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How much more efficient is it to put windows in the middle of the opening of a double-wall construction?

Tibor Breuer | Posted in PassivHaus on

It seems to me that sealing a window with flanges on the outside of a double-wall frame house against air and water penetration is easier and more bombproof than fussing with putting the window in the middle of the opening. I am not shooting for Passivhaus standards but want to do what is most practical and efficient.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Many factors affect the performance of a window. Installation details matter, but so do the specifications of your window.

    It doesn't make a lot of sense to come up with a Passivhaus-level of installation detail -- optimizing the placement of the window within your thick wall, or "overinsulating" the outside of the window frame -- if you are putting in run-of-the-mill double-glazed windows.

    So, the most important way you can improve the performance of your windows is to choose a top-notch window. Since you didn't tell us your climate or anything about the windows you want to install, it's hard to give you advice.

    If you live in a cold climate, the best windows will be triple-glazed casements, awnings, or fixed windows with orientation-specific glazing. If you can't afford windows like that, then the installation details are far down the list of your priorities. (That said, any window needs to be installed well, with attention to water management and flashing.)

  2. Tibor Breuer | | #2

    Martin,
    I suppose I wasn't clear enough.I will be installing triple glazed casements and awning types,the best my clients can afford.My question is--I have a choice of how to install them in my rough openings.Cascadia windows come with clips or flanges.How much more efficient are they when they are installed in the middle of the R.O. taking into consideration the cost of sealing a window set back like that, compared to a flange installment? I'm not comfortable with what I have seen so far in double wall installations with the windows set back.

  3. GBA Editor
    Martin Holladay | | #3

    Tibor,
    Thorsten Chlupp is the guru who can answer your question best. In his JLC article (http://www.insulfoam.com/images/stories/docs/JLC-finalfeature.pdf), he wrote,

    "Windows can be installed either at the face of the sheathing — in a recess — or out at the face of the wall. From a performance standpoint, a recess is better, because the window is somewhat protected from wind-washing and the interior glass is more easily warmed by the heat in the room. By contrast, windows installed at the face of the wall are in an interior recess, separating them from the warm air inside (especially if a curtain is drawn) and exposing the outer layer of glass to cold wind. I’ve observed that in extremely cold weather — when it’s 25°F below zero, for example — frost tends to form inside windows installed at the face of the wall, whereas frost rarely occurs on inset windows.

    "I’ve installed windows both ways, but because of the frost problem I now do only recessed installations. A recessed installation is more complicated because the sides of the recess must be covered with exterior jamb extensions. On vinyl-sided homes, we make the extensions from 20-gauge metal coil stock. The bottom is sloped to shed water, and there are flanges on both edges — an inner flange that gets fastened to the sheathing and an outer flange that laps over the 1x4 strapping that we install on top of the EPS around the window. We’ve also made extensions from wood and cellular PVC. These solid extension jambs are glued and screwed at the corners and fastened to the wall over a thick bead of sealant. We either toe-screw them to the framing or fasten them from the inside with metal clips."

    There is more information, and some good detail drawings, in the full article. Click the link about to read more.

  4. Tibor Breuer | | #4

    Thanks Martin.I'm helping a friend design a place in Alaska and the link is great.I'm in the rainy Pacific NW. and still wonder what the trade offs are.There does not seem to be a grove in the window frame or any other way to completely interlock an extention jamb on the outside without relying on caulk,which over time can fail.Any thoughts?

  5. GBA Editor
    Martin Holladay | | #5

    Tibor,
    Since the foam is outboard of the WRB, it can be argued that the jamb extensions don't need to be totally watertight. The foam is unlikely to be hurt be small quantities of water that may occasionally leak through such cracks.

  6. John Klingel | | #6

    Too, I believe Thorsten only uses tilt-turn windows, which seal incredibly well.

  7. Tibor Breuer | | #7

    John, I'm not concerned about the window itself.In the Pacific NW I don't plan on 6 inches of foam on the outside, so my windows would set back furthur than the wall sheating.I'm concerned about the connection between the window and the jamb extention be it metal or wood. Sure, I can put and will, if need be, a sticky membrane from the jamb wall to the window frame behind the jamb extension but I wish there was a clear interlocking way to make that transition.With driving rains and a recessed window ,I have concerns.So my original question for the Pacific NW; would a window mounted on the outside with flanges be very much of a compromise to the efficiency of a double wall home?

  8. Matthew Amann | | #8

    Tibor, I believe you are asking for trouble seting windows back into the opening in the Northwest. Where it is very cold and the moisture comes in the form of dry snow, it is a different story. I like the look of windows set back, but it creates challenges where we live.

