Everybody knows that windows account for a large portion of the heating and cooling expense of a house. Seldom is any type of energy audit completed on an older home where the first recommendation is not for better more energy efficient windows. While this seems to be common knowledge, some people want more than just to accept this as true fact and to understand why.
There are a lot of models used for a true home energy audit and they can be very expansive and in-depth to a level that quickly leaves the average person hopelessly confused or bored. In addition, it would require a series of articles to accurately explain how a full home energy audit is conducted and this is really just to answer a basic question about windows. The reason the question always comes up about windows though is because such a large portion of a home’s recoverable heat loss is through the windows.
To understand why the loss through windows is so important you have to understand a little of the math involved in the calculations. The most important number to be concerned with involves the R value and U factor. Most have heard of R value before- it is the value used when insulating walls and ceilings where the higher the number the better insulating value. It literally means “resistance value” and measures the resistance to conduction of heat through a surface.
The U factor is the exact opposite of the R value and measures the heat that is lost through a surface. To get an R value simply divide 1 by the U factor. For example, a decent standard double pane window may have a U factor of .40 (The U value can be found on the labeling of all windows in the energy efficiency panel). The lower the U factor, the more energy efficient a window is. In this example, you could convert the U factor of .40 to an R Value by 1/.40 = and R value of 2.5. Compare this to attic insulation in most homes in where they recommend a minimum attic insulation R value of 30. Simply put, while the windows are small in total percentage of the exterior of a building, they are by far the largest losers of heat.
When you sit down with a “window expert” to hear the sales pitch, the difference in the numbers seems quite small. If you have the old single pane double hung windows in an older home with a U factor of 1 and they start telling you for only $7k you can have windows with a U factor .40 it is easy to lose interest and decide to give it another couple years. This is particularly true if you are trying to do the work yourself and change your own windows which is becoming more and more common.
These random numbers have no meaning so to put them into perspective understand most homes lose about 60% of their heat through windows. If the annual heating/cooling cost of the home is $5000 then $3000 of that is attributable to windows. The difference then between your old heating bill and your new (being 2.5x as efficient – the difference between a U factor of 1 and a U factor of .4) will be $1800. You stand to save $1800 a year in energy costs (plus save all the associated carbon emissions with that energy) or pay for the windows entirely in this example in 3.8 years.
Where you really need to use these calculations is in deciding which new windows to get. While modern windows are often 2.5x as efficient as older windows, looking at the difference between good windows and great windows is where things get confusing for many. The difference between a decent double pane vinyl window with low e coating and the top of a line triple pane, argon or krypton gas filled, glazed window will be a difference in U factor from approximately .4 to .2.
While a quick look and a less than forthright sales person my tell you that means it is twice as efficient (and will reflect that by being at least twice the cost), you need to compare that to how much more efficient is it than the better window, not the original window. In this case, we started with $3000 in energy cost for windows, the “good windows” saved $1800, and the “Best windows” will save $2400. This means the “best available” window saved only $600 more than the “good energy efficient window”.
If the best windows available do in fact cost twice as much as the good windows which would be fairly typical, then now it will take 5.8 years to recoup the cost. While any energy savings is worthwhile, in the real world of budget based decisions it is important to understand the somewhat complex methodology that must be used to determine actual energy savings.
In an ideal world the best windows are still the best bet over the long term, but when working with budgets it may be worth considering what other savings could be found with that $7000 extra dollars used in this purely fictional example. Perhaps instead of the best available windows, looking to invest that $7000 extra into better siding or a better furnace or A/C unit would be a better investment for both you and the environment. The only way to know for sure though is to take the time to do the math and see. There are no shortcuts when it comes to saving energy and saving the environment.
