[Bldg-sim] Appendix G Window to wall ratio

Fri Apr 15 12:16:53 PDT 2016

Hi there K!

As far as LEED/GBCI reviews are concerned, the User's manual is a very strong a resource to cite in my experience, especially where it can serve to clarify something less clear in the Standard.

That said, I'll admit to being shifty at times on this issue (there are certainly grey areas the standard doesn't clearly address), but I would probably lean towards Jason's interpretation as well for most cases.  In my own phrasing:  Draw the WWR area line to exclude "conditioned envelope that faces dirt."  

If you are dealing with a building featuring tons of glass facade, do keep in mind that opaque glass façade areas (i.e. spandrel) may be excluded when tallying the window area (be sure your model reflects such a distinction).  

If the quantity of windows in your semi-underground levels is pushing you just past the "40% mark" for the building as a whole (I assume that's what you're referencing), then do acknowledge/communicate with your design team that they are toe-ing the line with the Standard.  Some project-specific sensitivity analysis may be helpful for your own understanding before waving a big red flag over the issue:  bumping slightly past that 40% mark often won't hurt performance rating much, but a building with 80% WWR is almost certain to feel the burn.  

Also note that if you are making more use out of the building's relatively high WWR from an energy perspective (i.e. natural ventilation for thermal comfort, daylighting controls, deliberate solar load shading considerations for certain climates...), there's nothing stopping your team from claiming a net credit considering everything together.  

Finally, as a fellow who LOVES energy modeling but also likes getting outside every so often, I personally reckon extra daylighting and views are subjectively "worth" a net energy penalty.  I think most building owners/tenants outside of the vampire population would generally agree.  Consider me a fan of higher window distributions, so long as you're keeping me thermally and visually comfortable (glare control please!).

As to whether Appendix G  intends to "penalize" below grade floors with windows... that might just be a stretch.  If you were to flip around the situation (include below grade envelope for WWR calcs) - it would be weird to generally "encourage/discourage" the use of below grade windows facing dirt, right?  As a general rule, I try to remind myself that it's probably safe to assume the committee drafting this language never had any specific/given project in mind, and were just trying to come up with a generally fair (if occasionally arbitrary/silly) line to draw in the sand that would suit most cases.

Hope this helps you enjoy the weekend a little more!

~Nick

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Nick Caton, P.E.

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  Energy and Sustainability Services
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  Schneider Electric

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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jason Glazer via Bldg-sim
Sent: Friday, April 15, 2016 1:17 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Appendix G Window to wall ratio

I would suggest that you count only the portion of the wall that is above grade. Just my opinion.

Jason

On 4/14/2016 5:17 PM, k bk via Bldg-sim wrote:

	Appendix G (LEED) indicates that when we calculate the window to wall ratio only the above grade walls shall be included.

	We have a building design where the conditioned basement wall partially protrudes above ground and also has windows that let in sunlight. It is unclear if in this circumstance we could count the whole wall in the window to wall ratio. Any body have any hints.

	Surprisingly, this definition of below grade wall appears to apply only to appendix G & LEED. As per the user manual to 90.1 the below grade wall can be included towards the window to wall ratio, for the prescriptive path and chapter 11.

	Frankly speaking, it is unfortunate that Appendix G & LEED, Would discourage the construction of below grade conditioned spaces in this manner. In essence aren't we increasing the efficiency of the building if we put conditioned spaces below grade. 

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