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Dear Pablo: I am thinking about replacing my windows to cut back on my home's energy usage. Will this yield a good return on my investment or is my money better spent elsewhere?
Most windows can be compared to a giant hole in your home's insulation. While a standard wall might have an R-Value (resistance to heat flow; higher is better) of 13-19+, a single-pane window is no better than 1. A gas-filled double-pane window can reach a R-Value of 3, or closer to 2 once the seals have failed and the insulating gas has escaped. Of course there are triple-pane, gas-filled, low-e windows with insulation-filled frames that would have even higher R-Values but most of us don't have that kind of money.How Much Will It Cost?
According to the National Energy Assistance Director's Association's 2009 Energy Assistance Survey, 37% of households pay $2,000 or more in home heating costs each year. Windows typically cost between $300 and $700 each but can be well over $1,000 for the fancy ones. Assuming an average window replacement cost of $500 for each 3'x4' window and ten windows to be replaced, you are looking at spending at least $5,000. Even if new windows could completely eliminate your heating bill (they can't) you can already expect a payback period of greater than 2.5 years, which is on the outer range of acceptability for more corporate decision makers. Luckily household economics are a little bit more lenient so we don't need to write this project off just yet.
How Much Will It Really Save?
Lets assume a 2,000 square foot (~45'x45') house with eight foot ceilings. This house would have 5,440 square feet of ceiling, floor, and wall space, of which 120 square feet represent ten 3'x4' windows. If your windows currently have an R-Value of 1 and the rest of the building envelope is insulated to R-13, your building's average R-Value would be 12.73. Replacing your windows with windows rated to R-3 would increase this to 12.78, or 0.4%. Saving a couple of dollars a year on your heating bill is probably not worth spending $5,000. In fact, a study mentioned in this TreeHugger.com article says that the payback period of replacing old wooden windows is up to 400 years!
Of course there could be additional reasons to justify your window replacement. Replacing windows, especially if the old ones are falling apart, can add value to your home if you are planning on selling in the near future. If your old windows are drafty, you will be losing far more heat through them than a few dollars a year, as well as compromising indoor comfort and air quality. Finally, if your windows are just plain broken and need to to be replaced, or if you are building a brand new house, it does pay to spend a little bit more on better windows.
What Should I Look For In A Good Window?
There are a lot of different metrics used with windows. Here is an overview of what they are and what to look for:
- U-Factor - The U-Factor is simply the inverse of the R-Value (1/R) and zero is the theoretical best, allowing no heat through. A window with a U-Rating of 0.35 is equivalent to a R-Value of 2.86.
- Shading Coefficient (SC) - The shading coefficient compares the solar heat gain through a window to a single sheet of 1/8" glass. In hot climates a lower SC will block more of the sun's heat, but in colder climates a higher SC is desired to let in more heat.
- Solar Heat Gain Coefficient (SHGC) - The Solar Heat Gain Coefficient takes into account the heat absorbed by the glass and is approximately 87% of the shading coefficient.
- Visible Transmittance - Visible Transmittance defines how much outside light is blocked by the window. A tinted window will have a lower transmittance.
- Low-e glass - Low-e glass has a colorless coating that transmits light but reflects heat. This keeps heat in or out and adds to the R-Value of a window. Normal glass has an emissivity of 0.84 and Low-e is defined as 0.35-0.05.
- Air Leakage - Air leakage ratings tell how much air passes through any gaps in the window assembly in cubic feet per square foot of window.
Where Might My Money Be Better Spent?
Unfortunately most homes do not have a lot of energy saving measures with a low ROI. Aside from the obvious, like unplugging your beer fridge, replacing your light bulbs, and turning the thermostat down (or up in the summer, if you have air conditioning), there is not much a homeowner can do without putting down a lot of cash with little reward. Of course energy savings is only part of the equation. Comfort level, which is hard to quantify and put a dollar value on, can be greatly improved by installing a heat recovery ventilator (HRV) or any other new appliance.
One undertaking that might have a more reasonable ROI is insulation. Since replacing wall insulation is a bit more involved and costly, we are typically limited to replacing floor or ceiling insulation. Attic insulation like cellulose or fiberglass can be blown in using what resembles a giant vacuum cleaner running in reverse. Some local equipment rental stores or home improvement warehouse stores even rent this equipment by the hour. Loose fill insulation typically has an insulating value greater than R-3.5 per inch. Assuming that our hypothetical house already has four inches of loose-fill insulation (~R-13) we can add another four inches to bring it up to R-26 for very little expense and a 37.5% increase over our previous average insulation value.
Pablo Päster is a weekly columnist for TreeHugger.com, an experienced greenhouse gas engineer and the Senior Environmental Program Manager at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit the via this form and connect to his RSS feed.
More TreeHugger Articles on Windows and Energy Efficiency
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Building The Green Modern Home: Looking At Windows
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