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Insulating Low-Slope Roof in Warm Climate

John_Be | Posted in General Questions on

I built a lean-to “shed” (10×12) that has a roof pitch about 2.4. I used 2×6 for trusses. The room will be conditioned (heat and cool). We live in southern California, so temps rarely drop below 34 degrees but can get over 100 degrees.

I added vent holes thinking I could just use batt insulation. After reading so many articles, I don’t think I can! It is hard finding a company that does spray insulation. Do I have other options? Can I use batts and seal the holes? I’ve attached a picture for reference.

Thank you in advance!

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    John,

    Unfortunately at this point you have a number of obstacles:
    - The framing for the overhangs and skylight interfere with possible venting in many of the rafter bays.
    - The pot-light enclosures are deep enough to short circuit any insulation.
    - The roof framing is too shallow for an effective vented assembly.
    - (I presume) the roof is on, precluding exterior insulation.

    I don't see many viable options outside spray foam. Whatever you decide I'd swap out those light for shallow or surface mounted LEDs.

    1. John_Be | | #3

      How about just applying a radiant barrier and leaving the air vents? Thinking the radiant barriers that have about 9mm foam. Would that help? If so, staple directly to sheathing or have a 1/4" air gap?

      1. Expert Member
        MALCOLM TAYLOR | | #4

        John,

        I don't know much about radiant barriers as I live in climate where they aren't effective. They may be of some use where you are.

        Depending on how much insulation you will be satisfied with, you could add a layer of rigid foam to the underside of the rafters too. Working in your favour is that the size of the shed is so small the heating or cooling loads will be pretty low no matter what you do. When I build my own very similar shed I clad all six faces in 2" EPS and had done with it.

        1. John_Be | | #5

          Oh, awesome! I see those boards have radiant barriers as well. Nice shed and thanks for the advice.

  2. John_Be | | #2

    Bummer, but that's what I suspected. Thanks

  3. jackofalltrades777 | | #6

    Last low slope roof design (skillion & lean) used closed cell spray foam (6"). Might want to vent/elevate the top OSB sheathing layer if doing the closed spray foam since the sheathing cannot dry inwards/interior side and can only dry outwards/exterior.

    1. John_Be | | #8

      Makes sense, thanks. Too hard to locate anyone who sprays here. Only company wanted $3600 for 4" on about 130sqft.

  4. exeric | | #7

    I think individuals (still) are confused about when to use radiant barriers and when its overkill. Again, they do work as advertised to block radiative heat, but they aren't a replacement for a substantial thickness of insulation. They do nothing to keep a building warm in winter. It's just another tool in a smart builder's arsenal and should be used when it's appropriate and used in its most elemental form, i.e. not attached to insulation.

    The easiest, fastest, and cheapest way to use it is in "new" builds where you install roof decking pre-wrapped with a radiant barrier. It's kind of surprising builders do not use that more. There are really no negatives except the possibility its hard to get a hold of in certain parts of the country. It's an insignificant up charge here in California over standard roof sheathing. I think it's way cheaper than installing cool roof shingles and are just as effective. It should be used everywhere on new builds in areas of the country that have substantial cooling loads.

    1. John_Be | | #9

      We could not locate the boards that had the barrier attached (in socal). We would have had to wait about 6 weeks and I want to get this done. I think I'm going with the 2" foam board with reflective side attached directly to rafters. That way I get some R insulation, a radiant barrier, and the sheathing can still breathe. I think that's my only option.

      1. exeric | | #10

        That's too bad. One can only try. The supply chain seems to be messed up now. It sounds like you've been headed in the right direction except for some glitches that you've described.

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