The foam cores are virgin, polystyrene foam tapered from 4" to 2.5" to allow water to run off. Each foam piece is then grooved and fitted with a galvanized steel channel for support along the hinge. Each foam core is then sealed with a 4 mm polyethylene sheeting to help prevent water absorption. Your spa cover will withstand the effects of sun and weather, saving energy with a lower total cost of ownership.