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How to deal with 1/2″ gap when installing 6″ of foamboard into 5.5″ rafter bay

michaelbluejay | Posted in Green Building Techniques on

SHORT QUESTION:  What, besides 1/2″ foamboard, can I use to fill in the 1/2″ gap between the last-installed layer of polyiso and the edge of the rafters?

DETAILS:  I’m installing 6 layers of 1″ polyiso on the underside of a roof, in the rafter bays and then on top of the rafter bays.  After installing the first 5 layers, there’s a half-inch gap between the last layer of foam board and the edge of the rafters.  Normally I’d install a 1/2-inch of foamboard to be flush against the end of the rafters, and then install the final 1″ across the rafters, but my local home improvement store is out of 1/2″ so I’m looking for an alternative, since I’m eager to move the project along, and if I can avoid having to secure a truck just to pick up some 1/2″ foamboard then that’s a plus.

One idea is to use non-moving air as an insulator.  I could spray-foam the edges of the rafters and other perimeter wood, then stick the final 1″ piece on the rafters.

Another idea is to use bubblewrap or soft white foam roll, which I happen to have a lot of.

Or should I really just get a truck, go to a farther store, and buy the 1/2″ foamboard?

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  1. Expert Member
    Akos | | #1

    That sounds like an unvented cut and cobble roof. Since the foam is not fully adhered to the sheathing, warm air from the house can make it there and condense, since it is unvented there is no way for that to dry out. These roofs are prone to failing.

    Cut and coble can work, but you need to keep a vent space (1.5" to 2.25" depending on local code) above the rigid. This space needs intakes at the soffit and exhaust vents at the ridge.

    1. michaelbluejay | | #2

      Thank you, I just found this article about cut-and-cobble:

      I'll start a new thread asking if canned spray foam is an acceptable alternative.

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