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Window condensation issue

Bowler222 | Posted in GBA Pro Help on

I’m in the middle of constructing my home and have discovered a serious condensation / bulk water issue. First some background: We’re in southern Maine. The envelope is a double 2×4 stud wall system 10.5″ thick insulated with Roxul. Exterior air barrier is taped zipwall and interior air barrier is ADA with taped or gasketed seems and joints. No vapor barrier. We put in Intus Eforte UPVC windows.

We’ve made a few errors which are now rearing their ugly head. First, we used 1/2″ plywood shims to install the windows which are the same depth of the window frames and therefore a perfect thermal bridge. These shims (currently exposed on the inside) are getting really wet, seemingly from condensation. The sills are getting really wet too. We just finished drywalling and painting and our HRV isnt installed yet so the humidity in the house is around 65% (I recently fired up a dehumidifier to help). I plan to put a heated fan on these shims to dry them out and tape or caulk them to prevent moist interior air from contacting them. Error number two is Intus windows have weep holes in their frames to let water that gets past their gaskets out. We foolishly blocked these with tape and trim. We had a fair amount of water build up in the frames channels. We’ve since exposed the weep holes and water is now draining. I suspect cold or frozen water sitting in those channels was part of the condensation problem

We seemed to have found the root of our condensation issues or have we? If not, what else am I missing? I attached a photo of one of the particularly bad windows.

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  1. Bowler222 | | #1

    A photo of one of the particularly bad windows...

  2. GBA Editor
    Martin Holladay | | #2

    This type of condensation is fairly typical during construction or when a house has just been closed in. Your house is humid because of construction moisture from the concrete foundation and drywall mud. After 6 or 12 months, everything will dry out and stabilize.

    During the winter, you'll find that it is cheaper to run a ventilation fan than it is to run a dehumidifier. I suggest that you turn on the home's bathroom exhaust fans and leave them running for 24/7 until your interior humidity drops. Of course, you need to keep the house heated.

    It looks like you didn't install any sill pans under your windows. 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 Sigu 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.

  3. Bowler222 | | #3


    Thanks for the reply. The sloped sill is just one of a long list of details I regret missing. Unfortunately I don't have bathroom exhaust as I intended on relying in my HRV in booster mode. However, the HRV is not yet installed.

    If I can dry out the plywood shims and my rough sill and then cover with a product such as SIGA Corvum to block condensation from reaching the shims and the rough opening am I in (relatively) good shape or is there more I should be doing?

    Also, why is Wigluv not adequate flashing?

  4. GBA Editor
    Martin Holladay | | #4

    A sill pan has several elements. 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.)

    The sill pan flashing needs to be able to accommodate the flashing that you install on your rough jambs. The jamb flashing needs to lap onto the lip of the sill pan, so that any water dribbling down your jambs is directed into your sill pan.

    The sill pan needs to extend all the way to the rough jambs on both sides, so it catches drips at the crucial leak locations -- the two lower corners of your window.

    Finally, the sill pan needs to have an interior dam.

    Siga Wigluv is an air sealing tape. It's possible that Wigluv is waterproof enough to be able to use to fashion a site-built sill pan. But you haven't made any attempt to build a site-built sill pan.

  5. GBA Editor
    Martin Holladay | | #5

    Here is a link to a series of GBA videos that provide information on flashing the rough opening of a window:
    Window Sills That Won't Rot: Sloped Sill or Backdam?.

  6. Bowler222 | | #6


    Thanks for the information, very helpful. Unfortunately my ship has sailed for many of these practices. I wrapped the entire rough opening with Wigluv per SIGA's typical detail (except l had Zipwall in lieu of housewrap). At this point is there anything I can / should do short of ripping the windows out?

  7. Bowler222 | | #7

    The Wigluv flashing detail that I referenced....

  8. Stephen Thwaites | | #8


    Windows, like bathtubs, will not behave well if their drains are blocked.
    By exposing the drain holes i would think you've solved your problem.

