Synopsis: As the building industry tightens up on air-sealing and energy efficiency, builders need to be more aware of the risks associated with moisture and especially with bulk-water entry. Windows and their installation is an area ripe for major improvement for saving energy and increasing durability. The best installations keep water out, give it an escape path for when it inevitably leaks in, and block unwanted air movement. In this article, builder Brian Knight demonstrates his methods for ensuring window installations are as tight as possible.

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As the building industry tightens up on air-sealing and improves energy efficiency, we need to be more aware of the risks associated with air and water intrusion. Materials that get wet won’t dry out as quickly or easily as they used to do in leaky, poorly insulated houses, and many of the materials being used to build houses aren’t as resistant to moisture as their predecessors. There’s no doubt that with windows accounting for such a large part of the building budget, and being one of the most direct connections to the elements, their installation deserves extra attention.

Windows are complicated, and so is their installation

Compared to solid areas of wall and roof, windows and their interface with the air barriers and water-resistive barriers (WRBs) of a house are complicated and vulnerable. Many builders and designers are confused about best practices, and inferior techniques are rampant. As windows, tapes, sealants, and WRBs evolve, so do the installation guidelines for these products. It’s not uncommon for products that are typically used together to have conflicting installation instructions. When in doubt, I follow the window manufacturer’s instructions as a bare minimum, and add improvements from there.

The best window installations include redundant layers of protection for keeping water out, incorporate measures for blocking the air movement that can draw that water in through weak spots, and provide an escape path for water that enters before it has a chance to cause damage.

Not all windows are created equal, of course, and it’s tough to predict long-term performance. But I don’t discriminate or try to predict. Every window I install — regardless of brand, material, or price — gets the same belt-and-suspenders treatment to ensure that air and water leaks aren’t going to be an issue.

Aim for waterproof, but plan for leaks

We should always assume that water will find its way into a window opening. Even if the installer does a perfect job with sealing and flashing, water can still leak through the window unit itself — especially as the materials, transitions, and sealants go through many cycles of expansion and contraction over time. Gravity, capillary action, blowing rain, and pressure differences can push and pull water into these weak points.

Water leaks from windows usually show up at the sill. Because this is the most vulnerable area of a window, it’s also where the weather-proofing efforts begin.

As long as there is an air gap in place, a sloped and flashed rough sill is the best way to protect this vulnerable area. Creating a back dam along the rear edge of the sill flashing is one way to ensure that leaks under the window can’t reach the interior of the opening, but I typically don’t use back dams unless specifically required by the window manufacturer. I’ve found that they can complicate rough-opening dimensions, interfere with shimming the window off of the sill pan, and make it difficult to air-seal the bottom of the window.

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