Weather barrier mastics are also called breather mastics. Breather mastics have a perm rating over 10; DP 5050 and DP 5060 have a rating of 20. Breather mastics allow built up vapor pressure to pass through the mastic, thus allowing it to breath. Breathing helps prevent corrosion. Vapor barrier mastics like DP 3040 may be used outdoors to protect insulation, but corrosion could become a problem. The general rule of thumb is that weather barriers should be used outside the building envelope and vapor barriers should be used inside the building envelope.
DP 5050 and DP 5060 should not be used for chilled water piping inside the building envelope.
For outdoor duct work and for smaller applications, the preferred mastic is the standard DP 5050. The mastic can be coated on the insulation directly, but fab cloth or fiberglass fab cloth is often used. The DP 5050 is troweled/brushed on the fab cloth, pushed on the insulation surface, then smoothed.
For larger surfaces, such as tank insulation, the preferred mastic is a low viscosity DP 5060. A thin tack coat is applied to the tank and the fab cloth is embedded. A finish coat is brushed into and over the fab cloth. After drying for about 5 -10 minutes, a wet brush is used to smooth the surface. From a small distance, good insulators will create a smooth appearance – similar to a painted wall. Because the surface area on tanks is large, a thick weather barrier will wear out installer’s wrists.
Weather barrier mastics are primarily for use on outdoor applications. Contractors in some areas of the country will substitute weather barrier mastic for vapor barrier mastic – DP does not recommend this. It is never a good idea to replace a vapor barrier with a breather mastic. In low humidity areas, the theory is that there is not enough moisture in the air to create condensation to damage insulation. We do not recommend testing this theory.