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Musings of an Energy Nerd

A Researcher Looks at Insulated Roof Assemblies

Investigating a variety of vented and unvented roof assemblies, Kohta Ueno figures out what works and what doesn’t

Rigid foam keeps the roof sheathing warm and dry during the winter. One way to build a code-compliant unvented roof is to include one or more layers of rigid foam above the roof sheathing. Warm roof sheathing is much less likely to accumulate moisture than cold roof sheathing, so this roof assembly doesn't need a vent channel. [Image credit: Building Science Corporation.]
Image Credit: Image #1 - Building Science Corporation

Kohta Ueno knows a lot about insulated roofs. As a researcher and engineer at Building Science Corporation in Westford, Massachusetts, Ueno has seen plenty of well-designed roofs, as well as plenty of rotten roof sheathing. For a building science researcher like Ueno, rotten sheathing isn’t a disaster; it’s data.

At a presentation on roofs at the March 2016 BuildingEnergy conference in Boston, Ueno shared the stage with Peter Marciano. Marciano is a roofing consultant; his half of the presentation focused on low-slope commercial roofs. Since GBA readers are more focused on residential work than commercial work, I won’t be reporting on Marciano’s presentation. (I can’t resist noting, however, that when Marciano develops specs for a roof replacement job, he almost always tries to include rigid foam above the existing roof sheathing. For a high-quality roofing job, Marciano said, “bring insulation into the equation.” He advised, “Lie to the owner. Tell him it’s required by code.”)

An unconditioned attic is easier than a cathedral ceiling

Most of Ueno’s presentation focused on unvented insulated roof assemblies. “Building Science Corporation has been doing a lot of work on unvented roofs in various configurations,” Ueno said. “We have a lot of field experience on these roofs.”

Ueno began his talk by praising unconditioned attics. “A ventilated attic is the classic approach. The attic is outside space with insulation down on the floor. The roof sheathing dries inward, to the ventilated attic. This is the most cost-effective way to build a roof. You end up with high R. It’s moisture-safe, even if there are roof leaks. The attic is safe from condensation in the winter, because the attic ventilation carries away any moisture. It’s…

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6 Comments

  1. Kohta Ueno | | #1

    Link to Presentation
    Martin--great job; thanks for a nice writeup! If anyone's interested, the full presentation is available on BSC's website:

    NESEA Building Energy Conference - March 10, 2016
    2016 NESEA Unvented Roof Research - Kohta Ueno

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Kohta Ueno
    Kohta,
    Thanks for sharing the link to your excellent presentation.

  3. Dan Kolbert | | #3

    Thanks
    Kohta - I was also at the presentation, and it was fantastic. One of those topics I obsess over and I'm very glad that you are doing the research to help us all out.

  4. Paul Austin | | #4

    Rigid Foam Board Over a Vented Cellulose Insulated Roof
    I am sorry I missed your presentation Kohta. I will have to read the presentation instead. But I have a slightly different situation with a roof. We are renovating a cape for a Habitat for Humanity family. The roof is vented and insulated with a dense pack cellulose. However, the original builder thought it would be a good idea to install a 6 mil poly barrier under the sheet rock. So far we have not found any indication of mold on the backside of the sheetrock or in the cellulose. I would like to add additional insulation to the roof (2"XPS), but am reluctant to do so until I understand the physics of this roof. It seems to me that the insulation cannot breathe to either the roof or the interior. Any moisture if it accumulates at all has to find its way out through the vents. If the roof side is already sealed to moisture passing through, what harm might develop by adding a layer of insulation over the sheathing?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Paul Austin
    Paul,
    First of all, you need to tell us your climate zone or geographical location.

    In Climate Zone 5, 6, 7, or 8, it's unlikely that the interior polyethylene will ever cause any problems.

    If you want to add rigid foam on the exterior side of the roof sheathing, you would need to seal up the soffit vents and ridge vents, and follow the guidelines in this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    If you want to add a layer of rigid foam insulation on the interior side of the assembly, go right ahead. You won't have any problems -- especially because the assembly has a vent channel between the top of the cellulose layer and the underside of the roof sheathing.

  6. User avater
    Dana Dorsett | | #6

    Insulation doesn't need to breathe- it does need to stay dry.
    The concern with unvented roof assemblies is the structural integrity of the structural roof deck. When that deck is directly under the shingles it's possible to get wetted from above by leaks, wind driven rain, etc., or from air leaks (and to a lesser degree, vapor diffusion) from the interior side when the roof deck is colder than the dew point of the conditioned space air. The vent space gives the roof deck some drying capacity from shingle leakage, but completely interrupts interior air moisture drives with a layer of outdoor air. The 6 mil poly eliminates vapor diffusion drives, but either the ceiling gypsum or poly needs to be detailed as an air barrier to eliminate all moisture transfer from the interior out to the roof deck.

    When the deck under a few inches of foam with a nailer deck (vented or not) above the foam, leaks from the exterior become nearly a non-issue for the structural roof deck, and it keeps the deck warmer in winter which reduces the vapor diffusion drive from the interior. Even if there isn't a sufficient ratio of exterior R to interior R to keep it's wintertime average above the dew point of the interior air, the higher temperature is protective. Using cellulose is also protective, since it "shares" the moisture burden, and redistributes the moisture. But it's clearly safer if you can hit a ratio of exterior foam-R to cavity fluff R that would keep it warm enough even if there were minor air leakage to the interior or no interior side vapor barrier.

    Note: For dew point control purposes don't assume more than R9 over the long term for 2" of XPS. It's initial R will be higher in winter than the labeled R10, but as it loses it's blowing agent it's performance drops to about R8.5 when the mean temp through the foam is +75F, R9-ish when the average temp is through the foam is +40F. If you use EPS of similar density it will have the same long-term performance as XPS, but zero drift- pretty much the same on day 1 as it is on day 20,000. The global warming impact of the blowing agents used for XPS are pretty high, but negligible for those used for EPS.

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