I am trying to determine whether a difference of .05 U-value in window performance is worth the extra cost of the higher U-value window. We have a 1908 house, which we are trying to preserve the period look of and get a red-colored clad double-hung window. We have narrowed the choices down to Kolbe and Sierra Pacific. The Kolbe windows have a U-.30 and the Sierra Pacific windows have a .35 rating. The total Kolbe window package is $25,500 and the Sierras are $17,302. We are doing our project in two phases, so between this year and next, we can get about $2,000 back in Federal rebates with the Kolbe windows, but hardly enough incentive to make Kolbe a compelling choice. Our mechanical guy did a heating load calculation when designing the system, and he came up with 22831 Btuh with double low-E glass. He estimates our total house load will be 39503. This is with R-28 average walls (double staggered studs at our addition, 2” polyisocyanerate over our existing walls, and 6.25” of Styrofoam in our basement ICF walls – we replaced our existing foundation), R-50 attic, and R-15 below the new basement slab. We asked him to do an analysis of the difference between the two window U-values and he said that his program isn’t that sophisticated, but that we could easily do a manual calculation ourselves.

Here’s our analysis:

Portland, Oregon
Winter design conditions
Outside temp 15°
Inside temp 70° DeltaT = 70° - 15° = 55°

DeltaT x U-value x Area = Heat Loss Btuh

Sierra 55 x .30 x 549sf = 9059 Btuh

Kolbe 55 x .35 x 549sf = 10568 Btuh

If calculating the window performance load is as straight forward as I have shown, there is about 1500 Btuh difference between the windows or about a 4% increase in total heating load.

Questions:
1. Are there other factors involved in comparing window performance?
2. Why is the mechanical engineer coming up with 22831 Btuh with the same window area?
3. With our ground source heat pump and hydronic radiant floor heating system, the engineer estimates our annual electric heating cost to be $337 (yes, we thought that hard to believe, but he has 20+ years of residential ground source examples to back that number up), so are we completely off our rockers to even be considering the benefits of saving 4% of that annual cost. We do plan to live in our house for many years and are trying to save energy in every way possible, so there is the “principle of the effort” argument.
4. If there is so little difference in the energy savings performance, why has the government set the bar at .30, when many existing window products fall in the .31 to .35 range?

Thanks All

The differences are so slight in your circumstances, I wouldn't worry about it at all. If you are going to seriously upgrade the windows from .35, I would be looking in the .20 range, if anything. If you have run the calculations correctly, $13 a year isn't that much energy, take that $8000 difference and invest it in a CD at 2% and make ten times as much each year. If you are trying to minimize energy usage and not money saved, put that money into solar panels, which don't make much financial sense compared to the mentioned CD, but will make you more than $13 a year.

Go to Builditsolar.com find the insulation calculator and plug in the numbers. It will give you a savings analysis ten years out. The federal tax credit is a max of $1500.

For the $8,000 difference I would go with the lower cost ones and then add insulated window shades. Insulated window shades can double the R-value of these windows at a fraction of the cost as well as give you control over it. Also is there a difference in the SHGC? The ones with the higher U-value might have a higher SHGC offsetting any saving in heat loss by gaining it, at least on your south facing windows.

Tom Pittsley
ecobuilder@aol.com
www.eebt.org

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Thanks for the input Tom,

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We are planning on putting in pleated shades.  However, they don’t help us with our HERS scoring, because they require user operation and are an uncontrollable variable.  I did not realize they could double the u-value of the window, so we’ll look into the best type that will reduce drafting around the window shades.  Do you have any direct experience with using them effectively in your projects?

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I had done my initial planning for energy savings on our house using a two month subscription to HVAC-Calc.  It was a very easy to use program and produced results within about 15% of what the mechanical engineer came up with. 

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I recently contacted Don Sleeth of HVAC-Calc (by email at first, which he did not respond to) by phone to see if he could tell me what the actual U-values that where used in his software.  He said he did not know, but they where based on the Manual-J calculations, and I could go to it to find out for myself.  He also said that “there’s a lot more to it” when calculating window heat loses than the NFRC ratings, but I got cold silence from him when I inquired as to what those where.  Does anyone know what the other factors for window heat loss are?  I do not have access to Manual-J and undoubtedly would not have the expertise to understand it anyway, but I have spent a bit of time searching the internet to determine what those other variables are with no success.

Thanks.