Installing Windows the Right Way
Even top-quality windows need a sloped, waterproof sill pan in order to prevent water damage
Brian Beaulieu would seem to be well on his way to enjoying a high-performance house in southern Maine. The double-stud walls are 10 1/2 inches thick and insulated with mineral wool. The exterior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. is the taped Zipwall system, backed up with airtight drywall on the interior for a second line of defense against air leakage.
Beaulieu has invested in top-quality tripled-glazed Intus windows suitable for PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. designs. And it's here that Beaulieu has run into a problem.
"We've made a few errors which are now rearing their ugly head," he writes in a Q&A post at GreenBuildingAdvisor.
Plywood shims on which the window sits are exposed on the inside, forming (in Beaulieu's words) "a perfect thermal bridge." The shims and now the rough window sills themselves are getting wet, apparently from condensation.
And weep holes on the exterior of the frames were accidentally blocked with tape and trim, causing water to build up in the frames. The weep holes have since been uncovered, and Beaulieu plans on drying out the sills and shims with a heat and a fan.
"We seem to have found the root of our condensation issues," he writes. "Or have we? If not, what else am I missing?"
His window problems are the start of this Q&A Spotlight.
The house will dry out in time
Beaulieu's house is showing typical signs of high humidity in new construction, GBA Senior Editor Martin Holladay writes. "Your house is humid because of construction moisture from the concrete foundation and drywall mud," he says. "After 6 or 12 months, everything will dry out and stabilize."
He adds that it would be cheaper for Beaulieu to turn on a bathroom exhaust fan and let it run 24 hours a day than it would to use a dehumidifier, as Beaulieu wrote that he was planning to do.
Stephen Thwaites of Thermotech Fiberglass Fenestration doesn't think the wood shims are the source of the problem, but adds that with the drain holes now unblocked, the problem should be solved.
But for Holladay, a more pressing concern is the apparent lack of a sill pan in the window opening.
"It looks like you didn't install any sill pans under your windows," he says. "It's fairly common for the rough sill under a window to get wet at some point. What you need to do is anticipate that wetness by flashing your rough sill. The Siga Wigluv tape in the photo does not amount to flashing, and it certainly doesn't amount to a sill pan. Now that your windows are installed, however, it's a little late to be discussing sill pans."
For best results, use a sloped sill or an interior dam
A sill pan is a waterproof barrier that redirects water to the exterior while protecting the window openings and adjacent areas from damage.
"A sill pan has several elements," Holladay says. "Ideally, it has a slope. On the exterior, it needs to extend to your water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB), and it needs to lap the WRB (or to be integrated with some type of flexible flashing that laps your WRB).
"Moreover, sill pan flashing has to be waterproof, and has to be able to hold standing water without wetting your rough sill. (Clearly, you current arrangement fails this test.)"
Jamb flashing should direct any water over the lip of the pan so it can drain harmlessly away. Finally, an interior dam will prevent water from backing up toward the interior of the opening and causing problems.
Siga Wigluv, Holladay adds, is an air-sealing tape. And although it may be waterproof enough to use as a site-built sill pan, Beaulieu hasn't made any apparent attempt to build a pan with it.
Although Beaulieu has followed the instructions provided by Siga for creating what the company refers to as a "sill pan," Holladay thinks that the Siga instructions are "irresponsible."
"No construction expert I have ever talked to would call Siga's use of folded-over tape a ‘sill pan,’” Holladay adds.
And what about Intus installation instructions?
Referring to installation instructions available at the Intus web site, Peter L points out the gap between the window and the sides of the rough opening are to be sealed with Tremco ExoAir flexible polyurethane foam. This tape, he says, expands to fill the gap and keep out water and air.
Judging from the photos that Beaulieu has supplied, it looks as though he used spray polyurethane foam and Siga tape to seal the frames into the opening.
"Each window manufacturer has their own personalized install directions, and I believe that if you had followed the Intus directions, you would not have had the issues you are experiencing," Peter L writes. "There is no way water would have backtracked into the interior of the home had you installed the Tremco ExoAir properly as shown. Quality windows like Intus have detailed install instructions and following them will result in a better install."
