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August 7, 2012
If you have spent any amount of gathering information on windows you have no doubt heard many different terms related to the performance characteristics of new windows. Things like double and triple pane, argon and krypton gas, and low E coatings are product options that are most often associated with a window's thermal performance. You might have also wondered which one of these are most important and how should a consumer evaluate the performance differences between 2 different windows ? Will 2 different double pane windows with argon gas always provide the same level of energy efficiency and comfort? The simple answer is not necessarily and that is one of the problems that homeowners face. How are they supposed to "test out" something before it gets purchased and installed in their home? Luckily there is a 3rd party organization that does a lot of that work for us and they are called the National Fenestration Rating Council or the NFRC for short.
The NFRC is like the Underwriters Laboratories of the window world and you will find labels attached to the windows of various manufacturers that reference their ratings. The NFRC tests windows for things like thermal efficiency and solar heat gain and then assigns numbers to the results so that consumers can then make more informed decisions regarding the windows they are considering for their home.
Depending on which part of the country you live in, one rating might be more important than another. For instance, Solar Heat Gain Coefficient (or SHGC), is a very important factor in the deep South where air conditioning consumes a lot of the energy over the course of the year. It might also be important where a home has a lot of west facing glass and is difficult to cool on summer afternoons. The lower that SHGC number, the less heat from the sun gets passed through the window into our homes.
In our Northern climate however we are a net heating environment and letting the sun in to help us warm our homes can actually be very beneficial. In order to get the best performance in cold temperatures, we should be focused primarily on the U-factor of the window. This number measures the thermal performance of the window unit. Most folks have heard of R-value before when talking about insulation. The bigger the number, the better the resistance to heat loss. Window folks use the inverse of that number or the U-factor to explain thermal performance. The smaller this number the better. A high performance window today will have a U-factor of around .20 (or R-5) or less. Unfortunately, over 90% of the windows being installed today fall far short of that. Most of these windows have a U-factor of .30 or higher and are 50% less efficient than what's available today. It's important to note that 50% less efficient not only impacts the energy efficiency, but also the inside temperature and comfort that the window can provide.
When evaluating window products, demand to see the NFRC ratings for U-factor and use that as a tool to compare the relative performance of the windows you are considering. The investment is too great not to utilize this readily available metric to compare your options !