u factor in replacement windows

Old 03-15-05, 05:54 AM
Gerald Luton
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u factor in replacement windows

I am in the process of installing replacement windows in my home. I was quoted .30 u factor in the sales pitch.
The windows are now at my home and the actual u factor is .32. How significant is the difference of .02?

In soil a change of one figure is measured exponentially,
that is a change from 6 to 5 is 10 times more acidic and a
change from 5 to 4 is 10 times more acidic. A change from
6 to 4 would be 100 times more acidic.

I have tried to research the difference from .30 to .32 on the internet and have had no luck finding an answer.

Could someone please help me with an answer or a web site?

Thank you,

Old 03-15-05, 06:27 AM
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Gerald Luton, Welcome to thae DIY Forums.
I have included a link to a study done by the U of Minnesota and it covers all areas of the country for window U factors and their recommendations. Good luck and I hope this helps you.

Old 03-15-05, 03:27 PM
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It would be practically impossible for a salesman to tell you what the u-value of your windows will be, since the u-value changes depending on the overall glass size. I'm amazed he was within .02. I think it's misleading when salesmen do that, unless they tell you up front that it's only an estimate- or- that it's based on a window of a set size, whereas your windows will be either larger or smaller, and the value will vary, based on that.

Regarding your question, .02 is not that significant of a difference. I believe it would be correct to say that a .32 window is 2% less efficient than a .30 window. (only when the temperature inside is 70 F, and 0 F outside.) As the temperatures become closer to being the same, the difference is negligible.

U-factor (U-value) is a measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-F (W/sq m-C). Values are normally given for NFRC/ASHRAE winter conditions of 0 F (18 C) outdoor temperature, 70 F (21 C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.

For a typical residential-size window (3 by 4 feet/0.8 by 1.2 meters), changing from a standard aluminum edge to a good-quality warm edge will reduce the overall window U-factor by approximately .02 Btu/hr-sq ft-F.
Old 03-15-05, 05:47 PM
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U factor

What he said! That was a really good reply...couldn't have said it any better.

Last edited by Oberon; 03-15-05 at 05:48 PM. Reason: none really, just more to say.
Old 03-15-05, 06:48 PM
Gerald Luton
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u factor

Thank you for all of the replies. I feel better about my deal already.


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