While the features and options that are best for replacement windows depend on the circumstances and personal preferences and priorities, most homeowners want increased energy efficiency.  Mid South Windows strongly recommends adding the discussion of visible transmittance (VT) to your discussion of replacement window energy efficiency. Visible transmittance (VT) measures the amount of natural light that the window allows to flow into the home.  VT is one of the common performance measures that are reported for replacement windows.  Each window company that you receive a proposal from should have this information readily available.

Consumers have become more knowledgeable about different measures of energy efficiency such as U-factor and SHGC (solar heat gain coefficient) during recent years. However, there is less understanding that achieving high ratings for performance measures, such as SHGC, can be achieved in different ways. Simply comparing ratings on these various measures of energy efficiency is not adequate to make an informed comparison of two or more replacement window options. It is important for homeowners to be fully aware of any tradeoffs that increase one or more energy efficiency performance measures at the expense of other factors such as visible transmittance, i.e. the amount of natural light that will flow through the windows.

Special coatings are used to drive the solar heat gain coefficient (SHGC) down. SHGC measures a window’s ability to block unwanted solar heat gain.  Some of the manufacturing techniques used to increase energy efficiency can create an unwanted decrease in the amount of natural light that the windows allow to enter the home.  It is important to understand the type of coating placed on the window. The amount of natural light that is blocked varies based on the coating that is used. Not all coatings are the same, even though the SHGC ratings may be essentially the same.

The amount of natural light that flows into the home can have a significant impact on a home’s value, appearance and feel.  Often, too little attention is given to VT ratings when homeowners are researching and selecting replacement windows. It is easy to remember what the VT rating indicates.  As the rating increases, the amount of natural light increases.