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Windows: Different Summer and Winter U values?
Anyone ever hear of a window, or unit of insulated glass, as having different winter and summer U values?
This wasn't on an NFRC sticker. It was on a glass manufacturer's spec sheet.
They listed it like this:
Winter - Air / Argon 0.35 / 0.31
Summer - Air / Argon 0.32 / 0.28
Our program requires a U value of 0.30 to receive an incentive rebate. So obviously, homeowners and contractors want to average the summer and winter Argon fill U values to come up with an average of 0.295 and call it good.
I think it is just a piece of insulated glass that they want to have installed/retrofitted into their existing wood windows, and get an incentive for that.
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AGC Glass Performance Data.pdf, 246 KB
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Replies to This Discussion
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Permalink Reply by christian kohler on -
The winter U-value is what is shown on the NFRC labels, and should be used for code compliance and incentive programs. It is calculated at -18C (about 0F) outside, and 21 C (about 70F), and a roughly 15 mph wind speed. The summer U-value is calculated with conditions where it is warm outside and hardly any wind. This means that the U-value will be a bit better, but it is not a representative condition for heat losss. The Solar Heat Gain Coefficient (SHGC) is more important during periods with solar gain. So think U-value for heating, and SHGC for cooling.
Christian
WIndows and Daylighting Group
Lawrence Berkeley National Laboratory
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Permalink Reply by Steve Waclo on -
Adam:
Currently in communication with this gentleman:
Andre Anders, PhD
Leader, Plasma Applications Group
Lawrence Berkeley National Laboratory
1 Cyclotron Road, MS 53, Berkeley, California 94720, USA
Tel. 510-486-6745, Fax 510-486-4374
He's circulating the information among his associates and will be in touch following the holiday. Will probably find it's way Christian at LBNL, who has responded since last I checked in.
BTW, Christian's advice to use the winter number is probably not what you wanted to hear regarding the available incentive. If 0.30 U is a common cut off # for incentives, I would expect manufacturers to work to that figure, although in a manufacturing environment, such changes may be a challenge. Perhaps a bit of latitude from incentive providers is in order...who is going to notice a U difference of 0.01 ?
Thanks for surfacing this mystery.
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Permalink Reply by christian kohler on
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Steve, Adam,
Andre forwarded your question to me, that is why I responded here. Many manufacturers have products that meet U=0.30, since that was the cut-off for the IRS tax credit.
If you are looking for even better windows, you can take a look at this website, where you can find listing of manufacturers that meet U=0.22 and less.
http://www.windowsvolumepurchase.org/
Christian
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Permalink Reply by Adam Zielinski on
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Thank you for the replies! Our program does require NFRC stickers, so this guy is going to be out of luck anyways.
But I was curious about the different winter and summer U values anyways. Thanks for the response and clairificaiton!
Adam
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