    It would be very surprising to me if condensation on the wood shims was the source of your water. They aren't that much more conductive, if at all, than a window frame.

    If you can, leave the rough sills exposed for a period of time to make sure the problem does not return.

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  9. GBA Editor
    Martin Holladay | | #9

    Concerning your immediate condensation problem, I agree with Stephen. Everything will dry out soon if you leave the interior of the rough sill exposed, and continue to heat the house. If you don't have any exhaust fans installed, crack a few windows to encourage a little dry exterior air to enter the house.

    Concerning Siga's definition of a "sill pan": I think it is irresponsible. No construction expert I have ever talked to would call Siga's use of folded-over tape a "sill pan." I'm reproducing two of their photos below to highlight the company's irresponsibility.

    You don't have to remove the windows and re-install them, in my opinion, especially if your house has adequate roof overhangs and is not located next to the ocean.

    Most windows leak, and most leaks eventually dry harmlessly. The problem is that some windows leak a lot, resulting in wall rot or sheathing rot. That's why it's a good idea to invest in (or make) a sill pan.

    It's one thing to slap a $189 window in a wall like this. But if you are buying triple-glazed Intus windows, and investing in Siga tape, the rough opening deserves proper flashing.

    Caption to the photos below: in spite of Siga's claim, this is not a sill pan.


  10. GBA Editor
    Martin Holladay | | #10

    I think that you are misunderstanding my objection to the Siga method.

    I am not questioning whether Siga tape is waterproof.

    I am questioning the fact that the photos show the following conditions:
    (a) the rough sill is not entirely protected by flashing (that is, the sill flashing is not wide enough);
    (b) the flashing installed on the rough sill has no slope, and
    (c) the flashing installed on the rough sill has no backdam.

    This is not a sill pan. This is a rough sill that is half-flashed. A sill pan must be able to hold water like a bathtub, and must direct the water to the exterior (usually, over the WRB).

  11. albertrooks | | #11

    Brian and Martin,

    I do think the Wigluv will perform well as a sill pan. I certainly hope so because we've (The Small Planet Workshop and our Siga trainer Nash Johnson) have just completed a round of three Siga workshops in Portland OR, Olympia and Bellingham WA where we have now advised about 50 builders and architects that Wigluv makes an excellent sill pan.

    Here is a little background: When I started working with Siga in 2010, I actually went over to the factory just outside of Lucern Switzerland and took a day long training along with other non German speakers from Norway etc... The next day I got to tour a couple of job sites including a large mixed use retail & multifamily Cross Laminated Timber project.

    Wigluv was recommended as a sill pan in the training workshop and then I got to see it in use as a pan at one of the job sites also. I'm saying this just to let you know that the recommendation is not new.

    Martin, when Patrick Haacke of Siga came over and you got a chance to visit and write "One air Barrier or Two?", Siga was just learning about what was going on in the US. By early 2012 they learned what an issue we have with windows leaking and that our pan systems are really lacking (my opinion). That's when those detail sheets were made for the US market.

    So... here is why it works, It's just 3 elements:

    1, The Wigluv backing material (the printed area) is water proof. 2, The Wigluv tape itself is really flexible and you can get it to conform well to the shape of a pan. 3, The quantity and type of adhesive creates a monolith when applied and "cured". (Cured is letting it "set" for a couple of days)

    During the Sill Pan portion of the workshops we discuss this "monolith" point because the lapping sequence question always comes up. The Siga Trainer (Nash in our case) says that the lap sequence becomes moot because the layers form a chemical bond and the tiny ridges are just bumps in the path to the exit of bulk water.

    While I can't see what going on inside the application, my overall experience has been really good. I have not heard of any delimitation of Wigluv and it's been used in some pretty bad conditions -Thorsten in Fairbanks at -40 deg, Hot tub lids, inflatable 6 person white water rafts as a hole patch in Oregon (BEFORE THEY SET OUT - crazy people!) , and a patch for the curtain of our own soft side delivery truck.)

