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Q&A Spotlight

Insulating and Air Sealing a Vaulted Ceiling

An owner-builder looks for advice on creating a trouble-free roof assembly

A new lakeside cabin will be built with scissor trusses in the roof, creating a vaulted ceiling over the kitchen and living area. The GBA reader planning the project provided this photograph to illustrate the cabin's basic construction while asking for advice on the roof stack-up.

Kevin is working up the design for a new lakeside cabin in Tennessee, with plans currently calling for a scissor-truss roof and a vaulted ceiling over the kitchen and living area. He wants a trouble-free roof assembly, but local advice on insulation and air sealing has been hard to find.

The roof starts with scissor trusses set on 8-foot centers. Between them, running horizontally, Kevin would run 2x6s on 24-inch centers (the 2x6s set on edge), and insulate the bays with fiberglass batt insulation. The bottom of the 2x6s would be covered with tongue-and-groove pine boards. On top, he’d start with OSB sheathing, then add 2 inches of rigid foam insulation, a layer of 1/2-inch plywood, then foil bubble wrap and, finally, metal roofing.

He’s ruled out the use of structural insulated panels (SIPs).

“There doesn’t seem to be anyone around here, in southern Kentucky, that cares much about good insulation and sealed houses,” Kevin writes in a Q&A post. “I don’t have any education other than applying different parts of roofing projects I have seen online and what I have seen in person. Please point out problems with this idea or a better way.”

That’s the topic for this Q&A Spotlight.

This plan has some problems

GBA editor Martin Holladay responded to Kevin’s question before Kevin had fully explained his plan to install rigid foam above the roof sheathing. Based on Kevin’s description of fiberglass batts installed between the 2x6s, Holladay noted a glaring weakness in plans: the lack of an uninterrupted air barrier in the ceiling.

“You absolutely need an airtight ceiling to prevent interior moisture from entering your roof assembly and causing moisture problems,” Holladay writes. “That means (a) You can’t…

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  1. Expert Member

    Above Sheathing Venting
    The article linked to support above sheathing venting shows its effects when used under stone-coated metal shakes. There is no mention of its benefits in general, or under any other materials.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Malcolm Taylor
    This GBA page contains lots of links, and I wasn't sure at first what article you were talking about. After some investigation, I concluded that you were talking about the link provided by GBA reader Jon R in the last paragraph of the section under the heading, "Combining fiberglass and rigid foam."

    For any confused GBA readers, here is the link: "The Impact of Above-Sheathing Ventilation on the Thermal and Moisture Performance of Steep-Slope Residential Roofs and Attics."

  3. Expert Member

    I should have been a lot more specific. Thanks.

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