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New roofing over unconditioned attic — any recommendations?

James Howison | Posted in General Questions on

We have an unconditioned attic and will need a new roof soon. I looked into creating a conditioned attic but for various reasons (old rafter structures, hard to find experienced contractors, cost) that is not going to happen. We have gone with ceiling mounted mini-splits (MLZ-KP09NA), so we did get (most) of the ducts out of the attic, just two for an UltraAire 98H (which will be buried under cellulose).

We’re in Austin TX so we get a lot of strong sun. Is there anything that can be done in a smart way during re-roofing? We will probably go with a metal roof, and I think I should be looking for high solar reflectance index (SRI) values. I’ve read about Kynar any other suggestions? Worth worrying about? We will likely go with a vapor open, 3D, sub-roof membrane, such as Solitex UM.

Beyond a roofing with high SRI anything else that can/should be done? We don’t have a ridge vent, should we be thinking a vapor vent, as discussed here, although that seems only relevant for conditioned attics? https://buildingscience.com/documents/insights/bsi-088-venting-vapor Perhaps having a vapor open membrane and 3D space above the membrane makes that irrelevant anyway.

Would a small layer of insulation under the roofing, even with an unconditioned attic, help with high radiant loads from sun, or is that screwy thinking?

Thanks y’all!

–J

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    James,
    In general, the temperature of a properly designed vented unconditioned attic is irrelevant. Who cares if it is hot up there? You don't live in your attic, right? As long as the attic floor is sealed against air leakage, and you have installed at least as much insulation as is required by the building code, everything is fine. You don't care if it is 140 degrees up there.

    Ah, but what if there are ducts up there? (Big mistake.) That changes things.

    If you are able to bury your ducts in deep attic insulation, you're back where we started. Who cares if it's hot up there? Lowering the attic temperature won't appreciably affect your energy bills, as long as you've installed enough insulation.

    For more on burying ducts, see this article: "Burying Ducts in Attic Insulation."

  2. Expert Member
    Peter Engle | | #2

    Full ridge and soffit venting is a good idea for any unconditioned attic. In your climate, people do claim some benefit from a radiant barrier under the roof, with the shiny side facing the attic. This can take the form of a reflective material hung on the underside of the rafters or a layer of roof sheathing with foil laminated to it. LP Tech Shield is one of the sheathing products with a reasonably long track record. The big benefit of the sheathing is that there are no extra construction steps and not much can go wrong.

  3. GBA Editor
    Martin Holladay | | #3

    James,
    Peter is correct that radiant barrier roof sheathing makes sense in your climate, but only (1) for new construction, AND (2) if there are ducts in the attic that aren't buried in deep insulation.

    In your situation, you will never see enough energy savings from retrofitting a radiant barrier to justify the hassle and expense of installing it.

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