    Based on all of that experience, I think Wigluv is fine for keeping the dripping water from window frames from getting to the framing for a few decades. Certainly as good as the thick, clumsy and inflexible products I typically see used.

  12. albertrooks | | #12

    Martin, Ah yes. I am misunderstanding the question. Pardon the long windedness.

    To that I can't really add too much. As you know most of the Euro windows instals are "innies" since the walls are thicker. I don't recall seeing many sloped sills in the projects and details that I've looked at. In fact I can't recall seeing even one.

    The applications that I know of are typically a flat like Brian's. It works because the widow quality is high and the weeps will hit a sloped metal pan that is joined to the window just below them and carries the "weeps" away and over the WRB. This is all at the window assembly and above the flat wooden window buck. When detailed in that way, I'm not used to seeing a slope or a back dam used.

    This would be the same detail that you got to look at on The North Project by the Artisan Group in Olympia.

    This should be fine on window quality like Intus. The exterior weep water should be carried away by the metal before it gets to the Wigluv flashing. It might help to see what the exterior of the Installation looks like.

  13. albertrooks | | #13


    In re-reading and looking at the pictures again I see you are making some good points. In effect: "Water happens". I can see the benifit of having all of the wooden sill covered in this application. After all, The point here is to protect the structure. I'll check into this with the Siga folks to see if they want to revisit a fully covered sill.

    While I don't think that flat "bucked" sills will go away on "innie windows", or that it needs to go away, Do you think that a flat buck needs a back dam? What do you think of the combination sloped metal and flat bucks on the higher end windows? There are typically additional layers of a tape or tape like membrane added both inside and outside that are designed to catch the water before it gets to the sill. (This is not applicable to the basic vinyl frames with welded corners).

  14. GBA Editor
    Martin Holladay | | #14

    Lots of windows installed in the U.S. have leaked enough to cause wall rot. While many builders are still slapping windows into rough openings without sill pans, GBA is trying to raise standards in the industry.

    If you live in a dry climate, it's possible that you can get away without a sill pan -- especially if you are building a one-story house with wide roof overhangs. But sill pans are relatively cheap, and they provide good insurance. If a house has a wet-wall problem, one callback can easily cost $10,000.

    If a tape manufacturer is recommending that their product can be used to fabricate a site-built sill pan, then it's not good enough to say "Intus windows don't leak." If you are selling tape, it has to work for all customers, even those who install Marvin or Pella windows.

    "Sill pan" has a specific, technical meaning. The Siga photos do not show a sill pan.

  15. GBA Editor
    Martin Holladay | | #15

    To defend aspects of Brian's approach, I'd like to remind you that this appears to be a condensation issue, not a rain intrusion issue.

  16. Bowler222 | | #16

    Peter, thanks for the links. Admittedly we did not follow all of the recommended practices and I regret that, however I do not believe that bulk water is making its way in through the rough opening. We taped the outside perimeter of the window frames to the WRB (face of zipwall) with Wigluv. (See photo)We also filled the rough opening with closed cell foam. I got believe the problem stems from blocking the weep holes. At least I hope so, otherwise I'm not sure what else I can do at this point.

  17. Bowler222 | | #17

    Sorry...the photo was larger than 2mb, will try to find a smaller one

  18. Bowler222 | | #18

    a couple of photos showing the windows from the outside. The first taken this summer pre-siding and the other taken today.

    It seems my problem is still far from solved: I noticed this evening that one corner of one window has gotten wet again. The weep holes have all been exposed and indoor humidity reduced to 40%. The weep holes have been exposed for a few weeks now and it hasn't rained in as much time (snow yes, rain no). I'm at a loss for whats going on!

  19. GBA Editor
    Martin Holladay | | #19

    The plot thickens.

    Now that I have finished chastising Siga for misuse of the term "sill pan" and bad flashing advice, it's time for me to chastise Intus.

    Remarkably, the installation instructions for Intus windows make no mention of flashing. 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.

