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Community and Q&A

Design for a ventilated low-slope roof

Timothy Denny | Posted in Plans Review on

We are building an energy efficient house in central Vermont. Part of the house is one story and has a “shed” roof with a 3/12 pitch in which the ceiling is parallel to the roof (the highest point of the cathedral ceiling is 12.5 ft). Our architect wants us to build a cold, ventilated roof based on ideas from Joe Lstiburek.

The roof section looks like this (from inside out): tongue and groove wood ceiling, cross-strapping, pro-clima DB+ smart vapor retarder, 2×12 rafters at 16″ o.c. filled with dense-pack cellulose, 1/2″ thick sheathing, 2″ of foil-faced polyiso insulation board, 1/2″ sheathing completely covered with ice & water membrane, 2″ thick strapping to form an air gap (with vents at bottom and then into a vertical wall above), 3/4″ thick sheathing, synthetic underlayment (e.g. Grace Tri-Flex) and a standing seam roof. (this should give an approximate R-50 value – we cannot make the roof thicker for various reasons).

I question whether we need all three layers of sheathing. I would rather omit the sheathing applied to the rafters and instead install the polyiso board directly on the rafters. I would then increase the next layer of sheathing to 5/8″ thick (e.g Advantech tongue and groove) and install this with long screws into the rafters (and then cover it with the ice & water membrane). I would also reduce the final layer of sheathing to 5/8″ thick (again Advantech T & G).

Will I be OK with putting the polyiso directly on the rafters and then having just two layers of Advantech sheathing?

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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues here.

    The most important issue is that the 2-inch layer of polyiso is wrong. It is the wrong choice of rigid foam for this application -- since polyiso doesn't perform well at low temperatures -- and it is too thin to keep your roof sheathing above the dew point during the winter. That is risky as well as a code violation.

    Here is a link to an article that discusses this type of roof assembly: How to Build an Insulated Cathedral Ceiling.

    In your climate zone (Zone 6), the rigid foam layer above the roof sheathing needs a minimum R-value of R-25. This could be accomplished with 6 or 7 inches of EPS. (It makes more sense to install EPS at this location than polyiso. To learn why, read this article: In Cold Climates, R-5 Foam Beats R-6.)

    For more information on installing rigid foam above roof sheathing, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

    In your case, you might want to consider installing 6-inch EPS nailbase.

    This type of roof doesn't need ventilation channels above the rigid foam, although the ventilation channels are an excellent idea and make for a great roof.

    One final point: If you can't accommodate the rigid foam because of height restrictions, you can switch to a vented roof assembly. From the bottom up, one way to do that would be: finish ceiling, air barrier membrane, rafters filled with dense-packed cellulose, plywood roof sheathing, vapor-permeable roofing underlayment, 2x spacers to create ventilation channels, another layer of plywood or OSB roof sheathing, roofing underlayment, and roofing.

  2. Timothy Denny | | #2

    Thanks Martin. Actually, I misread the section drawings I had, and my architect called for 4" total of polyiso, which would have been sufficient if it were not for the depression in the R-value at cold temps that you pointed out. Going without any foam will not give us the necessary R-value, so I guess we have two options: A) find some way to increase foam on the outside to 6" or B) move the polyiso inside the rafters where it will stay warm. If we went with option B, we'd tape the polyiso seams to make it the air-barrier and use a vapor-permeable underlayment above the first layer of sheathing (e.g. Pro clima Mento) so the cellulose could dry to the ourside.

  3. GBA Editor
    Martin Holladay | | #3

    You might consider an exterior foam sandwich with 2 inches of polyiso on the bottom and 3 inches of EPS on top. That way the EPS helps keep the polyiso warm.

  4. Charlie Sullivan | | #4

    The crossover point where EPS and polyiso are the same is about 43 F. For Martins 2+3 solution, from the middle of the polyiso out, you've got R6 of polyiso and R12 of EPS. On the warm side you've got R6 of polyiso and R38 of cellulose. So you've got R18 on the cold side and R44 on the warm side. The mean temperature of the polyiso, with the temperature 10 F outside and 70 F inside will be:
    10 + (18/62)60 = 27 F. That's low enough that the EPS is still a better choice than polyiso.

    I like the idea of EPS on the outside and polyiso on the inside, but when you have that much cellulose on the inside too, the polyiso is still pretty cold on a cold day in Vermont.

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