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How thick to encapsulate ducts in a vented attic?

Rich Singleton | Posted in Energy Efficiency and Durability on

I was wondering if there are any resources that provide guidance as to how much closed cell spray foam to use when encapsulating ducts in a vented attic? I haven’t been able to locate any specific guidance on the matter. Most seem to recommend a minimum thickness of 1.5″, but this seems to be based more on providing enough foam to ensure an adequate vapor barrier/air seal. I don’t know if there is a cost/benefit rationale for increasing the thickness further, but certainly there must be a point of diminishing return. Does the thickness of the ccSPF on the return ducts need to be same as the supply ducts or would less be ok? The ductwork is already wrapped in R8 insulation.

The house is in Climate Zone 4a in the Mid-Atlantic region with a conditioned crawlspace which contains the first floor ductwork and mechanicals. The second floor mechanicals are within the conditioned space but the ductwork is hung in an vented attic (preventing buried ducts) with radiant barrier roof sheathing.

Thanks for any help you can provide.

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  1. GBA Editor
    Martin Holladay | | #1

    The answer to your questions can be found in the last two paragraphs of a recent GBA article, Dealing With Ductwork in an Unconditioned Attic. Here are the two relevant paragraphs:

    "The next best option is to encapsulate the ducts. See recommendations for accomplishing this at the Building America Solution Center. Note all the tabs for various items of key information.

    "For more information, see two other suggested solutions: Ducts Buried in Attic Insulation and Ducts Buried in Attic Insulation & Encapsulated. Note that the former approach is not recommended in moist and marine climates while the latter approach is OK for all climates."

    The recommendation for 1.5 inch of closed-cell spray foam is in addition to the R-8 insulation that wraps the ducts before the installation of the spray foam begins. If the R-8 labels on the insulated flex duct are accurate, then these encapsulated ducts end up with about R-17 of insulation after the 1.5 inch of spray foam has been installed.

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