Vinny, the only thing I know of that is going to make foam spongy is water. Take some of the foam and squeeze it to see if you get moisture out of it. IF that is the case, it is not the cover, but you have a leak, keep digging until you find warm water which is closer to the source of the leak.
Ants love the moisture, any place you see them spray where they are, where they travel, and close and seal up any and all little access holes where little critters can get in, steele wool works great. The size of the little bits of foam are the size of the mouth that dug it out. Mice and chipmunks are the most common critters, you gotta lock them out.