WINDOW IDENTIFICATION
First and foremost, the question that you should ask yourself is, "Do I need this?" For some models, replacement is more trouble and cost than it's worth. A good rule of thumb: if you have 2 or more panes of glass, you probably don't want to replace the window. With most new windows you would end up spending between $500-700 to replace each window and frame, resulting in marginal savings [1].
First and foremost, the question that you should ask yourself is, "Do I need this?" For some models, replacement is more trouble and cost than it's worth. A good rule of thumb: if you have 2 or more panes of glass, you probably don't want to replace the window. With most new windows you would end up spending between $500-700 to replace each window and frame, resulting in marginal savings [1].
If your windows are older models or are mostly single pane, then replacing just the glass can give you savings over $15 a year with a cost of only ~$50-100 [1]*. These savings are determined purely from the U-values of a window. However, in southern climates like Houston, the solar energy gained often outweighs the passive energy transfer [3]. Don't forget the impact of the Solar Heat Gain Coefficient (SHGC).
NUMBER OF PANES The number of panes on a window, also called the glaze, has a profound effect. The difference between a single pane window and a window with two panes can often be a 50% cut in energy cost. Oftentimes the glaze of a window will have a much larger impact then the gas used to fill that window [2]. When deciding how many panes you need, you will want to determine whether the impact of a third pane is enough to justify the corresponding cost increase. |
MINIMIZING SOLAR HEAT GAIN
For warm climates like Houston, you have to keep in mind the effects of solar energy. You do this by getting a window that has a low SHGC. If your windows don't have a low SHGC don't worry, you can use external or internal attachments to conserve energy. Outdoor awnings and shutters are simple and visually appealing ways to reduce your solar heat gain. Internal drapes over the windows also have similar effects. But to decrease the SHGC for the lowest cost, you should consider solar screens. Solar screens come in many varieties and can reduce your windows SHGC drastically while also decreasing glare and providing excellent UV protection [4]. These screens are easy to install by yourself and often serve as the cheapest way of reducing your electricity bill.
REPLACING INEFFICIENT WINDOWS
There are two aspects to consider when discussing the incentives of window replacement: the economic cost and the environmental cost.
Economically, considering the U-factor alone, new windows should pay for themselves in 3-5 years by lowering your buildings heating costs [1].
The environmental ramifications of new windows may discourage potential buyers when they consider whether the energy needed to make new windows and the associated replacement energy costs will be offset by the longterm savings. To find the total environmental cost of a new window, our team contacted the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The ASHRAE advised looking at the total power drains of window-making factories and dividing by those factories' operating energy consumption. We were lucky to find a research report that did this and found the average energy cost of a new double paned window to be 27.1 kWh [5]. Just from changing your single pane windows to double paned windows you can expect a decrease 125.5 kWh [2]. That means that in the first year alone in the state of Texas you would stop 126.1 pounds of CO2 from entering the atmosphere by implementing the change[6][7].
For warm climates like Houston, you have to keep in mind the effects of solar energy. You do this by getting a window that has a low SHGC. If your windows don't have a low SHGC don't worry, you can use external or internal attachments to conserve energy. Outdoor awnings and shutters are simple and visually appealing ways to reduce your solar heat gain. Internal drapes over the windows also have similar effects. But to decrease the SHGC for the lowest cost, you should consider solar screens. Solar screens come in many varieties and can reduce your windows SHGC drastically while also decreasing glare and providing excellent UV protection [4]. These screens are easy to install by yourself and often serve as the cheapest way of reducing your electricity bill.
REPLACING INEFFICIENT WINDOWS
There are two aspects to consider when discussing the incentives of window replacement: the economic cost and the environmental cost.
Economically, considering the U-factor alone, new windows should pay for themselves in 3-5 years by lowering your buildings heating costs [1].
The environmental ramifications of new windows may discourage potential buyers when they consider whether the energy needed to make new windows and the associated replacement energy costs will be offset by the longterm savings. To find the total environmental cost of a new window, our team contacted the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The ASHRAE advised looking at the total power drains of window-making factories and dividing by those factories' operating energy consumption. We were lucky to find a research report that did this and found the average energy cost of a new double paned window to be 27.1 kWh [5]. Just from changing your single pane windows to double paned windows you can expect a decrease 125.5 kWh [2]. That means that in the first year alone in the state of Texas you would stop 126.1 pounds of CO2 from entering the atmosphere by implementing the change[6][7].
Click on the button to the right to determine What To Change, a comprehensive guide for our suggestion on when you should replace or supplement windows.
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* $15 per year value taken from a house with an IEER of 10, and an average delta T of 15. The window would be 13.5 sq. ft. going from a single to double glaze
Sources:
[1] Joe Perritt, "How Much Does It Cost to Replace Windows," [Online], 2016 December 2nd, Angies List, from: https://www.angieslist.com/articles/how-much-does-it-cost-replace-windows.htm
[2] U.S. Department of Energy, "Selecting Windows for Energy Efficiency," Whats New in Building Energy Efficiency, https://windows.lbl.gov/pub/selectingwindows/window.pdf, DOE, Pittsburg, PA (1997)
[3] "Energy-Efficient Windows," [Online], 2016 December 2nd, U.S. Department of Energy, from: http://energy.gov/energysaver/energy-efficient-windows
[4] "Exterior Solar Screen'" [Online], 2016 December 3rd, Efficient Window Coverings, from: http://www.efficientwindowcoverings.org/understanding-window-coverings/exterior-solar-screen
[5] G. Weir and T. Muneer, "Energy and Environmental Impact Analysis of Double-Glazed Windows," Energy Covers, volume 39, pages 243-256. (1998)
[6] “How much carbon dioxide is produced per kilowatthour when generating electricity with fossil fuels?” [Online], 2016 December 3rd, U.S. Energy Information Administration, from: https://www.eia.gov/tools/faqs/faq.cfm?id=74&t=11
[7] “Texas: Profile Overview,” [Online], 2016 December 3rd, U.S. Energy Information Administration, from: http://www.eia.gov/state/?sid=TX#tabs-4
[1] Joe Perritt, "How Much Does It Cost to Replace Windows," [Online], 2016 December 2nd, Angies List, from: https://www.angieslist.com/articles/how-much-does-it-cost-replace-windows.htm
[2] U.S. Department of Energy, "Selecting Windows for Energy Efficiency," Whats New in Building Energy Efficiency, https://windows.lbl.gov/pub/selectingwindows/window.pdf, DOE, Pittsburg, PA (1997)
[3] "Energy-Efficient Windows," [Online], 2016 December 2nd, U.S. Department of Energy, from: http://energy.gov/energysaver/energy-efficient-windows
[4] "Exterior Solar Screen'" [Online], 2016 December 3rd, Efficient Window Coverings, from: http://www.efficientwindowcoverings.org/understanding-window-coverings/exterior-solar-screen
[5] G. Weir and T. Muneer, "Energy and Environmental Impact Analysis of Double-Glazed Windows," Energy Covers, volume 39, pages 243-256. (1998)
[6] “How much carbon dioxide is produced per kilowatthour when generating electricity with fossil fuels?” [Online], 2016 December 3rd, U.S. Energy Information Administration, from: https://www.eia.gov/tools/faqs/faq.cfm?id=74&t=11
[7] “Texas: Profile Overview,” [Online], 2016 December 3rd, U.S. Energy Information Administration, from: http://www.eia.gov/state/?sid=TX#tabs-4