    One thing is clearly noted in the Intus instructions: "Windows shall be installed so that the weep holes are not blocked." Makes sense.

    I don't quite understand your new photos. The caption to one indicates "Caps over recently exposed weep holes drilled through Azek trim." I'm assuming that the picture-frame trim on the outside of this Intus window consists of Azek. If that's true, then the weep-hole cap in the photo is inserted into a piece of Azek trim, not the Intus window frame.

    If I'm right, this type of installation raises all kinds of questions. Did you decide to cover the Intus window sill with horizontal trim? If so, that isn't recommended. The sill should drain to the exterior; you don't want trim over the sill.

  20. Bowler222 | | #20


    We drilled holes in the Azek in the exact locations as the intus weep holes in the window. You don't think this will still allow draining?

  21. GBA Editor
    Martin Holladay | | #21

    The weep holes aren't the only location where water exits a window. 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.

    If you wanted an "innie" installation, you would need to install metal flashing under the sill to convey the rain water to a dripedge beyond the siding.

    You have "outie" windows, so you would (in theory) want the sills to extend beyond the siding, and to be able to drip unimpeded. But you have covered the edge of the sill with horizontal Azek trim. So where is the dripedge?

    Do you intend the new Azek to be a kind of sill? If so, you would need to find some way of making the joint between the sill and the Azek watertight for decades.

    Or do you intend the rain to drip behind the Azek? That would be strange, and it would require you to create a series of kerfs on the back of the Azek.

  22. albertrooks | | #22


    With all of the chastising of Siga and Intus going on as we head into the holidays I have to wonder: Did the Grinch arrive early this year?!? (In Germany it would be Krumpas doing the chastising. Ah… the diversity of the globe.)

    Ok seriously, I went and checked out the video series that you suggested:

    I am now more confused. I know where your going (advising for sloped sills and or back dam's for better quality) but the series is showing a flat sill for all of the peel and stick as well the moulded product videos. Both of the peel and stick videos (#3 & #4) don't have the site built sill pan going any farther back to the interior edge than the Siga detail advises.

    I really can't see any difference at all between them. Based on that I'm not sure that the recommended detail from Siga is any more irresponsible than what's advised in the Fine Home Building Series.

    The Siga detail goes back just as far. It's got uprights. And it lapps out and over the WRB to drain bulk water.

    What am I missing??

    Best regards from " Confused in Whoville".

  23. GBA Editor
    Martin Holladay | | #23

    I'm not going to defend every piece of advice in the video series on GBA, but on the issue of sill pans (discussed in the first video in the series), the advice is pretty unequivocal. I'll quote from the transcript:

    "Creating a self-draining sill pan is the most important part of window installation. Right now, if I put down sill flashing tape on top of a flat rough opening, the problem I’m going to run into is that half of the time the water can drain to the inside and the other half it can drain to the outside.

    "Now, I want to increase the likelihood that the water is going to drain to the outside of the wall. There are a couple of options that I can use to do that. The first and simplest is to use some scrap plywood or other wood to create a positive backdam, where the wood is built up along the back edge of the rough opening and the sill pan material is folded down to create a lip on the inside edge so the water can’t drain to the inside. Now, that runs into a couple of problems because you may end up with a rough opening that isn’t deep enough to accommodate the window itself plus this solid piece of material built up back there; you may end up with a height problem. An alternative to that is to use flexible or compressible types of backdam material; one of those is using a backer rod and the other is using a peel-and-stick insulating tape.

    "Another way to create a simple draining sill pan is to slope the surface of the rough opening on the sill. That can be done by applying a piece of beveled cedar siding or beveled pine siding. One of the things you have to keep in mind if you’re going to use a clapboard system is you’re going to end up kicking up the rough opening sill height by about 3/8 in., so you need to make an adjustment for that. On the advantage side, it’s sloped, so that’s going to drain the water. Many of the other systems—either that piece of plywood strip or the foam backing strips—are only creating a backdam, and it’s not going to create a slope to the bottom of the sill, so water isn’t going to tend to run out; it’s just going to tend to puddle there. And that can be a concern if you have a wood-framed window: That wood might be sitting in a puddle of water over a period of time.

