Window Flashing Tips
Windows are common problem areas for moisture. History & building science show we can't seal perfectly, but we can create a path for drainage, and divert moisture away from sensitive areas. MTI provides a belt-and-suspenders approach for rainscreen assemblies to reduce the risk of water intrusion at windows.
The Window Drainage Plane is installed on the flashed sloped-to-drain sill (a beveled piece of wood siding can be used to create the sloped sill). The channels in the Window Drainage Plane create a small air gap below the sill for drainage onto the WRB.
The Moisture Diverter is flashed with the water resistive barrier (WRB) and installed at a slope-to-drain above the rough opening. This diverts moisture on the WRB and in the rainscreen drainage plane away from the rough window opening, reducing the risk of moisture accumulation.
What about the air seal? Set the top and sides of the window flange in a sealant to help seal out moisture. The air seal is best installed at the interior of the window. with backer rod and a high-quality caulk. This limits the seal's exposure to wind, UV, and moisture while also creating a pressure moderated chamber beneath the window that assists with drainage.
These best practice details take a little more time and result in an installation that will last the life of the wall.