Rob Graff is getting a new roof, and with it an opportunity to turn his vented attic into an insulated, conditioned space.
But he’s also got some concerns.
“They will be spraying 5 inches or so of closed-cell foam directly against the decking,” he writes in a Q&A post at GreenBuildingAdvisor. “This will fully seal my attic. My question is, how does this work if you do not have air conditioning in the house, other than window units in the bedrooms and one in the living room?”
Graff’s two-story house includes a partially finished attic, and he’s not sure how that can be vented once the roof deck is insulated.
“My concern is that, because heat rises, the attic will gather heat. How do I get that heat out of the attic if it has no vents? Also, how would I integrate a whole-house fan into this design?”
Graff’s questions are the topic for this Q&A Spotlight.
The attic is just another room
Once the roof deck is insulated, the attic becomes a conditioned space just like any other room in the house, GBA senior editor Martin Holladay says. “If you install closed-cell spray foam on the underside of your roof deck, creating an unvented attic, there is no reason to believe that your attic will be warmer than a vented attic,” Holladay writes. “If anything, it will probably be cooler.
“Your attic will be within 5 or 10 degrees of your living room temperature. If your living room is hot, your attic will be, too. If your living room is cool, your attic will be, too.”
The closed-cell foam, Holladay adds, should have only a marginal effect on how long the new roof shingles will last. Although the insulation will make the roof shingles “somewhat hotter” than they would be over an unconditioned attic, insulation is only one of a number of factors affecting shingle temperature.
“The most important factor affecting shingle temperature is shingle color,” Holladay says. “If you want cooler shingles, out of the belief that cooler shingles last longer — and they probably do — then install white shingles, not dark shingles.”
Incorporating whole-house ventilation
Graff currently places a fan in an attic window for nighttime cooling. With downstairs windows opened, and the fan in the attic window running, warm air is pushed out of the house and cool outdoor air is drawn in to replace it. This is an effective, low-cost way of cooling down a house without the use of air conditioning.
With an insulated roof deck, Graff wonders, what happens to this strategy? “So how does warm air leave the attic?” he asks. “It sounds as if the air in the attic will never get changed, and I will no longer be able to bring in cool air at night.”
Holladay noted that nighttime ventilation cooling — using either a conventional whole-house fan or the NightBreeze system — is a good way to cool a house in any climate that has cool tempertures at night. Graff responded that the NightBreeze system seems like something that would be added to a central air-conditioning system, not a stand-alone solution.
“When you convert a vented unconditioned attic into an unvented conditioned attic, you are incorporating the attic into your conditioned space,” Holladay replies. “It’s just like another room in your house. It shouldn’t be significantly warmer or colder than any other room. It’s just like a second-floor bedroom or a third-floor bedroom. But If you want nighttime ventilation cooling using a whole-house fan (usually installed in the ceiling of the upstairs hallway), it’s much easier to do with a vented, unconditioned attic.”
But, James Morgan writes, there’s no reason Graff wouldn’t be able to continue using an exhaust fan installed in his attic window — if he really needs to. “I see no reason why this arrangement will not continue to work in the new attic regime,” Morgan says. “To get the proper benefits, of course, there has to be an open air path to the attic space where the window is located and all window AC units need to be shut off.
“A whole-house fan is simply a bypass arrangement to enable nighttime cooling in homes without such a convenient layout as you currently have. It’s not so much that it won’t work, but that you don’t need it.”
Insulation plan has some disadvantages
Spraying 5 inches of foam on the bottom of the roof deck has at least a couple of problems, says Dana Dorsett.
Because closed-cell foam is vapor-impermeable, it would trap moisture at the roof deck, Dorsett says, and using foam at that thickness is a lost cause from an insulation point of view because of thermal bridging through the rafters.
“What’s more,” Dorsett says, “the blowing agents of most 2-lb. foam have a huge life cycle global warming hit, making anything beyond the first inch or two net negative for the climate.”
Instead, Dorsett recommends spraying 1 inch to 2 inches of foam against the roof deck and finishing with damp-spray cellulose or high-density fiberglass, a strategy that’s “the best of both worlds.” In support of his suggestion, Dorsett refers Graff to a Building Science Corporation report on the subject.
