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Proper Window Installation

Updated: Jul 22, 2020

Accepting That Windows Will Leak

The second half was our dawning realization that windows are, by nature, failure-prone. It’s not a matter of if windows will leak, but when. Windows are complex, full of junctions, corners and seams — rife with opportunity to leak moisture.

Easily 20 percent of windows are already leaky when delivered to job sites, based on sill dam tests. A certain percentage of windows will leak coming off the assembly line. More will leak after being packaged and shipped, yet more after unloading at the warehouse. Still more will leak once the windows arrive at the job site and are moved around, and even more after they’ve been handled and installed. Then come the forces the windows will be subjected to in situ: structural shifts as the building settles, years of wind load and weathering, aging materials and sealants.

After sanding the rough edges, Leak Doctor applies a joint-filling to seams and penetrations to reinforce against potential movement at joints.

At some point in a window’s life, it will leak. To ignore this reality is to play Russian roulette with your building envelope, yet that has been the traditional approach. When a wall rots under a window, the traditional response is “that window is defective,” not “that opening wasn’t properly flashed.”

We realized we had to detail rough openings knowing that windows will leak sooner or later. Rough openings need to be completely water-resistant to keep water from penetrating the wall assembly, but also vapor-permeable to let construction moisture, seasonal moisture fluctuation and exterior moisture evaporate from the wall. That is true for conventional installations with windows at the exterior edge of rough openings as well as high-performance installations with windows centered in the wall.

In our climate, the simplest way we found to achieve water-resistance was with vapor-permeable liquid-applied flashing. Any rain that hits the opening or leaks through the window drips down to the liquid-applied membrane and out to the exterior moisture-management layer.

Leak Doctor installs an expanding foam tape into a flashed doorway rough opening. Under compression, the tape expands to create an airtight, insulated seal that sheds water.

Keeping Water Out in Five Steps

The beauty of this approach is that it is easy to install: simplicity over complexity.

Gone are the layers of sheet goods and traditional tape membranes building layer upon layer. No “tents” of tapes in the corners. No fish mouthing to funnel moisture where it shouldn’t go. No inadvertent reverse-lapping.

A flashed rough opening awaits water-resistive barrier and window installation.

As an installation method, liquid-applied flashing is as close to foolproof as anything we know. It is much harder for a carpenter having a bad day to make mistakes with a liquid-applied membrane than with tapes, peel and stick, and papers. And errors, like inadequate coverage, are easy to identify because they aren’t hidden by layers of tapes and paper. A little spot-touching fixes the problem.

Installation involves five steps. (Note: for a detailed, illustrated description of our approach to window flashing, see the “New Window Installation” of our Best Practices Manual.)

1. Prepare the Rough Opening:

Start by ensuring that the rough opening is ready for the install. It should be dead level and dimensioned precisely with a consistent installation gap — we shoot for three-eighths inch (0.95 cm) around the perimeter of the window unit. Double-check the measurements of the rough opening and modify as necessary. If the opening is too large, for example, you can add filler pieces coated with liquid-applied flashing.

We recommend budgeting time into projects for corrective action to make sure openings are ready to accept a window. The opening should be rough-sanded and dust-free. A few minutes extra prep time makes flashing application smoother.

2. Reinforce Structural Seams:

The traditional way to do that is with fabric mesh tapes, but they go on slowly and are prone to ripples and fish mouths. We’ve seen testing that shows that a thread poking through the flashing material can serve as a wick to bring water through the membrane.

We prefer a vapor-permeable fluid-applied joint and fastener treatment. We apply it over fastener penetrations and on all sheathing joints and seams, ends and corners in the rough opening. Make sure the treatment you use can withstand building movement, as well as filling joints and seams.

3. Install Window Sill Pan:

Window and doors are typically installed using one of two approaches: 1) barrier system installation or 2) a drained system installation. Barrier installations do not provide for forgiveness to water infiltration through or around the window or door opening and should be limited to areas of low rainfall potential or areas where the area is protected (such as under overhangs or porches). Drained installations are designed to manage small amounts of water infiltration through or around the window or door opening, by collect and draining the water back out to the exterior.

Central to the performance of a drained opening is a pan flashing. A pan flashing is an element installed below every window and door designed to collect and direct any water that may leak through or around the opening back out to the exterior. As such the sealing of the window or door assemblies into the rough opening (through the use of membrane flashing and/or sealants) should be made at the jamb and head of the window, however the sill should be left free to drain to the exterior.

