Finding effective ways of beefing up under-insulated roofs is a perennial problem, affecting countless houses built to minimum energy standards.
Even roofs that meet minimum code requirements are susceptible to thermal bridging when only the rafter bays are insulated.
So imagine starting with a New Hampshire beach house whose cathedral ceilings contain not a speck of insulation. “There is no insulation except for 1/2-in. Homosote sheets between the trusses,” Michael Buckley writes in this Q&A post at GreenBuildingAdvisor. “The roofing is relatively new (5 yrs). So I would balk at tearing it off.”
Although Buckley posted his question last September, it continues to attract comments, as well as questions from other homeowners facing similar problems.
Buckley’s budget is limited. He has no objections to putting in a new ceiling, but he wants a cost-effective way of insulating the 5 1/2-in.-deep bays between the framing. He intends to do the work himself.
As GBA senior editor Martin Holladay points out, Buckley is in Climate Zone 5, where code requires a minimum of R-38 ceiling insulation. “That amounts to about 7 1/2 in. of XPS foam or about 6 in. of polyiso foam,” Holladay writes. “Or you could use about 12 inches of fiberglass batts.”
What’s the best answer? That’s the subject of this week’s Q&A Spotlight.
One plan that won’t work
One option Buckley has been considering is a combination of polyiso foam board and a product called Prodex Total 48, which consists of two layers of reflective aluminum on a 5 mm (3/16 inch) layer of closed-cell foam. Retailers handling Prodex claim the material has an R-value of 15.67, prevents 97% of potential radiant heat transfer, and even acts as a vapor barrier.
Don’t believe it, both Holladay and David Meiland warn. “Their claim of R-15 in a 5 mm thickness is pure BS. Don’t even consider it,” Meiland says.
“Prodex is a scam,” Holladay writes. “Their R-value claim is totally fraudulent.”
Rigid foam will work
But Meiland adds that Buckley’s idea to add rigid polyiso insulation in the 4-ft. wide spaces between trusses has merit.
“If they are on 4-ft. centers, then installing foam board from below might be the right choice,” he says, “although it might be time consuming to cut and fit all of the pieces, and you are not doing anything to deal with thermal bridging that way. You will also need to install a new ceiling finish.”
Although the rafter bays are only 5 1/2 in. deep, Holladay says there is no reason the insulation has to be limited to that depth as well.
“Since you’re planning to install a new ceiling anyway — and since your 4-ft.-on-center framing means that you need new framing members of some kind anyway — there doesn’t appear to be any reason that you need to stick with a 5.5-inch-deep ceiling,” Holladay says. “There are all kinds of ways to sister new rafters to your existing rafters, or to furr down the framing.”
Such a configuration might include rigid foam between the trusses, followed by another 1 in. to 2 in. over the bottom of the trusses, which would increase overall R-values while reducing thermal bridging. Drywall would complete the assembly.
Even better, Meiland says, would be adding insulation on top of the roof deck, even if the roofing is only five years old. “You get continuous insulation with the framing completely covered, and you don’t have do much work inside,” he says.
Neither way would be especially easy as a DIY project.
And while we’re on the subject
Buckley’s predicament soon brings two similar posts, one from Carolyn Smith in Atlanta and another from Sharon, both of whom are looking for ways to increase insulation in the roof.
Smith lives in a 1955 ranch with cathedral ceilings. The ceiling in half the house is drywalled; in the other half it’s open to the roof decking. Two years ago, she added a metal roof, but no new insulation. “I now realize I missed an opportunity,” she writes, “but of course I am not willing to take it off.”
Sharon is remodeling a room with a cathedral ceiling in which 10-in.-thick unfaced batts have been installed between the rafters. “I know 10 in. of insulation is not enough,” she writes. “There have been no problems with ice damming or anything, but with all the new energy initiatives I want to add more insulation from the inside.”
A potential fix in both cases is to use rigid foam board, cutting it slightly smaller than the cavity into which it fits and sealing the edges with expanding polyurethane foam. Air-sealing would be an important step.
When Smith asked for advice on the “best approach” to solve her problem, Holladay responded that the best (although not the least expensive) approach “would be to install the 5 inches of polyiso on top of your asphalt felt underlayment, followed by 1×4 strapping from soffit to ridge, screwed through the polyiso to the rafters. Then install nailers on top of the strapping (at 90 degrees to the strapping), 2 ft. on center, and then install your metal roofing.”
Three houses with essentially the same problem. If nothing else, their common complaints prove that insulating correctly at the time of construction is always the cheapest and most trouble-free option.
Our expert’s opinion
We asked GBA advisor Jim Sargent for his opinion on Buckley’s situation. Here’s what he had to say:
KISS — as in, keep it simple, stupid.
The roof structure seems to be in good shape and performing as it should. The owner would like to increase the thermal performance. Adding foam or rock wool to the underside will do this without having to redo parts that are still performing.
Either material would have to be covered with something else (fire-rated for foam and something cosmetically appealing for the rock wool). It will not be perfect, but it should make a big difference It is also simple enough for a DIY project.