Re: Window Rough Opening Moisture Management
Leighann Cobetto
Twenty years ago, my construction company, Hammer & Hand, built precisely according to the blueprints, signed off by professional architects and vetted by the building science experts. Yet some building enclosures failed anyway, full of moisture, rot and heartache. It was expensive and frustrating.
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.
Meanwhile, the potential consequences of design and installation flaws grow increasingly serious. As we move to buildings that are more airtight and well-insulated, the natural drying potential of building envelopes decreases. Moisture intrusions that were once a minor problem become potentially catastrophic. Hygrothermal modeling, to understand how humidity and temperature will interact in the building, is suddenly mission-critical.
What we eventually realized is that no bulwark of tapes and sheets can beat the reliability of a properly installed liquid-applied flashing. Ironically, it was through our work constructing buildings to meet a most rigorous energy-efficiency standard that we discovered the value of this simple approach.
Driven to overcome moisture problems in the building envelope, my firm knew we needed to be both builders and building science technicians, guided by the laws of physics. And we needed to continue pushing forward, working to transform the energy efficiency and thermal performance of our buildings.
Simply put, Passive House design provides builders with a foundation in building science and high-performance building techniques necessary to construct buildings that outperform and outlast conventional structures.
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.
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.
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.
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.
Lisa,
If you want to install vertical furring strips (to create a rainscreen gap) and horizontal wood lap siding on your brick house, I would recommend that you first install a layer of plastic housewrap (for example, Tyvek or Typar).
Your trickiest details will be the flashing details, especially at the windows. Ideally, you would remove the windows and either replace them with new windows or re-install them in properly flashed rough openings. If you aren't planning to remove the windows, you had better hire a good contractor who thoroughly understands flashing issues and moisture management issues.
Thank you for your response. The windows have an existing frame that sticks out about two inches. I am wondering if this could be used as one of the furring strips? We will be stuccoing the rest of the house therefore the stucco should come out a bit and we don't mind if the rain screen comes out a bit from the house. Would this avoid our need to remove windows? And even though the house doesn't have tyvek now - I am curious as to why you use it in the rain screen application. Is it protecting the house? Or the wood? Thanks
Lisa,
The housewrap would help keep water off your bricks. It acts as a water-resistive barrier (WRB), a layer that is required by building codes. The WRB needs to be integrated with your window flashing and door flashing as part of your water management strategy.
The reason I advised you to think through what you want to do with the windows has nothing to do with whether or not you will use the existing window casing as a furring strip. The need to install flashing in the window rough opening is a water management detail. Unless you know how to take care of the water that runs down your window during a rainstorm, your work may end up channeling rain water into your walls, leading (potentially) to mold or rot.
This historical idea of diverting moisture away from sensitive details with architectural details can be applied today, but with a twist. A thin veneer with a predictable rain screen drainage plane affords an opportunity to apply this moisture diverter technology on the inside of the void of the rain screen drainage plane rather than by adding architectural details on the outside.
The next step involves moving water away from the top. Remember that moisture moves downward. If we follow the examples from history that created external details away from the top of windows and doors, much of the moisture problem can be eliminated from the window rough opening. The only difference is that the moisture diverting mechanism is inside the building envelope.
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 (BSC 2009).
Too often, an attempt to seal around a window or door unit is made by stuffing the gap with fiberglass insulation. However, because 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 backer rod, caulk or nonexpanding foam should be uniform without any visible gaps.
Blower door testing, conducted as part of whole-house energy performance testing, may help indicate whether windows have been successfully sealed. With the blower door pressurizing the house, use a smoke pencil to check for air around windows. A smoke trail moving away from the smoke pencil into the wall around the window or door unit indicates a leak that should be sealed.
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