You could call it the $6,500 problem, because that’s what it’s going to cost Kacey Zach to re-insulate a cathedral ceiling with closed-cell polyurethane foam and hang new drywall.
Writing at Green Building Advisor’s Q&A forum, Zach explains the situation: a cathedral ceiling framed with 2x12s and insulated with fiberglass batts to R-38 “with no regard to air sealing.”
The result? Nasty ice dams where two sections of roof intersect. “I’ve gotten quotes to spray foam the ceiling to R-38 with closed-cell foam for $4400,” Zach writes. “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?”
That’s the subject of this Q&A Spotlight.
Despite the cost, foam is the best option
Even though the closed-cell foam isn’t cheap, GBA Senior Editor Martin Holladay thinks it’s Zach’s best bet for curing the problem.
“It can be installed from below — a job that will require new ceiling drywall — or it can be done from above — a job that will require new roofing,” Holladay says. “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.”
Zach thinks the foam will be between 6 inches and 7 inches thick, and plans on filling the rest of the cavity with R-19 fiberglass batts. In order to spread the costs out, Zach wonders if it’s OK to leave the insulation exposed for the winter and tackle the drywall later.
Unfortunately, that’s not an option, Holladay says. Exposed foam is a fire hazard that must be covered with gypsum drywall to meet code requirements. There will be some thermal bridging through the 2×12 rafters, he adds, but not enough to cause any real problems.
Less foam, more fiber insulation
Dana Dorsett has a slightly different suggestion: “Rather than an ungodly R-38 of closed cell foam, it’s fully code-legal, moisture-safe, and cheaper to go with R-20 in closed-cell foam against the roof sheathing, and fill the rest of the cavity with fiber insulation to bring it up to code-minimum R-value.” Dorsett adds a link to the section of the International Residential Code dealing with roof assemblies.
The reason to dial back the amount of closed-cell foam is the HFC-245fa blowing agent it uses, Dorsett says, a chemical with 1,000 times the global-warming footprint of carbon dioxide. “Using any more than the minimum required for meeting the other goals is the way to go,” he adds. “This is getting there with half the foam in a safe and responsible manner that can’t be argued by inspectors.”
Another option, Dorsett adds, is an Icynene water-blown product called MD-R-200, which has an R-value of 5.2 per inch and is a Class 3 vapor diffusion retarder (1.8 perms at a thickness of 3 in.), according to the manufacturer.
“You’d still be able to get to R-38 even in 2×10 framing without burning up the planet,” Dorsett says, “and it would be sufficiently vapor retardant to protect the roof deck.” Aloha Energy, in Saratoga Springs, New York, also has a water-blown closed-cell foam, he adds, and isn’t too far from where Zach lives.
Vented or unvented roof assembly?
As David Sheard points out, there are two competing theories on how to insulate this kind of ceiling.
“The two different theories are to leave an air space above the foam, below the roof 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,” Sheard says, summing up a classroom discussion in his Canadian Home Inspectors’ course. “The second theory is to fill the entire cavity tight with foam making contact with the bottom side of the roof sheathing.”
One key question about the unvented assembly, he adds, is whether it affects the life expectancy of asphalt shingles on the roof, and whether manufacturers will honor warranties when shingles are installed over an unvented roof.
“Either way will work,” Holladay replies. “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.”
As to the warranty issue, he says, it varies by manufacturer. Many won’t provide a warranty if there is no ventilation channel beneath the sheathing. “There is little logic to justify their position, however,” Holladay says. “Fortunately for contractors, the roofing warranties are almost worthless, so this issue doesn’t matter too much.”
Installing rigid foam is yet another option
Zach has found a deal on 3-inch-thick foil-faced polyisocyanurate board, and suggests putting two layers of this insulation against the roof deck between the rafters. “This will be a cut-and-cobble job from below, but each of the 11-foot runs are straight and wide open, so putting up rigid foam seems like it won’t be too bad of a job,” Zach says. “Rip the sheet on the table saw, spray foam the rigid to the rafter bay.”
That approach will work, says William Geary. The foam can be cut so it fits tightly between the rafters, or loose with foam filling the gaps around the edges of the foam.
“If your interior trim details allow, I suggest you strap across the bottom of the rafters with 1×3 or 1×4 (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,” Geary adds.
Holladay advises that, If Zach insists on the “cut-and-cobble” approach — a method that some homeowners like, but that no professional contractor uses — it would be better to combine “cut-and-cobble” with a continuous layer of foam on the interior, on the underside of the rafters, to address the problem of thermal bridging. As long as there’s enough rigid foam against the roof sheathing, Zach shouldn’t have to worry about any moisture problems in the ceiling with this “foam sandwich.”
Our expert’s opinion
GBA technical director Peter Yost credits the good points made by Martin and other commenters, adding these points:
Use a certified spray foam company and trained installer. Many if not most of the problem spray foam installations I have seen are due to a substandard level of expertise by the installer or the company in general. In the U.S., we don’t require certified spray foam companies and trained installers the way the Canadians do, but we should. Look for certification and training such as this one from the Air Barrier Association of America.
Go with water-blown foam. The benign water-blown spray foams have way less of an environmental footprint than the R245fa-blown foams. But you should be aware that switching to water-blown spray foam is not likely to save you any money. It should also be mentioned that flame retardants are still an issue even with water-blown spray foams.
It’s more important to get a continuous air seal than it is to vent. Venting all those valley rafter cavities is really tough to do, of course. But if you don’t have that topside drying potential of the venting, than the vapor profile of your roof assembly becomes critical. Take a look at this blog I wrote on the subject.
An unvented roof assembly must dry in one direction. I like the idea of continuous rigid insulation on the interior, as both an interior air barrier (if all seams and margins are taped/sealed) and a thermal break on the roof framing. But since the top side of the assembly — your asphalt roofing shingles — form a layer with very low vapor permeance, using foil-faced rigid foam eliminates any drying potential to the interior of your roof assembly. To provide some ability for the assembly to dry to the interior, you will need to pick a rigid insulation with at least some vapor permeability, such as EPS or rigid mineral wool. (I must say, I have never used rigid mineral wool on the interior of a roof assembly and can imagine there is a learning curve there).