“All insulation systems are compromises,” Holladay says. Installing a continuous layer of rigid foam insulation over the roof sheathing or under the rafters would control thermal bridging, he advises.
“Unlike Dana Dorsett,” Holladay adds, “I don’t think that installing 5 or 6 inches of closed-cell spray foam on the underside of your roof sheathing will cause any moisture problems. It’s true that the roof sheathing won’t dry to interior to any degree — but as long as the roof sheathing is dry on the day that it is insulated, the system should perform as designed.”
Our expert’s opinion
Peter Yost, GBA’s technical director, adds these thoughts:
First, it is always helpful to know the climate that any assembly is facing, whether there is any HVAC equipment in the attic, and what the roof cladding is. I am going to assume that this is a cold climate, there is no HVAC equipment in the attic, and the roofing consists of 3-tab asphalt shingles.
Given these assumptions, here is my hygrothermal assessment (that is, my assessment of the energy and moisture performance of the attic space and roof):
1. Net energy efficiency gain by moving the air and thermal barrier to the roof line.
In almost all climates and building types, it is a net gain to “cathedralize” the attic, even if this space is not going to be used as fully conditioned space or living space. This is especially true for hot climates, but in many cases holds for cold climates as well, in large part because the roof plane is usually the easiest plane to get these two barriers continuous, and because the increased volume of the overall inside space of the building typically represents a relatively small energy penalty. Particularly in a cold climate, sealing air leaks at the eave is critical to combating ice dams.
2. Using the same attic window(s) to ventilate at night.
There is no reason that Rob Graff can’t continue to pull air from the living spaces below, up and through the cathedralized attic space. In fact, the significant moderation in temperatures afforded by the roofline air and thermal barriers will reduce the need for night-time cooling.
3. Impact on attic solar heat gain of roof cladding type and color.
The thermal performance of the roofing (its reflectance — the fraction of solar energy the roof cladding rejects — and its emittance — the fraction of absorbed energy the cladding can re-radiate) is important. But take a look at the numbers below for different roofing materials. White shingles are much better than black, but white metal roofing is far better than white shingles at rejecting the sun’s energy and shedding any that it absorbs. Interestingly, while Galvalume has a really high reflectance, the small portion that it absorbs is not easily re-radiated or emiited, so a white metal roof is far better at keeping an attic cool than a bare metal roof. We don’t know the age or condition of Rob’s roofing, but switching to white shingles is not nearly as effective as switching to white metal, albeit at a much higher initial cost.
- White 3-tab asphalt shingles (representative numbers from Energy Star): Initial reflectance – 0.66; Initial emissivity – 0.85.
- Black 3-tab asphalt shingles (representative numbers from Florida Solar Energy Center): Initial reflectance – 0.034; Initial emissivity – 0.90.
- White metal roofing: Initial reflectance – 0.84; Initial emissivity – 0.91.
- Galvalume metal roofing (bare metal): Initial reflectance – 0.90; Initial emissivity – 0.06.
4. Drying potential of the closed-cell spray-foamed roof assembly.
Given that the vapor permeability of both the spray foam and the asphalt roofing shingles is very low (both are Class I or Class II vapor retarders), the new roof assembly has little to no drying potential. While Martin Holladay is correct in saying that if the roof is dry at the time of spray foaming, it can stay dry, that is really dependent on just how much confidence Rob Graff has in the ability of his roof to shed bulk water and not leak. A very simple roof plane with few penetrations and in a sheltered location is one thing, but a complex roof plane with lots of penetrations in an exposed location is a whole other matter. I generally recommend strongly against assemblies without one dedicated direction of drying potential, unless the roof assembly has been designed and constructed (and will be maintained) flawlessly.
5. Need for ignition barrier on spray foam.
One code inspector may give a homeowner a pass for leaving the spray foam exposed in a non-living space;another may not. But frankly, if the pitch of the roof brings any utility to the newly enclosed attic space, Rob Graff will end up storing stuff or inviting people to this space, and it will need an ignition barrier, easily provided by gypsum board as the attic is finished.