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Community and Q&A

Uninsulated barn with wood siding: does a radiant barrier make sense?

ideaconstruction | Posted in GBA Pro Help on

I have a 20×30 barn with wood siding (pine boards) and asphalt shingles over 3/4″ T&G decked gambrel roof. The 2nd floor loft has a 17ft height at the ridge with zero insulation. I read your article about radiant barriers and an example of an uninsulated barn with steel siding and roof was mentioned. Would a barrier be functional toward reducing air temperature in my barn and if so, where exactly should it be installed? I would try to do this myself. Thank you.
Rob Cramer

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  1. GBA Editor
    Martin Holladay | | #1

    In this application, a radiant barrier will work to reduce summertime heat gain. Of course, so will insulation. If you think that there might be a chance that the space will eventually be conditioned (heated or cooled), then insulation makes more sense than a radiant barrier.

    If you decide to install a radiant barrier, the best place to install it would be on the underside of the rafters (with an air space between the radiant barrier and the roof sheathing).

    For walls, it should be installed on the interior side of your posts (or other wall framing).

    Remember, radiant barriers are fragile, and must be protected from abuse. They also lose effectiveness if they ever get dusty.

  2. Expert Member
    Dana Dorsett | | #2

    Since the dust can't accumulate much on the bottom side of the RB, the hit in performance isn't really very much even when the top-side get's dusty.

    Painting the underside of the roof decking and the sides of the rafters with an aluminized "radiant barrier paint" (and NOT any of the over-hyped nano-particle products that you may come across) before hanging the radiant barrier will improve both short & long term performance, even though it's emissivity is much higher than the pure aluminum coatings/foils used in radiant barriers.

    Aluminized woven-fabric type radiant barrier is pretty rugged stuff compared to foil-only products or aluminized paper, and goes up pretty quickly. One-side low-E products like Tyvek Thermawrap come in very wide rolls, but aren't quite as effective as the shiny-aluminum-both-sides products.

  3. ideaconstruction | | #3

    Hi again, it's Rob (I didn't know how to respond to the (excellent) answers given so I started a new post.) Here goes: I was reading your answer to the query about the 1929 San Diego bungalow insulation. My barn also has siding nailed directly to the studs. So I shouldn't then for example use open cell iso sprayed on the back of the siding because that will interfere with the drying of the wood after heavy rains (which we're about to get soon)? Thank you.

  4. GBA Editor
    Martin Holladay | | #4

    You are correct. It isn't a good idea to install spray foam insulation against the back side of your siding.

    If you want to insulate this type of wall with spray foam, it's best to have an air space between the foam and the siding. One way to do this is to first install some battens or spacers (for example, 1"x1" sticks) to create an air space, followed by a piece of 1-inch-thick rigid foam. Then you can install spray foam against the rigid foam.

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