Smoothing Moisture Troubles at Rough Openings

PUBLISHED BY: MTI

AUTHOR: Tyler LeClear Vachta

POSTED: April 12, 2018

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.


Window Drainage Plane

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.

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Moisture Diverter

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.

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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.


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Call 1-800-879-3348 or contact us.

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