    "One thing you can do—and I’ve done in some cases—is actually install both. I’ve put down a piece of beveled cedar siding and then put a piece of foam to give me both the backdam protection and that sloped sill."

  24. jackofalltrades777 | | #24

    If one was installing an "innie" window within a wood frame buck, couldn't a "sloped sill" be fashioned by ripping the wood at an angle to create a natural slope away from the window? One would of course wrap the wood with some peel & stick like Grace or even the Siga tape would work.

    Scenario two: If one was utilizing an ICF buck like the "Fox Buck" which is a fully integrated EPS buck that is held in place by the concrete (no wood whatsoever). One can actually rip the EPS by shaving the EPS with a "cheese grader" or some from of a rasp and creating an angled slope from the window. One can then stucco the EPS up to the window frame, making sure not to block the weep holes on the window. This way you would have a sill that is angled from the window frame and of course waterproof since it would be EPS with concrete underneath and a synthetic stucco finish to further enhance the water shedding capabilities.

    I hope that comes across clearly and makes sense without having to draw up a detail...

  25. jackofalltrades777 | | #25

    Here is the link to the Intus Windows tech page that has the install details for the uPVC windows:

    It is clear that the first potential problem was that you blocked the weep holes, which in the install instructions are noted as NOT to be blocked. The other issue is that the instructions show that the Tremco ExoAir open-cell flexible polyurethane foam was to be installed in the rough opening. The tape expands to fill the gap/void between the window frame and the rough opening. It serves as an air and water barrier to prevent those elements from coming back into the interior of the home. 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. 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.

    I can see in your photo that you used spray foam and Siga Tape to seal the window rough opening to the window frames. One easy way to test this is to grab a hose and turn it on high and spray around the area to see if your rough opening is leaking. The Intus windows are air and water tight so the leakage is clearly coming from the rough opening and the install.

    Here is a video link explaining the window install:

  26. jackofalltrades777 | | #26

    Intus is working on developing more detailed install directions that should be available in early 2014. As with any install, a lot depends on the individual install since different wall assembly materials are used as well as different exterior finishes. There are always new techniques and products coming out to help with detailing window installs. One great thing with uPVC windows like Intus is that unlike wood, they do not rot from moisture exposure. Of course the wood sill is another story but once the water drainage is addressed, the wood will dry out and all should be well.

  27. Stephen Thwaites | | #27

    It should be obvious but, to some it isn't,
    - a self-draining sill pan should not include a barrier than impedes water

    As an example, that means on flanged windows, bottom flanges should not be taped, caulked or otherwise sealed to the exterior sheathing or building paper.

    In Brian's case, if he wanted to implement a self-draining sill pan, one of the steps would include removing the exterior tape between the window sill and the sheathing.

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  28. jackofalltrades777 | | #28


    How would you accomplish a sill pan design with a barrier that does not impede water in my ICF innie-window install? I can't see water getting underneath the EPS and peel&stick membrane. Therefore, there would be no need to design a drainage system in that area.

  29. GBA Editor
    Martin Holladay | | #29

    Peter L,
    All windows leak. Some leak on the first day they are installed; others begin leaking after a few years.

    The most typical seams for leaks are seams between the sash and the frame.

    Of course, an innie window will probably receive less wind-driven rain than an outie window. It also helps to have wide roof overhangs to reduce rain exposure.

  30. Stephen Thwaites | | #30

    Peter L;

    Like Martin, i'd be cautious about eschewing a self draining sill pan.

    A self-draining sill pan is like insurance. You shouldn't need it, but you have it because if you don't and there's a problem – it's often a very big very expensive problem.

    In the end it's your nickel,
    if you think that for your situation, sloped EPS below your window will do the trick, then I hope you are correct.

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