The way these foam boards get their high R value is because they have things other than air trapped in them. When you break them up you release the gasses. There is an article on GBA about taking polyiso scraps, grinding them up and using them as attic insulation. Apparently the R value is almost nothing and it's messy and time consuming. The only way this can work is to actually foam the pieces together without breaking them up. If you're dead set on using EPS you can buy huge sheets of it cut to any dimensions you want at a local plant. Ask around. I'm sure there's one near you.
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