The polystyrene and the foam are one in the same. The polystyrene core is water-resistant. The ASTM test proved maximum water absorption of (2.5%) for (1lb density). E.P.S. is an inert, organic material. Polystyrene provides no nutritive value to plants, animals, or microorganisms. The polystyrene will not rot and is highly resistant to mildew. Aging has no effect upon the performance of the polystyrene E.P.S. is able to withstand the abuse of temperature cycling 180° assuring long term performance. Please refer to fire rating for flame spread information.