The Basics of Window Energy Rating (ER)
Windows and patio doors are the most expensive building components in your home. They are also a major operating expense because they account for the most heat transfer (loss) between your living environment and the outdoors. Where a typical insulated wall has an R-factor of R-20, standard double glazed glass has an R-factor of R-2. So we gain or lose 10 times as much heat through our windows and patio doors as through our walls. There are several things that can be done to windows and patio doors to minimize heat transfer, but these all add to the building cost. One simple thing that is free and will save you money is to minimize the windows and patio doors with a westerly exposure.
There are a wide choice of windows available. In this article I will provide some targets for you to think about.
There are important government and industry standards that will help you choose the right windows for your climate and your budget. The two most important standards are the Resistance to heat flow or R-value, and the Energy Rating or ER. Fortunately, these standards are consistent across the US and Canada making window comparisons possible.
The ER standard is a valuable consumer tool because ER is based solely on a window's performance regardless of it's materials or how it was built. The higher the ER the better.
A common, poor performance, fixed window, with an aluminium spacer between the glass panes and double glazing (2 panes of glass), might have an ER of -35. The minimum ER for what is considered to be a high performance fixed window is +2 and the ER for the opening version of the window is -11. A moderate cost, high performance, fixed window, with insulated spacer, double glazing, low-E coating, and argon filled should have an ER of +5 and the opening version should have an ER of -8. The highest performance, fixed window, with insulated spacer, triple glazing, low-E coating on 2 surfaces, argon or krypton filled gap has an ER of +15 or more. It is also the most expensive.
If you are concerned about energy costs to heat or cool your home, you must consider the ER number when choosing your windows. There are many other considerations including appearance, price, durability, condensation resistance, wind resistance, water resistance, lower sound transmission, and sunlight reflection coatings. But, generally speaking, if windows are certified to have high ER numbers they will include many of these items.
Be sure to buy windows that have a visible certification on the window, certified by a trusted standards agency, and are guaranteed to meet the standards that you require.
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