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Community and Q&A

Simple Modeling for Choosing Windows

montlamothe | Posted in General Questions on

I’m in the design phase of building a house in Washinton State (Trout Lake) Hot summers and heavy snow winters. Working with an architect and builder, and we’re going for Pretty Good House energy goals/methods.We’re at the “find a window company” stage. Is there a basic way to model the house for general guidelines on what U-values, SHGC, and coatings to spec for different locations around the house? Not looking to do a full energy model, but something more than just the same windows specs for the whole house.

Most window sales people I’ve spoken with don’t offer that as a service (Loewen, Sierra Pacific, Marvin, Andersen) , at least to my knowledge.


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  1. Expert Member
    Deleted | | #1


  2. moe_wilensky | | #2

    Are you thinking along the lines of varying SHGC and/or U-Value based on a window's orientation and location within the building? While this strikes me as a fairly complicated analysis with likely minimal payoff in the end (along with possible downsides in terms of window cost and install complexity), it sounds fun.

    First, what are you optimizing to? life cycle cost, energy cost, energy use or thermal comfort?

    Second, I'm not sure i would bother optimizing/varying the U-Value by orientation/location, if you've got any large picture windows where conductive heat loss is less fame dominated and the marginal cost per delta UA is significantly lower than other smaller operable windows in the home, than you could consider maximizing the U-value of the large window so that the area weighted average U across all windows is roughly equal to code (or some other target that makes sense)

    Next, For SHGC, I think the hardest part will be accounting for shading from the building's surroundings. Ignoring this for a minute, i think BEOpt might be your best bet. i would set up a run with all glazing on one orientation, then define a range of window properties and orientations to iterate though. BEOpt will simulate each combination of orientation and window (so, if you have 10 window options and 4 significant orientations, you’d run 40 models). then I’d group the run results by orientation and select the run that gave the best outcome relative to whatever it is your trying to optimize.

    Lastly, I would then set up a run with roughly accurate WWR defined by orientation and assign the “best” window found above to the correct orientation. I would then run this model, again allowing the orientations to vary. The difference between the best and worst run will show you the impact of the optimization effort.

    (glossing over lots or nuance and complexity here, but this could give you a rough idea if its worth pursuing further)

    PS I'm pretty sure that you would be better off going with code min windows and spend you time and effort on keeping the total glazed area to a minimum and employing shading to control seasonal heat gains.

  3. Expert Member
    Michael Maines | | #3

    I use BEopt for exactly this. It is a whole-house energy model but that's what you need. BEopt is a free download from NREL: (Or, depending on the project, I sometimes work with a consultant who uses the WUFI Passive modeling program.) They don't tell you what coatings to use, but you can try different U-factors and SHGCs to find the best balance for your project, then tell the window company those are the values you're looking for and they will provide their closest matches.

    I model code-minimum values and then run additional models with different configurations to find the best overall performance, then (ideally) compare that against the cost of the windows. Most of my projects have more glazing than ideal from an energy point of view so the choice of glazing has a relatively large impact on performance, as well as comfort.

  4. montlamothe | | #4

    Thank you both for your responses, I’ll look into BEopt. We have more glazing on the house than is probably seen as high performance, so I’m trying to maximize comfort and to a lesser degree performance through glass choice.

    1. Expert Member
      Michael Maines | | #6

      If you want to focus on comfort, there are tools for that, such as this one: I am somewhat dubious about the results but they look pretty official. A few basic rules that affect comfort:
      1. The area of glazing. Large windows matter more than small windows.
      2. How close you will be to the glazing. I usually worry less about comfort at an entry door than at a dining room.
      3. How clothed you will be. Comfort matters more in a bathroom than in a living room (usually).

  5. moe_wilensky | | #5

    Modeling the thermal comfort impacts of different glazing options is likely not achievable with any "Simple modeling" approaches. You're probably just better off applying best practices and moving on. Given your climate (and assuming the building design is locked in), I'd probably locate your low e coating to minimize radiant losses in the winter, plant some well placed deciduous trees to control gains in the summer and be done with it.

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