But the Intus installation instructions raise issues of their own.
"Remarkably, the installation instructions for Intus windows make no mention of flashing," Holladay writes.
"These instructions remind me of the type of installation instructions that window manufacturers used to provide in 1990. It's almost as if Intus is totally unaware of developments in window installation methods over the last 23 years. Clearly, it's time for Intus to wake up."
And apparently Intus has. The company is developing more detailed installation instructions that should be available early in 2014, Peter L says.
Exterior trim is another problem
In addition to the lack of a correctly detailed sill pan, Beaulieu's window installation seems to suffer from picture-frame trim that he's installed over the edge of the window frames on the outside of the house. Beaulieu has drilled holes in the trim to match the weep holes in the frame, but this may not be enough to forestall problems.
"The weep holes aren't the only location where water exits a window," Holladay says. "Windows are designed to shed rain off the sill. Windows need to be installed so that the sill extends beyond the siding. (This is a typical ‘outie’ installation.) When water drips off the sill, it drips beyond the siding."
The trim Beaulieu has installed now covers what would normally be a drip edge. Unblocked weep holes and a gradual decline in indoor humidity may be part of the solution to Beaulieu's problems, but the exterior trim detail seems to be unresolved.
Our expert's opinion
Here's what GBA Technical Director Peter Yost had to say:
There are several issues I would like to chime in on:
1. Moisture of construction. As we build tighter and more energy-efficient homes in cold climates in the middle of winter, I am seeing more and more cases of wintertime condensation, on windows and (worse) in walls. Moisture of construction has always been there; it’s just that we have changed the how and when of our building. And nine times out of ten, this issue comes to a peak as the drywall crew fires up their propane heaters for their taping. The best thing to do is get that central heating system hooked up ASAP (instead of propane heaters producing a ton of new moisture) and, as Martin says, get some 24-7 exhaust ventilation up and running as well.
2. Responsible high performance manufacturing and sales. One of my pet peeves is the manufacture and sales of non-integrated products and components. High-performance builders should only purchase from manufacturers that integrate all of their own products and integrate their products with other components or products that fit into a system.
How come high-performance skylights come with flashing kits and detailed installation sequences and most windows don’t? If all windows leak — and they will — then the integration of flashing, the window, and the weather-resistive barrier is the responsibility not of the installer alone, but of each and every manufacturer that contributes to the water-resistive barrier system. I am waiting for someone on the residential side of our industry to do what Tremco has done on the commercial side. Tremco’s Engineered Transition Assembly integrates all of their air and water barrier components into a guaranteed assembly. Now that is green building high performance.
3. Sill pans. Martin and I were brought up with the same background on this: Every window installation must include a sloped or back-dammed sill pan weatherlapped with the water-resistive barrier (WRB). That weatherlap means the WRB tucks under the vertical apron of the sill pan and the WRB covers the vertical wings of the sill pan at the jambs. The sequencing of these weatherlaps depends on which gets installed first, the window or the WRB. I have never been able to do this correctly myself without first mocking up all the components and rehearsing the sequence, and the same is true for most installers.
4. Integrating window flashing with the WRB, not the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. . When I was a builder in the 1980s and knew absolutely nothing about building science or continuous water, air, and thermal barriers or control layers, I thought keeping water out of an exterior wall meant fitting cladding (clapboards, board-and-batten, you name it) tight enough together and then caulking around penetrations and weatherlapping drip edge to the cladding. And never mind vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding; I just sort of punted and thought maybe the J-channel carried water like a gutter.
It never even occurred to me that all flashing details should be integrated with the WRB, not the cladding. After meeting Joe Lstiburek and working with him for just a couple of months, I understood that you create a dedicated drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. by always integrating and weatherlapping your WRB to your penetration flashing, not the cladding. And yet, now many years later, I still routinely drive by job sites where there is no consistent integration of flashing to the WRB, there are reverse laps at the heads of the windows, and the builder is hoping for the best.
Until we can train water to do what we want, it will remain cheaper and more effective to train our crews to weatherlap and integrate the WRB and window flashings.
- Brian Beaulieu
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