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Selecting Energy Efficient Windows

Many of you who have been following the progress of the cottage at the Garden Home Retreat know the story of how a trio of chance circumstances surrounding a window set the project in motion.  The chain of events kicked off when I saw a window design by the early American architect, Asher Benjamin.  Later I was further inspired by the great windows of the antebellum home Oak Alley Plantation during a visit to Louisiana and the final chapter happened during a meeting with Marvin Windows and Doors while we were discussing a design project I was working on.  When I asked to see some of the windows Marvin had constructed over the past 100 years, there was my Asher Benjamin window, classically built with all the modern conveniences and energy efficiency of today’s technology. 

It was a big day when the windows were installed in the cottage at the Garden Home Retreat because it represented a milestone in the journey toward completing the project.  Being a visual person I was primarily focused on the aesthetics of the windows, so I was bowled over when I discovered just how weather-tight they really are. It’s pretty windy on the ridge where the Garden Home Retreat is located and there isn’t even a whistle around those windows.

Manufacturers have come up with many designs to increase weather resistance and decrease air infiltration to make us much more comfortable during the seasonal highs and lows we experience every year.  For instance, there is single, double or triple pane glass sealed into the frame with insulating air spaces to slow down the transference of heat.  Or we can choose to have these spaces filled with a colorless, odorless gas to increase the insulating performance.  There are also spectrally selective low-e windows that are tinted or coated to block selected radiation wave lengths.  This coating helps improve thermal performance depending on whether we prefer high, medium or low solar gain.  Whew!  That’s a lot to think about!  With all the different combinations to choose from, selecting new windows can be quite challenging. 

In order to alleviate the confusion the National Fenestration rating council was formed to present an unbiased rating system in order to help out the consumer.  Fenestration simply refers to the design and disposition of the exterior openings in a building collectively.  I asked the people at Marvin if they could help me out with an explanation of that rating system and I’d like to pass that information on to you. 

Windows, unlike walls and insulation, are affected not only by the outside air temperatures, sunlight and wind, but indoor air temperatures, air flow and solar radiation around and through them.  The r-value used for walls and insulation do not accurately reflect this interaction.  For windows the energy efficiency is measured with the u-factor and the solar heat gain coefficient (SHGC).  While the r-value measures the resistance to heat (an insulating factor), the u-factor measures the rate of heat transfer or how well a product prevents heat from escaping.  The lower the number, the slower the heat transfers. The u-factor is especially important during the winter heating season and in the northern areas of the country where winter heating is of prime concern.  The solar heat gain coefficient measures the percent of solar radiation (heat) that enters through the window. The smaller the number, the better the product blocks heat from the sun.  This is especially important during the summer cooling season and in the southern areas of the country where summer cooling is the bigger concern.  Those of us that are in the middle and have both heating and cooling concerns have to weigh these two concerns together to approach a happy medium and apply this information according to our own personal preferences.  While we like to stay cool in the summer, we do not necessarily want to block all of the solar heat gain in the winter.

Additional performance ratings are for visible transmittance, which is the measure of how much light comes through the window.  The higher the number, the more light is transmitted. 

The rating for air leakage measures how much outside air comes through the product.  The lower the number, the better. 

And last is the condensation resistance to measure how well a product is able to resist the formation of condensation.  The higher number is better.

The ratings for air leakage and condensation are optional ratings at this time.

With all this in mind, the best action is to select a manufacturer of a window that has a reputation for service and stability, provides a warranty, participates in the NFRC certification and labeling process and energy star windows program and provides a product that is energy efficient.