing, you'll need to install very rigid insulation baffles before the foam is installed. For more information on this issue, see How to Build an Insulated Cathedral Ceiling.
Q. "Will manufacturers of asphalt shingles warrant the product considering there is no air flow below the sheathing?"
A. The answer varies from manufacturer to manufacturer, but many asphalt shingle manufacturers won't provide a warranty if there is no ventilation channel. There is little logic to justify their position, however. Fortunately for contractors, the roofing warranties are almost worthless, so this issue doesn't matter too much.
I came across a deal on some foil faced polyiso. 3" thick. Can I put two layers of this against my sheathing? This will be a cut and cobble job from below, but each of the 11' runs are straight and wide open so putting up rigid foam seems like it won't be too bad of a job. Rip the sheet on the table saw, spray foam the rigid to the rafter bay. I don't have a straight shot soffit to ridge on over half the roof so it would be unvented.
Yes, you absolutely can put in two layers of polyiso insulation against the roof deck. You can cut the foam tight, or even quicker and perhaps better is to cut it loose, support it with nails against the sides of the rafters, and spray the gaps with foam making sure you seal deep into the gaps against the sheathing.
If your interior trim details allow, I suggest you strap across the bottom of the rafters with 1x3 or 1x4 (this will help reduce thermal bridging and reduce nail pops from seasonally moving 2x12s) and fill the remainder of the cavity with dense-pack Spider fiberglass or cellulose under Insulmesh netting.
Billy
Kacey,
The cut-and-cobble method can work, but it doesn't address thermal bridging through the rafters, so it isn't as good an approach as one that includes a continuous layer of rigid foam.
If you can combine cut-and-cobble with interior continuous rigid foam on the underside of the rafters, you'll get better performance.
Martin and Bill have me thinking now.... the ceiling that I tore down had a few nail pops. I figured that screws may help that when the new drywall goes up. But maybe the 1x4 (16 or 24 OC???) strapping combined with rigid in between the strapping would be the best. I would only loose 3/4" of ceiling height, we have a story and a half so the upper loft looks over this area under the cathedral but not full height (knee wall at the top) so I don't want to loose too much ceiling height.
Martin,
If I put foam on the underside of my rafters, won't this become a foam sandwich? I was intending on 6" polyiso with 5" of fiberglass in the bay. The polyiso will be spray foamed into place via froth-pak
Kacey,
You're right that it's a type of foam sandwich. But as long as you have enough rigid foam on the exterior side of the sandwich -- and 6 inches is definitely enough -- and as long as the rafters are dry when the work is being performed, this type of sandwich carries a very low risk. I think it will be OK.
Martin,
Thanks for all the great help! I'm thinking about how I will sell this to the wife...
How thick of rigid foam on the underside to you suggest?
Any problem with 1x4 across the rafters (16" OC) with 3/4" rigid foam in between the 1x4s? I realize this is not ideal but it may help my sale to the wife.
The reason this will be a hard sale is that we have a loft area that overlooks the area under this cathedral ceiling. The loft ceiling is about 2' higher than the peak of the cathedral ceiling which means by lowering the cathedral height I will be blocking part of the view from the loft.
Kacey,
Continuous insulation is much better than insulation that is cut into narrow strips and fit between furring strips.
If you are worried about the effect of lowering your ceiling, you might install just 1 inch of polyiso under the rafters. Even 1 inch will help with thermal bridging. Of course, 2 inches would be better.
It's even possible to skip the furring strips, and screw your ceiling drywall through the polyiso to the rafters. I've done it. It is a little tricky, but it's possible. And that way you can save 3/4 inch.