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How to insulate unvented attic with foil faced rigid foam in 6 inch bays?

Andrew Shu | Posted in Green Building Techniques on

Hello,
Im currently remodeling my house in southern california. Im interested in building an unvented attic since all my HVAC equipment and ductwork are in the attic. In addition, we’ve had rats and mice nesting in the fiberglass insulation before we tore it all out. This is a single story, 3200 sqft ranch house that will have quite a few vaulted ceilings. With that in mind, what is the best way to use foil faced rigid insulation to create an unvented attic?

Option 1: 5 inches of JM AP foil-faced polyiso screwed directly to the undersides of the rafters and foamed and taped to air seal

Option 2: 4 inches of Foamular in the bays glued to the plywood and 2 inches of JM AP screwed to the rafters

Option 3: 6 inches of Foamular in the bays, Radiant barrier attached to the rafters.

Thanks!
Andrew

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Andrew,
    The best way to insulate your roof with rigid foam is to install the rigid foam on top of the roof sheathing. But I'm guessing you don't want to do that, perhaps because the roofing is in good shape and you don't want to replace it.

    Working from indoors, the best way to insulate your roof is with spray polyurethane foam.

    A distant third in the "which is best?" contest is either Option 2 or Option 3. This approach is never used by professional insulation contractors but is often suggested by homeowners. The nickname for this approach is "cut-and-cobble." It's fussy, time-consuming work. Good luck.

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