I understand you concern. Heat entrapment never occurred to me though, probably because over the course of 10 years and 2 different rv's I have not experienced any ill affects with this application and we live in South Florida where the sun can be rather brutal. I suppose if you notched the edges of the styrofoam or put a small hole or two that might allow for limited air flow. It seems to work for us, especially with the shower sky light. This method also eliminates UV discoloration (yellowing) of surrounding materials.

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