Waterproof membrane that I can apply to styrofoam?

I installed a "Toilet Tank Anti-Sweat" kit, which is just a bunch of styrofoam that insulates the cold water from the warm, humid air. Now I'm concerned that this styrofoam will degrade over time and would like to seal it over with some kind of membrane. I read online in various places that RedGard is a good membrane, but it's used for more industrial applications and I'm concerned that it might melt the styrofoam or that it might not adhere. Does somebody have a good suggestion? I live near a Lowes, an Ace Hardware and a Home Depot.