Seattle Passive House – Windows can be such a pane in the glass…

Diligence is key in building a Passive House.  Making sure all the details are incorporated as planned is necessary.  When Dan’s windows showed up with the wrong stickers on them there was more than enough heartburn to go around.  Windows are a very important part of the Passive House configuration.  Figuring out where they are located, how much sun they let in (or keep out), how large they are, what their physical properties are in terms of materials & construction are all very important.

As the truck pulled up Dan noticed immediately that there was a problem.  A call to the manufacturer determined that it was most likely a sticker mixup, but just to be sure Dan did not install them until the engineer determined that they were in fact the right windows.  Phew!  I am happy to report that they are in and I have the pictures to prove it.  Dan’s friend Dale even came up from Portland to help put them in.  Additionally there is now a set of stairs to the 2nd floor and I was able to go inside for the first time and I took more pictures.

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If you notice, the windows are not flush mounted to the outside as you would expect.   There is a reason for this.  The windows perform better thermally if they are placed closer to the center of the window well.  There is a program available from the Lawrence Berkely National Laboratory called THERM that can be used to test for the thermal capabilities of construction materials, primarily windows and after testing the position and types of materials, it was found that where the windows are located within the assembly is an important factor.

With the windows in, the Tyvek and siding going up outside, we are getting close to another Passive House feature – the airtight layer.  Dan designed his Passive House to have the airtight layer on the inside of the structure.  He will be using OSB over the larsen trusses on the inside, sealed, and then will blow in dense pack fiberglass insulation into the cavity.  Once the doors are in and any other openings are covered it will be time for the blower door test.   If you want to be sure you don’t miss any updates, you can register for email notification on the right hand side of the page.



12 responses to “Seattle Passive House – Windows can be such a pane in the glass…

  1. Kristina Hestenes Stimson

    Nice artical about windows. This comment is a bit off subject, but it’s about glass:

  2. Kristina Hestenes Stimson

    Passive hause could use some Prism Glass
    to get light deeply into the building:

  3. Linda, do yo know mfr and SHGC/VT/U-values?

    • The manufacturer was Cascadia. I looked at a copy of Dan’s PHPP (I don’t know if it is the most recent) and see that the average window info is SHGC = 57, Avg U Value = .21 (Range from .25 Mbed north window, to .17 South & West Door) I’ll check with Dan to see if anything changed.

  4. The REAL specs regarding window performance are :
    SHGC (insulated glazing unit only)- 0.57
    VT (IG unit only)- 69%
    Center of Glass = 0.143; Edge of Glass = 0.199(Fixed), 0.196 (Tilt/Turn) and 0.232 (Awning); Frame = 0.285, 0.289 and 2.261; Overall (NFRC ratings) = 0.180, 0.17, 0.220

    The windows are great, but I wonder if Cascadia understands the heart-skipping moment they put me through… Dan

  5. Dear Linda and Dan,
    I’m not sure it is a given that the windows need to be placed in the centre of the wall for best performance. Can you justify this with any reasoning or a study? One reason I ask this is because I can see little reason for this claim. I can imagine that it is advantageous to place insulation on the outside of window frames, which means placing the windows flush to the exterior is a problem, but moving them say 2 inches back from the exterior will allow that much ridgid foam to be placed in front of the window frame. Moreover, this 2″ thickness can be flashed with something attached to the window frame. Placing the windows deep inside a 20″ thick wall (just an example) means flashing the seam between insulation and window frame becomes difficult unless there is a step, in which case, the window is reduced in size, and you have a similar amount of insulation covering the frame as in the 2″ reveal situation.
    I’m familiar with Therm, but still don’t see the reason. Perhaps it is a wind exposure issue?
    Certainly thermal gains from deep-set windows are significantly reduced.
    Best wishes on your project. I look forward to reading the rest of your blog.

    • Hi Lyndon,

      This spring Passive House Northwest had a remarkable conference and one of the fabulous speakers was Bronwyn Barry of She gave a wonderful talk on windows with a lot of emphasis on placement. You can download a PDF of her PowerPoint at the PHNW site and see a THERM example on page 8 that specifically addresses this. Her point was that the U value changes may seem insignificant when you see them, but when added to all the windows within the project it can be significant. Page 9 of the PDF shows how she was able to save 22% of the annual heat demand of the project she was working on just by moving the windows into the center of the wall assembly. The rest of the presentation that dealt with over insulating window frames, flashing the windows, and step over thresholds was very well done. One comment she made at the conclusion was that if you did use the example of the new flashing detail she offered on pg 19 of the PDF that you should be sure to note the increase of the rough window opening in your PHPP. (Her point was you trade a little window performance for a better flashing detail.)

      Thanks for the comment!


  6. Dear Lind and Dan,
    Thanks for the reseonse. I didn’t realize there was one, as it didn’t come to any email…..
    Someone told me the reason for placing the windows in the middle of the walls is that the isotherms remain as parallel as possible – I think it was Katrin K. This is able to be seen in a THERM graphical output. This makes sense now.
    Thanks for Bronwyn’s presentation – I look forward to reviewing it.

    • Hi Lyndon,

      I’m sorry you didn’t get the response earlier – have you signed up to be notified when this thread has new comments? I really liked Bronwyn’s presentation – she did a wonderful job of presenting the information. Thanks for checking back in!


  7. marco lissandrello & lisa maxon

    trying to build a passive house in italy is practically impossible meeting every roadblock imaginable with absolutely no buy-in from any of the parties you try to get involved. this kind of article is motivation…

  8. marco lissandrello & lisa maxon

    this is a clear article…we wish it were so in Italy, where there is total lack of comprehension of the passive house possibility, roadblocks around every corner, and a general ‘no-can-do’, extrememly cynical attitude about the idea from those related to the business of construction. thank you for the positive note. it serves as motivation.

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