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Reducing WWR to 40%: which approach?

asked 2015-09-02 17:59:03 -0500

updated 2015-09-02 18:23:23 -0500

When you have a lot of glazing and are following ASHRAE Appendix G, you might have to reduce the glazing area in the baseline model to be no more than 40%, respecting the proportion of the proposed model for each orientation.

I see two ways of doing this:

  • Add windows on all exterior (non-adiabatic) walls, and set a WWR ratio by facade. This would be akin to using the OS/sketchup measure "Set Window to Wall Ratio")
  • Downsize your existing windows in order to be at 40% WWR overall

I'm leaning strongly towards the second option, but I might have missed something.

What do you say?

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which software are you using?

obuchely's avatar obuchely  ( 2015-09-03 08:48:57 -0500 )edit

I use eQuest, OpenStudio, E+ in IDF editor. This question is software agnostic in my mind.

Julien Marrec's avatar Julien Marrec  ( 2015-09-03 09:05:07 -0500 )edit

Some packages can automatically reduce WWR to meet 90.1. (Trace 700, IES...)

obuchely's avatar obuchely  ( 2015-09-03 09:23:06 -0500 )edit

They do it by shrinking the existing windows?

Julien Marrec's avatar Julien Marrec  ( 2015-09-03 11:26:38 -0500 )edit

4 Answers

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answered 2015-09-03 08:16:44 -0500

I have typically taken the second approach, adjusting/reducing the height of all windows on all faces to get the model down to the 40% limit. We have an excel file that determines the reduction needed and calculates the new heights that can be pasted back into eQuest. [Also handy because it checks & adjusts for the requirement for a Chapter 11 model.] I am potentially missing something, but the first approach would require both creation of new windows and a decent number of calculations to determine the WWR by façade; yes? Not a criticism, but a question since I typically am in eQuest and not OpenStudio [yet - diving in soon though!]. Thanks for any insight into this approach

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Glad to see I'm not the only one feeling that way.

For eQuest I've got user expressions based on a global parameter that do that per orientation for me (it's easy, they are always rectangular). For OpenStudio I just wrote a measure to shrink the windows.

Julien Marrec's avatar Julien Marrec  ( 2015-09-03 11:31:12 -0500 )edit

I have also used global expressions based on a global parameter to resize the window height to change the window to wall orientation.

Daniel's avatar Daniel  ( 2015-09-03 12:24:03 -0500 )edit

answered 2016-05-30 10:03:54 -0500

updated 2016-05-30 10:04:12 -0500

I've finally came to add this OpenStudio measure to shrink windows around their centroid on the BCL: you can find it here.

Note that it should always work for shrinking windows, but if you use it to increase the size of the windows, it could theoretically result in windows that extend from their base surface, so just be aware of that.

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answered 2016-12-14 11:08:07 -0500

Nick Caton's avatar

TLDR: I'm in complete agreement with Daric (he got my vote!), but don't see this as a binary question with only one right answer so I'm contributing some additional perspectives. I'm going to diverge a little from LEED / Appendix G and the 40% rule to tackle the topic of window distribution during model development more broadly as well, so I'm not expecting this to get upvoted as a best answer - just adding to the discussion:

I typically also take the latter approach, adjusting each window's area in-place thereby retaining the proportionality of the window distribution by any metric: facade/surface/orientation. Appendix G contains fuzzy phrasing (subject to interpretation) with regard to maintaining the proposed cases' distribution proportion, and LEED/GBCI have interpreted that to mean you should maintain WWR by major orientation (still a fuzzy line for complex floorplates)...

However my reading of Appendix G is that you CAN ultimately take either approach, and this decision is a good framework to consider for workflows both inside & outside of the confines of Appendix G. At times (early schematic/box-model studies come to mind), you might not know anything about the eventual window distribution (or what you do know is subject to major revisions), so uniform distribution could make the most sense when you don't have a better guess.

More often, you as the modeler can make the informed decision that a strategy of uniform distribution for geometries is a "fair enough" estimation for model development (where the actual distribution is or could be relatively uniform), to allow you to smartly invest more of your limited development time in more pressing/important areas of the simulation. Consider: Appendix G has nothing to say about a "distribution accuracy threshold" in discussing the baseline model's fenestration requirements - it is more broadly concerned with ensuring the baseline/proposed case are on equal footing (at least at/below the 40% mark).

In practice however, I have found aiming for "equal distribution" by facade/surface (by ensuring glass in on each opaque air-facing exterior surface) can become problematic, particularly in the proposed case.

One reason is this can introduce "disconnects" with details of the proposed case HVAC system(s) design: You can end up for example with zones/systems that weren't designed to handle the new skin/solar loads introduced by "spreading the windows around," and conversely systems/zones designed to handle a lot of glass are now "oversized" in your model's representation. This leads to concessions either with more autosizing and/or more unmet conditioning hours taken as an acceptable threshold (potentially butting heads with App. G's thresholds).

Minor reasons may include that the models can end up looking substantially different from the design (which is a big deal for some individuals), and also it can make for additional necessary documentation/explanation in a review process.

I hope this helps suggest a few new ideas - In my mind there's a time & place for ... (more)

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answered 2016-12-15 07:12:28 -0500

yongqingzhao's avatar

updated 2016-12-15 07:13:51 -0500

I have ever posted a topic aobout manipulate WWR using can find it

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Asked: 2015-09-02 17:59:03 -0500

Seen: 1,842 times

Last updated: Dec 15 '16