Polyisocyanurate has about the highest R value. Expanded and extruded
polystyrene is substantially lower, but still higher than fiberglass
batt insulation per inch.
In addition to the R value, look at the price. If you have the room to put
thicker insulation, the polystyrene is usually the most cost effective even
if you have to use more of it. If you are constrained by space, the other
foams give the best insulation.
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