Replacing Old Windows with “Green” Windows

 

According to the U.S. department of Energy, heat transfer through the windows accounts for big losses in the energy bill.  They say that you can reduce your heating or cooling bill ten to twenty-five percent by simply replacing old, single-paned windows with new, energy efficient windows.  Anyone who is looking to install new windows in an existing house should examine the cost and see if by replacing the windows there is an acceptable payback period.

I will do analysis of a typical case that a potential customer might face when looking into installing new windows in an existing house.  I will analyze the how an old, single-paned window performs against a triple pane window with ¼” gaps between panes (6) and then how cost effective this swap could be.  The first step is to determine how much heat transfer occurs through the windows, which corresponds to the energy in kWh, that is wasted through the window. To get the most accurate results, an average heat transfer rate from the sun was obtained over the course of a year for the Austin area.  The data would be biased if we were to look at heat transfer for one month in particular, for which there could be extreme heat or extreme cold.  Using an excel sheet created by Colorado Energy (3) the annual energy loss was calculated for both the new and old window.

This excel sheet determines the loss in energy based off of the U value of the windows, the area of the windows, the heating and cooling days per a year, the energy cost, and the cost of the new window.  The rate of heat loss is indicated in terms of the U-value of a window assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating properties (2).  The area of the window I chose is 49 square feet.  The heating and cooling days were found from HomeinSight (5) and the U-value was determined from (2).

Using the excel sheet with the aforementioned data an old, single-paned window would have an annual energy loss of 927 kWh compared to an annual energy loss of 612 kWh with the new window.  The result from installing the new windows is that 315 kWh of energy are saved each year.

With this amount of energy saved, the amount of money saved and the length of the payback period can be calculated. Assuming that energy costs around 11 cents/kwh, which is conservative, approximately $34.00/year is saved from installing this new window.  The installation of a new window with the all the material properties stated above, costs around $400 (8).  Using this information the calculated payback period is around 11.5 years, and this time does not account for inflation in the energy prices.  So if you are planning on staying in your house for this amount of time, replacing windows could be a smart idea, but consider how long it takes to justify buying new, energy efficient windows.

References

(1)    Calculating Heat Transfer Through Windows. (n.d.). Retrieved March 27, 2012, from http://gaia.lbl.gov/‌btech/‌papers/‌12486.pdf

(2)    Efficient Window. (n.d.). Retrieved April, 2012, from http://www.efficientwindows.org/‌ufactor.cfm

(3)    Energy Usage Calculator. (n.d.). Retrieved April, 2012, from http://www.google.com/‌url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCkQFjAA&url=http%3A%2F%2Fwww.azsfb.gov%2Fsfb%2FEnergy%2520Efficiency%2FExisting%2520Schools%2FEnergy%2520Usage%2520and%2520Payback%2520Calculator-Insulation%2520Gas%2520Heating.xls&ei=JlV7T_TpK-OC2AWX04itAw&usg=AFQjCNEO53KJljTgLAvbyJ3aTQTqxPdRUA&sig2=r1TcEruwBe_y-B2A04iRew

(4)    The Facts About Solar Heat Gain & Windows. (n.d.). Retrieved March 27, 2012, from http://www.nfrc.org/‌documents/‌SolarHeatGain.pdf

(5)    Home in Sight. (n.d.). Retrieved from http://www.homeinsight.com/‌home-value/‌tx/‌austin.asp

(6)    R-value table. (n.d.). Retrieved April 2, 2012, from http://www.coloradoenergy.org/‌procorner/‌stuff/‌r-values.htm

(7)    Sustainable By Design. (2009, January). Retrieved March 19, 2012, from http://www.susdesign.com/‌windowheatgain/‌index.php

(8)    triple pane systems. (n.d.). Retrieved April, 2012, from http://www.triplepanesystems.com/

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