  9. Albert Rooks | | #9

    Hi Tibor,
    I wish I could quote the numbers but at the last Passive House Northwest Spring Conference I watched Bronwyn Barry's presentation of THERM studies outlining your exact question. She placed the same window in various points from flange to most of the way in and the performance was substantially increased (according to THERM) when it was placed 50% from the exterior. This was based on a Enersign Window (pretty darn good!). Please feel free to contact her at Quantum Builders in Berkley California. You can find them at: http://www.quantumbuilder.com.

    I know she would be happy to discuss it and share the findings. They are the US importer of enersign and this is not an unusual question of issue.

    PS... It's not a problem to do this in the Northwest (where I live) or Alaska. This is a typical european installation detail and method. It's quite different that what we are used to. To seal the exterior I can suggest Siga Wigluv 60 all around behind trim & pan. It's driving rain proven in Switzerland. and works fine in Norway in this detail. If you want more info check http://www.siga.ch, or http://www.smallplanetworkshop.com

  10. Skylar Swinford | | #10

    Below is the link to Bronwyn Barry's presentation "Window Porn for Passive House" from the 2010 conference, which is similar to the presentation Albert mentioned. I hope the spam filter doesn't kill this post;)

    http://passivehouse.us/passiveHouse/2010_Passive_House_Conference_Presentations,_November_5_files/2010%20Conference-Windows%20Roundtable-Bronwyn%20Barry.pdf

    This presentation offers specific installed Psi values. For those that speak German see "Protokollbund Nr. 37" published by PHI.

    Dan Whitmore (Blackbird Builders) used Cascadia windows on his Passive House in Seattle. More info can be found at the following link: http://existingresources.wordpress.com/2010/05/12/seattle-passive-house-windows-can-be-such-a-pane-in-the-glass/

    I've uploaded a couple of key slides from Bronwyn's fantastic presentation.

    Hope this helps.

  11. John O'Brien | | #11

    Would the location of the window installation have any effect on the solar gain? I would think the double stud wall jamb would act as a slight overhang itself. I read through Bronwyn's slides, but I'm sure she explained it far better in person.

    This center location would make it much simpler to install exterior blinds though.

  12. John Semmelhack | | #12

    Yes. When the window is set in the middle of a wall, the head and jambs will increase the shading, as compared to a window set to the outside of the wall assembly.

  13. John O'Brien | | #13

    It'd be interesting to see if the loss of solar gain potential, was balanced out by the increased thermal protection of the recessed window.

  14. Tibor Breuer | | #14

    Yes ,John, that is a very interesting question for the Pacific NW where i live and build.I had just encouraged my clients to agree to 1 bigger window rather than 2 small ones side by side because of the light and the restricted view from an angle with a foot thick wall(as well as the cost ).That would happen weather the window is set on the outside of the wall or in the middle of the jamb.But you are right, the solar gain would be compromised if the window was in the middle.Bronwnyn Barry, in her slide presentation, states that narrower tall casement windows perform the best. That would compromise both view and solar gain. What are the tradeoffs? Hmmmmmmm.

  15. John Brooks | | #15

    I had the same thoughts about trade-off for lost solar gain
    and if that was figured in

    I noticed that Thorsten Chlupp is "overinsulating" his window frames both inside and outside.
    makes me wonder how an "Outie" window with interior frame "overinsulation" would compare.

    I sure like the simplicity and potential enhanced durabilty of Outie Windows

  16. Thorsten Chlupp | | #16

    There a many factors which flow into the equation of were the best placement of a window is within a wall assembly. The more one studies and experiments with the placement the more complex it all becomes...solar gain/loss is one big consideration and there a many more. The best publication about the topic is the Protokolbund Nr.37 by Passiv Haus Institute in Darmstadt - they researched this topic extensively. One can model the different scenarios in THERM. I have experimented with this for a few years now and see a direct efficiency increase of the window assemblies of 20-22% if correct placement and installation detail is followed. This being your weakest link in your wall assembly in regards to thermal performance it is just too much of a gain to ignore it. Over-insulation of the frames - it works on both sides. Breaking the thermal bridging of the frame on the outside and increasing the edge temperatures of the window frames results in the best performance - but is hard to achieve in a traditional casement style window frame. Over-insulating the inside frame will still increase performance. In very cold climates I recommended to treat both sides for optimal gain.

    John: an outie window with interior frame over-insulated is missing on key factor - an air pocket which creates a micro thermal boundary you create once the window is inset...
    Properly installed and flashed there is no difference in durability of your assembly - if anything you avoid putting the window frame through extreme flex cycles which - at least in cold climates can put tremendous stress on the window.

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