At Leak Doctor we use a PVC Sloped Sill pan, Manufactured to provide level surface for continuous support of doors and windows and a slope for draining water to the exterior. Sloped Sill Pan designed to protect your home from moisture intrusion, avoiding the potential for related rot, decay and mold. meets and exceeds NAFS—North American Fenestration Standard/Specification for windows, doors and skylights, we call it NAFS-08 even if the current Alberta Building Code doesn't not require this standard yet.

4. Apply Liquid Flashing:

Now we’re ready to apply the liquid flashing to the rough opening. Create a consistent layer of the product throughout the rough opening. We recommend extending this layer of flashing out onto the face of the building at least 9 inches (23 cm) in all four directions, tying into the building’s water-resistive barrier (WRB) system.

We do this for two reasons.

First, the areas around window rough openings receive lots of fasteners and penetrations, and traditional WRB products don’t have the ability to self-gasket around those fasteners. Liquid-applied flashing, which does self-gasket, protects the integrity of the exterior sheathing immediately around the rough openings. That’s a primary line of defense from the elements.

Flexible nailing flanges, or fins, on some windows can be hard to embed in a liquid flashing system. Here, a water-resistive tape provides an additional layer of protection over the nail heads.

Note that self-gasketing, or a material’s ability to close tightly around a fastener, shouldn’t be confused with self-healing. We don’t know of any products that are truly self-healing. If a nail pulls out, no membrane I’ve seen can heal itself and cover that hole.

Here’s the second reason for the 9-inch extension of flashing around the rough opening: If water does get driven between the exterior sheet goods layer and the liquid-applied flashing, gravity can take over and pull that water down the assembly, where it can be managed by a transition membrane. Nine inches is the magic number because, based on test chamber results, it gives enough horizontal room for wind-driven moisture to be drawn down by gravity. An exception to the 9-inch rule is if the flashing ties into a liquid-applied or otherwise fully structurally adhered WRB. In that case, 4 inches (10 cm) should be enough.

The transition membrane can be either a liquid-applied WRB product, or an embeddable transition sheet that shingles onto a more traditional WRB sheet good. In either case, water will lap out over the WRB system and drain off the building.

5. Install the Window:

We use a PVC sill pan with back dam, no beveling wood sill, and place the window dead level on setting blocks, with a consistent 1/2" gap between window and opening. We then install a closed-cell foam backer rod around the perimeter of that gap and apply a high-quality sealant compatible with the flashing.

This approach creates an airtight, waterproof interior sealant joint, connecting the interior edge of the window with the liquid flashing in the rough opening. Any water that leaks through the window will hit outside that seal and drain to the exterior. Not only does bringing the seal so far to the interior create a last line of defense on the inner plane of the window, but it also is less exposed to temperature extremes and UV light.

The interior sealant joint (white) is easily inspected from the interior. Special care must be taken to seal around window clips. Blower door testing can help reveal pinhole leaks at this critical seal.

That seal will outlive the window.

On high-performance projects where thermal performance is particularly important, we take extra pains to insulate the cavity between window and rough opening. One approach is to install a one-component low-expansion foam around the exterior joint. In high-exposure applications, we seal that seam with a vapor-permeable exterior flashing tape to protect the foam from the elements. Another approach is to set the window into the rough opening using an acrylic-impregnated expanding foam tape, which, under proper compression, creates an airtight, insulated seal that sheds water.

On conventional projects, windows with nailing flanges need extra attention to integrate into the liquid-applied system. While leaving exterior nailing flanges unsealed technically won’t compromise the interior seal, we need to take care to avoid saturating wooden frames. Tying exterior flanges into the liquid-applied product therefore becomes key. For many window systems, it’s easy to embed the top and side window flanges into the liquid flashing. However, for windows with flexible nailing flanges, it’s trickier.

In those cases we use a high-performance all-weather exterior flashing tape over the flanges. The tape need not be vapor-permeable, but it must stick well in damp, cold weather and effectively bridge flange to flashing.

For an extra layer of insurance, the upper tape edge at the head of the window can get a small bead of liquid-applied flashing to prevent fish mouthing and funneling water behind the connection.

Keeping It Simple, Reliable.

We’ve found these window installation methods simple in design and easy to install. They create airtight assemblies that repel liquid water but let vapor moisture escape, ensuring durability at a traditional weak spot in building envelopes. By eliminating layers of tapes, peel and stick, and papers, it’s easier for installers to do a good job, and for construction company owners like me to sleep well at night.

Our firm has used these techniques countless times, from window replacements in conventional kitchen remodels to über-high-performance window installations in demanding Passive House projects.

Simplicity triumphs over complexity, and reliability wins.

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