Flash point: a 10-step program for window installation

Tools of the Trade, March-April, 2005 by Bill Robinson

Before I became a field-training consultant for builders, I was a builder and remodeler. Even then, long before mold was a hot topic, it bothered me how many different methods were used to install windows. Stucco guys did it one way, East Coasters another, and production framers in California had their own techniques. So I set out to find best practices for my own sites to prevent problems and to streamline production.

The Building Environment & Thermal Envelope Council (BETEC) was at work on this then, too, trying to corral all the various methods of window installation around the country. Their research resulted in the American Society for Testing and Materials (ASTM) E-2112 window installation standards and American Architectural Manufacturers Association (AAMA) Installation Masters program (see "Resources," page 26). These are both excellent resources for detailing a window installation, and they're the standards and practices I teach builders nationwide to stay ahead of callbacks, leaks, rot, and mold. While these organizations offer several recommended methods, I describe only one of them here, mostly because if I was still building, it's what I'd use, no matter where I built.

EAST COAST WRAPPERS

Four main regional practices have been identified, which can be further boiled down to two: East Coast and West Coast. Typically, East Coast builders install the weather-resistive barrier (housewrap, asphalt impregnated paper, etc.) first and the windows second; the West Coasters set the windows first and wrap after.

While local conditions affect which method might be the best choice for your sites, I like the East Coast method best. It's easier to integrate the window into a weather-resistive barrier already in place. Before I start, though, I observe one rule automatically: I read and follow the window manufacturer's instructions. They trump any other installation methods because deviations from the instructions can void warranties and protections companies might provide for their products.

In many cases, however, manufacturers' instructions fall short. For example, I've seen many windows arrive with no instructions for fastening and flashing or instructions on ensuring the window is plumb, level, and square. In other cases, instructions don't even exist. If there are missing pieces in manufacturers' instructions, my fallback is ASTM E-2112 or AAMA Installation Masters guidelines.

Getting Ready. Here are my four simple guidelines before installation:

* If possible, schedule window deliveries as close to the installation date as possible. Ideally, they should arrive 24 hours before installation to prevent damage.

* Have a spec sheet on site outlining the installation process.

* Check products for damage before installation.

* Have the proper flashing materials available and be sure the installers are familiar with the process.

METHOD A1 INSTALLATION

Materials. ASTM E-2112 and AAMA Installation Masters call the "East Coast method" Method A1. They specify that the weather-resistive barrier--house-wrap or building paper--be any code-approved product designed to serve as a moisture-resistant membrane on exterior walls. Flashing--any sheet material, flexible or rigid, used to bridge the gap between doors, windows, and the adjacent moisture barrier--should be 9 or more inches wide and be resistant to water for a minimum of 24 hours. Verify with manufacturers that all caulks, sealants, and tapes you select are compatible with all the other products you're using.

Here's my 10-step program for installing windows:

1. With the weather-resistive barrier in place covering the window opening, use a marking gauge made from a piece of flashing (see "Window Head Marking Gauge," right) to cut across the window head with your utility knife. Then make a 45-degree-angle cut up and out from each of the top corners of the window. Lift the flap; temporarily secure it with tape. Next, cut an upside-down Y: Begin at the top center of the opening and cut three-quarters of the way down. Then cut a 45-degree "leg" to each bottom corner. Fold the flaps into the opening and staple to the jamb; cut off excess.

2. Install sill flashing on the sheathing below the sill. Extend it a flashing's width on either side of the jamb. Use a J-roller with peel-and-stick flashings to get a secure bond. Note: AAMA and ASTM E-2112 don't address corner or sill details that may be prudent in especially harsh or wet environments, but many manufacturers and building scientists have. If you're building or remodeling a house slope-side in Vail or on the windward side of Nantucket, you may want to take a belt-and-suspenders approach. For extra defense, nail a beveled strip to the rough sill (pitched toward the outside), before wrapping the sill and jambs with "flexible-wrap" type products.

3. Check the sill for level; shim as required. Place a continuous (3/8-inch nominal) bead of sealant on the back of the window's mounting flange covering the pro-punched holes. Be sure the sealant is compatible with the wrap and window.