So I am a bit worried since I have read that foam gives off fumes when heated. I just built my first rock wall and basking area from left over styrofoam found in packaging. I did about 3 layers of grout and 2 of mod podge. However the whole back and bottom are just bare foam in order to save on material and weight. Will this bare foam cause fumes to come out if left lime that? Or do I really need to cover it?