So let's say you have rafters that are 16" on center. The bays are 14.5" wide. You add a couple of inches to the foam board you're working with per this video's instructions. That's 16.5". You can only get two lengths that wide out of a 48" wide board and are left with another piece that's about 15" wide. I guess you could adapt and use inch-high sticks for this last bay, but it does seem a bit annoying.
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