Foster Vapor-Safe Mastic 30-90 is a white, high solids water base, fire resistive, flexible vapour barrier finish for most types of thermal insulation, including polystyrene foam. It may be used over dry concrete, finishing cement, and most metals. Vapor-Safe Mastic has the water resistance and low water vapour permeance normally found only in solvent based products. It can be used in high humidity environments, and greatly retards water vapour permeation.