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Extract current insulation before spray foam in the attic?

milwaukee8 | Posted in Energy Efficiency and Durability on

I am going to spray foam my attic, underside of roof, and all penetrations with open cell foam to air seal that portion of my conventially built and insulated house. My contractor is telling me I must extract the current fiberglass loose insulation now on the attic floor. While I am willing I can’t fully understand the science of needing to extract the current insulation. I am being told there would be two thermal barriers and with AC ducts in the attic the would be a moisture problem.

Anyone have advice or help with the science of this decision.


  1. GBA Editor
    Martin Holladay | | #1

    You can leave the insulation on the attic floor if you want.

    However, open-cell spray foam installed on the underside of roof sheathing has been (in some cases) associated with damp roof sheathing. Here is a link to an article with more information on the issue: Open-Cell Spray Foam and Damp Roof Sheathing.

    The bullet points below come from the article I just linked to.

    Here are my recommendations for builders who use spray foam to create a sealed, conditioned attic:

    • Recognize that this type of roof assembly carries more risk than a vented, unconditioned attic. Keeping ducts within the conditioned space of a building (not in the attic) is preferable to installing spray foam on the underside of roof sheathing.
    • If you want to lower the risk of damp OSB, choose closed-cell spray foam, not open-cell spray foam, to insulate the underside of the roof sheathing.
    • To further limit your risk, consider installing ventilation channels above or directly below your roof sheathing.
    • If you choose to install open-cell spray foam against the underside of roof sheathing in Climate Zone 5 or colder zones, make sure that you include an interior vapor retarder.
    • If you choose to install open-cell spray foam against the underside of roof sheathing in a humid climate, Joe Lstiburek recommends that the HVAC system be designed to condition the attic air and lower humidity levels in the attic.

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