Solar Heat Gain Coefficient

Window shopping for new windows

I remember driving with my father in his 56 Ford pickup to the local lumberyard some time around 1960.

My dad was a home builder, and like most builders at the time he would buy windows at the lumber yard. At that time these windows were made of yellow pine, real wood mullions, and a single pane of glass in between each mullion. The bottom line: they didn’t perform very well under harsh weather conditions. I remember the ice build up and the constant re-painting that needed to be done.

However, in this day and age we have aluminum and vinyl maintance free exterior products available. These are more reliable than the windows of yesteryear. When shoping for windows, look for the NFRC (National Fenestration Rating Councel) label. These labels are attached to windows, skylights, exterior doors etc.

So what’s on the label that is so important?

AL (air leakage) for one. AL equals heat loss and gain that occur by infiltration through cracks in the window assembly. The lower the AL (.30 or less) the better.

Next we have SHGC (Solar Heat Gain Coefficient). The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward.

SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. U-factor (U- value) is the rate of heat loss. The lower the U-factor, the greater a windows resistance to heat flow and the better it’s insulating properties.

Last is VT. VT (Visible Transmittance) is an optical property that indicates the amount of visible light transmitted. VT varies between 0 and 1, most values among double – and triple – pane windows are between 0.30 and 0.70. the higher the VT, the more light is transmitted. A higher VT is desirable to maximize daylight. Certain zones in the US have different ratings. Check with the NFRC to find the adjustments in values. If you rember in my past articles I always mention listed and labeled material. This means products are tested in a laboratory under all kinds of conditions.

So when working on all my projects I always read a copy of the National Evaluation Services Report on new and existing products as far as product longevity reliability and performance are concerned.

This helps go above and beyond the call of duty if you will. I like to be thorough when doing something so I don’t upset a customer and have to spend money tearing things out later.

Best Practices. Solar Heat Gain Coefficient, U-Value Definitions ...

Century: Best Practices article, which guides its reader through a basic understanding of this daylighting “code.”   

 While the guidelines for daylighting in laboratories are somewhat different than in offices and public spaces, the article takes a look at conditions for both.  When it is well planned from the beginning stages of the design process, successful daylighting has the ability to save energy, improve productivity and reduce building energy costs.  So, with the proper type of daylighting, well thought-out visible transmittance, a good u-value and properly calculated solar heat gain coefficient, it is possible for buildings to successfully reduce their energy consumption by large amounts.  But what do all of these terms really mean?  Here’s a brief breakdown:

  Side Lighting : energy-efficient layers applied to windows or skylights to reduce the u-factor through absorption or reflection of solar radiation