# Fenestration and Window to Wall Ratio

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When calculating the window to wall ratio for your baseline building, should you calculate this percentage as

* an entire building?

* each building elevation?

* every floor elevation?

Has anyone had an interpretation on this issue? I was reading through the 90.1 User manual and discovered that both above grade walls and below grade walls should be considered for the wall to wall ratio which leads me to believe the calculation should be done as a whole building percentage. Just curious if someone knows the correct answer.

Thanks,

James M. Newman, EIT, LEED AP

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For LEED purposes you should be calculating the WWR using above grade walls
on a whole building basis, i.e. you don?t need a WWR for each fa?ade to
check against the 40% max glazing requirement.

Anthony Hardman, PE

The Green Engineer, LLP

Anthony Hardman, PE

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Hi James,

As per my understanding, window wall ratio is considered for above grade walls only for each orientation of building (not floor to floor).

Thanks,

Sunayana Jain

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This paragraph comes directly out of the ASHRAE 90.1-2007 User Manual:

"Window-Wall Ratio: The window-wall ratio is the ratio of vertical fenestration area to gross exterior wall area. The fenestration area is the rough opening, i.e., it includes the frame, sash, and other nonglazed window components. The gross exterior wall is measured horizontally from the exterior surface; it is measured vertically from the top of the floor to the bottom of the roof. The gross exterior wall area includes below-grade as well as above-grade walls. It is necessary to calculate the window-wall ratio with all compliance options, since this information is need with the prescriptive option, the trade-off option, and the energy cost budget method."

James M. Newman, EIT, LEED AP

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90.1 Appendix G states: "Vertical fenestration areas for new buildings and additions shall equal that in the proposed design or 40% of gross above-grade wall area, whichever is smaller."

If you are using 90.1 as your State's code, then you can include the below-grade wall area. If you are modeling using Appendix G, then you cannot include the below grade wall area in your calculations.

Dana Troy

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Correct, so if your proving prescriptive compliance you can use below grade walls, while if your proving compliance through performance modeling then you can only usie above grade walls per ASHRAE 90.1 Appendix G.

The question that I still have is should the whole building be averaged for fenestration percentage or should each face have its own percentage?

James M. Newman, EIT, LEED AP

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This is an important point to clarify. While the baseline WWR is a
percentage of total wall area, the amount of glass on each exposure is
to be proportional to the amount with which each exposure in the
proposed case is glazed.

----------------------------
Arpan Bakshi

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The whole building is the way I have always read it.
So you can have an all glass North wall (100%) and then 10% glass for E,W,&S and have a total window-wall ratio of 32.5%. But if you have a North wall with 100% glass and an E,W,&S wall with 40% glass, you now have to prorate each exposure down.
--> (1+0.4+0.4+0.4) / 4 = 55% which is greater than 40%. So, multiply each exposure by 0.4/0.55, and your new window-wall ratios would be N=72.7%, E,W,&S = 29.1%.

Joe Fleming
E.I., LEED AP BD+C, BEMP

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I?d agree ? I?ve always taken the 40% WWR mark in context to mean the gross above-grade building surfaces (minus the roof/skylights of course). Pretty simple.

If 90.1 intended to assign a prescriptive maximum WWR per face, it would say so. Further, rather than a single value I would expect to see a chart showing different maximum ratios for each orientation, and also broken out either by ?solar zone? (using a solar map), or perhaps more simply by latitude.

~Nick

PS: I realize Joe?s example is just that, an example, but if you?re ever modeling a 100% glass orientation, it?s worth nothing that any spandrel glass constructions that don?t transmit light aren?t ?vertical fenestrations? as defined in the definitions, and so wouldn?t count towards the WWR ratio check/calcs.

NICK CATON, E.I.T.

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