Best Foam Insulation?

I am curious if all foam insulation has about the same R-value or if some types are much more effective than others? Is anyone here knowledgeable on this topic? Thanks!
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Polyisocyanurate has about the highest R value. Expanded and extruded polystyrene is substantially lower, but still higher than fiberglass batt insulation per inch. http://www.progress-energy.com/custservice/carres/billtoolkit/rvalues.asp
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wrote:

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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