if your going to spend the money on mylar mount it on some pvc pipe so it not on the walls , it will cause them to sweat and cause the walls to mold. NOw flat white paint needs some anti mold added to the paint or your walls will mold. When that happens you will need to replace the walls, not cheap.
Remember cheap paint is cheap and expensive paint is cheaper bottom line of cost. Anti mold with the paint will save lots of problems:dance: