Foam plays a large part in the life of your outdoor cushions since any outdoor fabric is only relatively water proof in a rain storm, and will eventually let water into the cushions one day. Water sitting in the foam of your cushion can mold your fabric and create harmful mildew.  If you are willing to cover your cushions or bring them in to protect them from rain and moisture then do so. If however you are not, then here are a few types of outdoor cushion foams that may work for you.