Modifying existing historic windows

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Dear all,

I am modeling a historic building and the scope of work is to modify the
existing windows by adding a storm window to the existing industrial steel
sash windows. The storm window is one layer Low-E glass with aluminum
framing with brake (assembly U-0.67).
Assuming the existing one layer clear glass window has U-value of 5.8, can
I assume the assembly R-value of the window system as the total R-values of
both of the windows as following, and would it be acceptable by the LEED

Total R-Value: 1/5.8 + 1/0.67 = 0.172 + 1.49 = 1.662
U-Value = 1/1.662 = 0.6

I would appreciate your help in this matter,

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The ?assembly 0.67? for a single layer of glass leaves me a little uncertain ? I think if you dig deeper you might find the manufacturer may have some helpful context around that figure. Do they suggest the net impact of the low-e glass, associated framing, trapped air layer, and conductive transfer of the seals in contact with the existing assembly nets an effective U-0.67 for a given/standard window size? If so, your math looks pretty okay for the assumptions stated (just expand on what that 0.67 means in documenting your approach)! If not or if this isn?t made very clear, you may need to perform a more involved analysis to account for one or more of those items.

For savings accuracy with existing/historical window retrofits (not always a priority with LEED certification, understandably), I might also be interested (resources permitting) to test a mockup installation before & after to explore the impact on infiltration reduction in your model. That may more fully capture the cost:benefit ratio if the value of the measure is under active consideration.


[cid:image001.png at 01D327FA.71FB7AD0]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering Manager
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
F 913.564.6380
E nicholas.caton at

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

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Joined: 2016-07-15
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Thank you Nick,

Your recommendations are very helpful as usual. I am trying to get more
accurate numbers from the manufacturer, just wanted to be sure that
assuming the selected values are valid, the calculation would be acceptable.

Thanks again for your kelp


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