When it comes to choosing foam for outdoor cushions, there are specific types of foam you can use. Each of these foams has different features and properties. Here are some of the types of foams you can buy from vendors like The Foam Factory and their different properties:
While not exactly foam, compressed polyester is a compressed form of batting that is a common replacement for foam products. The compressed product is non-allergenic and resistant to mildew, which makes it a good choice for outdoor use.
Again, like the above, polyester fiberfill is not foam, but blown polyester fibers. One of the more affordable options that see use pillows and is also mildew resistant and non-allergenic. Be careful when washing as the fibers can bunch up/
Open Cell Foam
Open cell foam has pores that allow water and other liquids to drain through, making it ideal for outdoor applications. In fact, one of the best outdoor foam replacement options is Dry Fast, which will drain almost all the liquid immediately. Dry Fast is expensive but is probably the best outdoor foam you can buy.
Closed Cell Foam
Unlike any of the above products, closed cell foam does not allow liquid through, making it float. While closed cell foam is good for marine use, it is not ideal for outdoor use because it is not as comfortable as the other foam types.