But trying to protect openings from moisture by adding complexity opened the door to design flaws and installation defects. Even today, most installers of high-performance windows are expected to be origami artists as they manage multiple layers of tape, peel and stick, and sheet goods.
Window Rough Opening Moisture Management
For thermal reasons, windows are usually placed at the center of the rough opening in Passive House projects, rather than at the exterior face of the building. That transforms the way the window attaches to the building and the way the window assembly manages water. You are no longer securing the window to the outside of the building with its nailing flange. Nor do you rely on the nailing flange to act as the water management layer.
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 Pacific Northwest, 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.
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.
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.
4. Install the Window: We use a level sill, no beveling, and place the window dead level on setting blocks, with a consistent three-eighths-inch 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.
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.
Sealing the gaps between window and door units and the framing rough openings requires care and precision. Unlike other parts of the air barrier on exterior walls, which have layers of redundancy, the seal around a window and door unit stands on its own: usually only a single closure separates the indoor air from the outdoors (BSC 2009).
Too often, an attempt to seal around a window or door unit is made by stuffing the gap with fiberglass insulation. However, fiberglass is not an air barrier; air can readily seep through the insulation fibers. Instead, the gap should be filled with one or more of the following materials (DOE 2000):
Air sealing window and door rough openings is typically done by the insulation contractor, but in some cases it may be done by the window and door installer or by the finish carpenter prior to installing window and door trim.
Visually inspect the seals between the window rough openings and the window and door units prior to installing interior finish materials. The seal from caulk or nonexpanding foam should be uniform without any visible gaps.
Poorly air-sealed homes are less comfortable and cost more to maintain because they provide a pathway for drafts, cold spots, moisture, and insects into the home. Comprehensive draft protection includes a continuous air barrier around the whole house along with caulking and sealing in all holes and cracks. This includes around wiring, plumbing, ducts, and flues; where wall framing meets flooring; around windows; where drywall meets top plates and sill plates; where rim joists meet foundation walls and subfloors; etc. Spray foam insulation can be used at rim joists, floors above unconditioned space, and in attics to insulate and air seal at the same time.
Structural Insulated Panels (SIP) can create a strong, well-insulated, airtight, building envelope. They can, however, be unforgiving when it comes to moisture management. Most SIP dry slowly if or when they get wet, so back-vented claddings, robust flashing details, meticulous sealing of penetrations, and effective spot/whole house ventilation are all essential.
Any window is, by nature, failure-prone. They are complex, full of junctions, corners, and seams: rife with opportunity to leak moisture. Although high quality windows may take decades to fail, it is not a matter of if windows leak, but when. And given that super-insulated, airtight wall assemblies do not dry as readily as the leaky walls of conventional construction, Hammer & Hand realized early on that we had to detail our rough openings to be (1) completely water-resistant to keep water from penetrating the wall assembly, (2) vapor-permeable to let construction moisture, seasonal moisture fluctuation and exterior moisture evaporate from the wall, and (3) airtight. Airtightness is crucial in high performance construction partly because airtight detailing also helps ensure proper detailing for energy performance and prevention of water intrusion. If you are controlling for air, you are controlling for energy and moisture.
In the Pacific Northwest, the simplest way Hammer & Hand has found to achieve this is through vapor-permeable liquid-applied flashing. Any rain that hits the window opening or leaks through the window drips down to a liquid-applied membrane installed around it into the rough opening (with the window air-sealed to the inside) and out to the exterior moisture-management layer. On the Glasswood project, and most other Hammer & Hand projects, we used a liquid applied flashing system manufactured by Prosoco called FastFlash.
Window installation at Glasswood involved four steps:i. Prepare the rough opening: The first step was to ensure that the rough openings were ready for the install. In this retrofit context Hammer & Hand placed a secondary sloped sill to span the framed sill, exterior EPS, and cladding layers. This served both to facilitate drainage outward and to provide a fresh, uniform surface for prep of the rough opening.
There are two reasons for this approach. First, liquid-applied flashing self-gaskets, protecting the integrity of the exterior sheathing immediately around the rough openings, an area that receives lots of fasteners and penetrations. Second, 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 the Sure-Flash transition membrane. Nine inches, based on test chamber results, provides enough horizontal room for wind-driven moisture to be drawn down by gravity, ensuring that water laps out over the WRB system and drains off the building.
iv. Install the window: Hammer & Hand placed the windows dead level on setting blocks, with a consistent three-eighths-inch gap between window and opening. The team then installed a closed-cell foam backer rod around the perimeter of that gap and applied a high-quality sealant compatible with the flashing (Air Dam).
This approach created 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. That seal will outlive the window.
Matt and Cameron have been making flashing kits and flashing the openings for a couple of days. They cut the ProtectoWrap Sill Pan Flashing tape 12 in. longer than the rough opening is wide so the legs can wrap up the sides. Then they score the bottom strip of release paper about 2 in. in from the back edge so that it can be removed as separate strips at different times. This score-line corresponds to the back of the window sill once the unit is installed, where we want the pan flashing to fold up and form a back dam.
Set the window into the rough opening, level, and fasten to the substrate per window manufacturer's recommendation. Membrane should be visible below the sill flange. Do Not apply Tamlyn Window Tape over the bottom flange! This area is left open to allow water to drain in the event of a window leak.
Adhere vertical sections of Tamlyn Window Tape over the jamb (side) flanges and to the shear wall or house wrap with a foam roller or hand aplicator. The vertical tape should overlap the tape at the bottom and extend 3" above rough opening at the top, preferably onto the exposed sheathing.
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. 2ff7e9595c
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