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Community and Q&A

Hybrid Insulation for an Unvented Roof

KevinKeegan | Posted in GBA Pro Help on

Climate zone 4. Scope of work is old house renovation involving the insulation of a roof area.
Please tell me if you see any problem with this method.  I would like to install 3 inches of closed cell spray foam to the underside of the  roof sheathing between the 2 x 6 rafters, and on the ceiling joists below the roof area install unfaced fiber glass insulation. The space between the underside of the roof at the peak side of the roof is high so I can install R38 and at the eve side I will have R19 depth as the ceiling joists are 2x6s There will be an air space between the topside of the ceiling joists and the underside of the roof. I understand that dew point and air movement are the most important issues.  I also understand that a vapor barrier should not be placed on the interior room facing side or heated side. I’m not concerned about R value for code compliance however I do not want to cause a moisture trapping condition with the proposed insulation install. I assume the closed cell spray foam will stop the air movement and the fiberglass insulation will add to increasing the overall R value. Do you see any harm to this approach? The reason I do not want to use all closed cell spray foam is due to budget costs. Also the soffits are closed and no ridge vent, so there will be no venting of the roof.
Thank you!

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Kevin,

    If I understand your plan correctly, you are talking about the “flash-and-batt” method. This article provides a deep dive into the process, as well as Zone 4-specific code requirements (just 1 inch CCSF for 2×6 walls). It also addresses your concerns around moisture.

  2. Jon R | | #2

    You should follow IRC code, even if not required. Specifically, do not create semi-conditioned space by splitting the location of insulation.

    R806.5 Unvented attic and unvented enclosed rafter assemblies.
    Unvented attics and unvented enclosed roof framing assemblies created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters, shall be permitted where all the following conditions are met:
    1. The unvented attic space is completely within the building thermal envelope.

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