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How should I insulate this roof assembly?

Mike Spooner | Posted in Green Building Techniques on

I am building a 24′ x 8.5′ “tiny house on wheels.” The roof is pitched at 3.2 degrees and is sloped the long direction (perpendicular to the rafters). So as far as insulating it is concerned, it is essentially a flat roof. The rafters are 2×6. Sheathing is 1/2″ plywood, and the roof will be covered with ice and water and a low-slope metal roofing.

My original plan was to spray foam the whole assembly (closed cell) and leave it unvented. Now I am rethinking this due to the expense and possible health concerns with foam (or at least improperly installed foam).

My plan now is to add 2 1.5″ diameter screened vents to each end of each rafter bay, leave a 1.5″ air space under the plywood sheathing, and fill the rest with fiberglass. Under the fiberglass, I would do a continuous layer of 1/2″ or 1″ foil-faced polyiso foam board (or possibly 2 layers of 1/2″), with all seams taped to form a vapor barrier. Under the polyiso will be tongue and groove pine (or other wood). There will be few, if any, penetrations in the ceiling (no can lights, just an electrical box for a ceiling fan and a pendant light).

Is this a good approach, or is venting a flat roof like this hopeless, even with a good vapor barrier on the interior?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mike,
    The type of vents you describe won't work with low-slope (flat) roofs -- the low slope precludes significant air flow through the vents -- so your approach is risky. For more information on this issue, see Insulating Low-Slope Residential Roofs.

    Closed-cell spray foam (installed as you originally planned) will certainly work. By far the best solution, however, is to install a continuous layer of rigid foam above the roof sheathing. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

  2. Mike Spooner | | #2

    Thanks Martin. That is what I was afraid of. The foam on top of the sheathing is ruled out due to the height limits of the structure. If I do interior spray foam, how thick of a layer do I need?

  3. GBA Editor
    Martin Holladay | | #3

    Mike
    Q. "If I do interior spray foam, how thick of a layer do I need?"

    A. The answer depends on the climate zone where the tiny house will be parked, applicable codes (if any), and your personal goals.

    In most of the U.S., codes require a minimum R-value of R-49 for roofs. That's about 8 inches of closed-cell spray foam. For safety reasons, foam this thick has to be installed in two or three lifts rather than all at once. (It can catch fire and burn down your house if you ignore this safety step.)

    Warmer areas of the country have less stringent requirements -- generally R-38.

    All that said, most tiny houses don't fall under building code requirements. Tiny houses tend to overheat, so you probably don't need to worry much about the R-value of your roof.

  4. Mike Spooner | | #4

    Thanks. Achieving R-49 would be pretty difficult with 2x6 rafters. My question was more aimed at determining how much foam I need to make sure that I won't have any condensation issues. If I can get away with just 2" of spray foam and then fill the rest up with fiberglass batts, that might be more practical.

  5. GBA Editor
    Martin Holladay | | #5

    Mike,
    Again, the answer depends on the geographical location where the house is parked. This article will explain the minimum ratios for foam when combined with fluffy insulation: Combining Exterior Rigid Foam With Fluffy Insulation.

    (Although the article discusses the ratio rules for rigid foam and fluffy insulation, the same ratio rules apply when you are talking about closed-cell spray foam combined with fluffy insulation.)

    Note that the ratio will change depending on the climate zone.

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