Andrew K has a project that should ring a bell with lots of homeowners: What’s the best way of insulating a room over the garage so it becomes useful, conditioned space?
Andrew lives in Massachusetts, in Climate Zone 5A, and would like to turn the space, what real estate agents used to call a “bonus room,” into an office. The roof is framed with 2×8 rafters and already has both soffit and ridge vents, but no insulation whatsoever.
“I have looked at closed-cell [spray foam] insulation, but quotes have come in at about $5,000-$6,000 to insulate the entire roof deck,” Andrew writes in a Q&A post at GreenBuildingAdvisor. “So I’m looking for cheaper alternatives.”
What he’s got in mind is adding fiberglass batt insulation to the rafter bays, leaving a 1-in. space for ventilation, plus 2 in. of rigid foam on the interior, then drywall.
Should Andrew install a vapor barrier? Should he be using faced batts? And which kind of rigid foam insulation would be the best choice, extruded polystyrene (XPS) or polyisocyanurate?
Those questions are the start of this Q&A Spotlight.
More ventilation, better insulation
Both GBA Senior Editor Martin Holladay and Dana Dorsett would opt for a ventilation gap greater than 1 inch gap that Andrew is planning, but they recommend slightly different approaches to insulating the rafter bays.
First, says Holladay, a 2-inch gap will be better than 1 inch. As far as insulating the rafter bays, he writes, dense-packed cellulose insulation will perform better than fiberglass batts, and polyiso insulation is more environmentally friendly than XPS.
“If you install 1×3 or 1×4 strapping (furring strips) on the underside of the polyiso,” Holladay adds, “you’ll have an easier time installing the drywall. And if you choose foil-faced polyiso, the air space will provide additional R-value to the roof assembly.”
Dorsett suggests the code minimum is 1 1/2 inch when using vent channels, although that’s not enough on low-slope roofs.
“Standard rock wool batts or high-density ‘cathedral ceiling’ batts can work if installed to near perfection,” Dorsett says. “Dense-packing cellulose in a vented assembly is damned-near impossible to achieve without adding something both rigid and vapor permeable to maintain the the vent channel without it collapsing due to the pressure while dense packing.”
One alternative, Dorsett says is a “flash-inch” of closed-cell spray foam on the underside of the roof deck, followed by dense-packed cellulose. In this case, the roof would be unvented.
“This does not meet the letter of the code,” he adds, “but there is ample evidence that it works… If you go unvented you can’t use foil facers on any interior-side foam. Fiberglass-faced roofing polyiso would work. EPS would be fine too. XPS has a heavy greenhouse footprint (heavier per inch than closed-cell foam) but that too would cut it. If XPS, 2 inches is the absolute max, or it becomes too vapor retardant.”
Air baffles and an air barrier
What if, Andrew wonders, he used a rock-wool batt, rated at R-23 at a thickness of 5 1/2 inches? Would he need baffles between the top of the batt and the roof deck to preserve the ventilation channel? Or are the batts rigid enough on their own?
Rock wool insulation is air-permeable, Holladay replies, and it requires an air barrier on the top.
“The air barrier separates the warm air held between the insulation fibers from the cold air passing through your ventilation gap,” he says. “Your ventilation baffles need to be installed in an airtight manner, because these baffles are your top-side air barrier.”
In addition to AccuVent baffles, there are a number of options, including plywood, OSB, cardboard, and rigid foam. “Your suggested material — 1-inch XPS — probably has a high enough R-value and a high enough permeance to keep you out of trouble,” Holladay says.
Not quite, replies Dorsett.
Although rock wool is air permeable, he says, “from a moisture transport point of view, but (unlike low-density fiberglass), rock wool batting is sufficiently air-retardant to not lose performance to convection, and will pretty much perform at its specified R-value as long as there is an air barrier on at least ONE side of the assembly.”
The same can be said of high-density cathedral ceiling fiberglass, Dorsett says. It doesn’t hurt to have an air barrier on the top side, it’s not as important here as it might be in other assemblies.
“Where it’s simple to add a top-side air barrier, go for it, but don’t sweat it where it’s difficult/awkward/impossible,” Dorsett says. “If it’s wide-open, using strips or chunks of 2-inch foam as spacers to the roof deck and 1 to 3 inch EPS rough-cut to fit, sealed at the edges with canned spray foam, works pretty well. EPS is preferred, since it’s 3 to 5 times as vapor-permeable as XPS.”
Filling the rafter bays with closed-cell foam is expensive and risky, he adds, because it would create a “moisture trap” for the roof deck, “between fairly impermeable roofing and barely permeable foam.”
Are AccuVents a problem?
Matt Zahorik, facing a similar building problem at this southern New Hampshire home, wonders whether the use of AccuVents might pose a moisture problem.
“Do you have a concern,” he asks Holladay, “in this setup that the AccuVent represents a vapor barrier, thus having a vapor barrier above and below the batts, giving the batts nowhere to dry should moisture be driven in?” he asks. “Would something like Tyvek on top of all or part of the batts but below the venting space act as an air barrier but allow vapor permeability?”
“I still think AccuVents can work fine,” Holladay says, “and here’s why: moisture problems in cathedral ceiling bays are almost always due to ceiling air leaks, not vapor diffusion. If you can make your ceiling airtight (or close to airtight), you’re not going to have moisture problems.
“The small amount of moisture that enters cathedral ceiling bays via vapor diffusion is absorbed by the rafters. The rafters distribute the moisture, which evaporates from the tops of the rafters into the ventilation gap at the top of the rafters.”
Dorsett isn’t so sure.
“I’m not totally convinced the extremely low permeance of AccuVent won’t be a problem if you have an actual air leak into the insulation from the interior side,” he says. “It’s not a simple thing to model, but it could be tested. With more permeable baffles the assembly becomes more resilient.”
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost sees it:
First of all, Martin Holladay already has a great GBA blog on this topic: “How to Build an Insulated Cathedral Ceiling.”
By all reports, the vapor permeability of in-place 3-tab asphalt roofing shingles is quite low, most likely a Class II vapor retarder (0.1 – 1.0 perms). That means either this cathedral roof assembly must either dry to the interior, or one side of the roof sheathing must be ventilated (with soffit-to-ridge venting).
Whether or not you choose an unvented or a vented cathedral roof assembly is way less important than getting the details right for the water, air, and thermal barriers in the assembly. It’s hard to get building scientists to agree on the required depth of cathedral roof assembly venting, but they all agree that workmanship — the quality of the insulation, air sealing, and venting — is quite often the key.
And let’s not forget that drying potential, particularly in roof assemblies, is about protecting against all three types of wetting: bulk water (roof leaks), air leakage, and vapor by diffusion. As you specify each component of the roof assembly, worry about the assembly details in that order.
Finally, there is considerable variation in vapor permeability of EPS insulation by type; the permeability essentially varies according to the foam’s density. It’s important to keep that in mind when considering the roof assembly’s vapor profile or directional drying potential.