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Q&A Spotlight

Installing Windows the Right Way

Even top-quality windows need a sloped, waterproof sill pan in order to prevent water damage

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The rough sill of this newly installed window is already wet. The owner/builder of a house in southern Maine is a little concerned about the puddles on his rough sills. Is this condensation? Is the problem due to blocked weep holes? Or is some other phenomenon responsible?
Image Credit: Brian Beaulieu
The rough sill of this newly installed window is already wet. The owner/builder of a house in southern Maine is a little concerned about the puddles on his rough sills. Is this condensation? Is the problem due to blocked weep holes? Or is some other phenomenon responsible?
Image Credit: Brian Beaulieu
Picture-trim exterior flashing covers the window sill.
Image Credit: Brian Beaulieu
Instructions from Siga, a manufacturer of tape, describe this type of folded-over flashing as a “sill pan.”
Image Credit: Siga

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 barrier 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 Passivhaus 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 barrier (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 cladding. 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 vinyl 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 plane 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.


  1. Aaron Ciesielczyk | | #1

    Window flashing
    I have been a remodeling contractor in South East Wisconsin for almost thirty years. My business is Aaron's Building Services. This looks like a flashing issue and not a new construction moisture issue. Martin and Peter hit it on the head. There needs to be a sloped or dammed sill pan, the pan needs to extend past the WRB. This talk about advancements in window technology is interesting. Before the last 1/3rd of the 20th century, windows had built in sloped sills. Manufactures have moved away from this. As Peter has pointed out, the lack of integrated flashing from window manufactures. The windows need to be removed and re flashed. Do not use picture frame trim at the bottom of exterior of the window. And lastly. Is there any head flashing? If water comes in at the head of the window you already have a issue.

  2. Todd Noice | | #2

    Seals, vents and drains
    Water has a long lists of states of being. It falls, runs, drips and pools. Water also condenses, evaporates, it's in vapor and it freezes. Plastic, vinyl, glass and wood all move.

    Deciding where and when things will move and which states water will be and when in assemblies that sit out in the sun, wind and rain and contain warm moist air to keep us happy isn't always as simple as it looks.

    I'm wondering if the windows have flashing over the flanges or under. Where are the seals, vents and drains around the window so the assembly can move and keep everyone happy for a long time?

    Horizontal surfaces need drains so gravity will prevent water from pooling. Horizontal surfaces also need seals - with drains being the first place for water to go next.

    Since water condenses and it evaporates and it floats in the air as humidity it needs a vent.
    Is the envelope a barrier or a rainscreen? When the wall heats up where will the air escape? When it cools how will the condensation drain and dry?

    When it rains where does the water on the glass go? When I watch windows in the pouring rain the water mostly finds the bottom corners. Picture frame joinery is a good way to get water behind the trim. It looks like the weep hole covering trim has a reverse lap bonus at the bottom - the trim on the vinyl extrusion creates a pool where you need a drain.

    My vote is with Aaron's assessment. Pull the window out and use a rainscreen barrier system and get the trim behind the outer drainage plane of the window. Maybe you could have a custom sloped sill pan made out of copper - why not go all out? Or try it this way:

  3. Russ Chapman | | #3

    Condensing Surface Within the Frame Assembly itself?
    Looking at the interior photo and where the water is pooling, it appears most of the wetting is happening at the mull between the two window units (most of it here) and along the outer frame. I'm assuming this is a factory mulled window unit and not a stud pocket built in the field? Follow the water marks on the rough sill. Looks pretty dry directly below the glass so seemingly the interior glass surface is staying warmer than the frame/mull? Or perhaps that’s where the sun has dried out the rough sill before the photo was taken? Is the builder seeing excess condensation on the glass itself in the mornings? Is it possible that there's a piece of structural aluminum or steel vertically within that mull and its surface temp is lower than the glass and thus is the condensing surface for the construction humidity? It's hard to eliminate thermal bridging in this application. If that's where the moisture is condensing, I'd expect that will always be the weak spot in the thermal envelope for this home unless someone pokes a new hole somewhere else. Are the wet sills showing up more on mulled units than singles and does the pattern tend to indicate the moisture is running out from the mull itself rather than off the glass? Last I checked, even with the highest quality windows, the frames are still the weakest link in their overall performance and drag the performance of the glass down. Is there any possibility of adding insulation to that mull post before you cover it all up with finish trim? Would you expect in the future that whenever this homes humidity increases water is going to condensate in these same locations? It doesn't seem like having excessive condensation WITHIN the window frame itself is the most durable assembly long term.

    A new years salute to the importance of mechanical ventilation!

  4. Brian Beaulieu | | #4

    More info
    Not exactly how I want to see my name in print but I suppose it could be worse...
    I think the first attached photo is a bit misleading. After an intense driving rain late last fall, the inner channel within the window frame backed up into the house as they had no where to drain to prior to opening the weep holes up. This was the cause of this particular water stain. Although we should have used a sloped sill or backdam approach in our window installations, I do not believe bulk water is making its way past our flashing. I've attached a photo of the flashed windows prior to siding. The Wigluv tape went on quite smooth without bubbles or folds and once it was on, there was no getting it off, it made Ziptape look like painters tape. We did not use flanges, we flashed the window frames to the face of the zipwall. Additionally, we did install head flashing over each window.

    To answer Russ' question, the mullion in the photo was a field built 2x4 box with a 2" XPS insulation core, it does not appear to be showing signs of condensation.

    We have not noticed any issues since opening the weep holes and installing our HRV. Things have dried out sufficiently. We plan to leave the interior window stools off until spring to allow further drying. We also intend to remove the trim in the spring to see if there is anything alarming behind there. The next driving rain will be the big test but its going to just be the white fluffy stuff around here for a few more months.

  5. Peter L | | #5

    Year Follow-Up

    It's been over a year since this article. What did you conclude? I would be curious to hear if it was the flashing or something else that caused the water issue.

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