Shedding Some Light On Windows

by Dave on May 15, 2009

As seen in Coastal Sussex Weekly, April 23, 2009:

Not long ago, the most important things to consider about windows was muntin pattern and if there were enough of them focused on the homes particular view.  While those factors are still on the list, a focus on energy efficiency has added new items – and more science, as well. New to the list are a string of intense sounding terms – U-Value, Low-E, SHCG, VT – yikes – and materials used for the construction of the windows. These factors combined can increase the energy efficiency of your home, reduce heating and cooling bills, and determine how much maintenance is required to take care of these.  Let’s break down the terms so that we all can understand them and not feel like we were just thrown into a College Engineering class.

The U-Value is the measure of heat transmission through a wall or window. What this means to the rest of us is: the lower the number, the better \at keeping heat inside a building. For our region (Northern Central – requiring heating and cooling) the suggested value for windows is 0.40 or less.  The newest ENERGY STAR 5.0, for our region, will require a U-Value less than or equal to 0.32.

To assist in lowering this value are Low-E (Low- Emittance) coatings.  Basically, these are thin, virtually invisible, metal or metallic oxide layers deposited on the window glazing surface to suppress heat flow. Most windows that have Low-E coatings will state this either in the name or in the product literature.

On to Solar Heat Gain Coefficient (SHGC) – an ominous sounding term describing the fraction of solar radiation admitted through a window, directly transmitted, absorbed and subsequently released inward. The lower a window’s SHGC, the less heat it transmits.  Within our region, it is suggested that windows have SHGC of 0.55 or less.  ENERGY STAR 5.0 will soon require a SHGC less than or equal to 0.40.  While windows with lower SHGC values reduce the need for summer cooling, they also can reduce winter solar heat gain.  Solar heat gain in the winter allows us to not have to run our heaters as often.

This is where Visible Transmittance (VT) comes into the spectrum (yes, pun intended).  VT is the measurement of how much light within the visible spectrum is transmitted through the glazing of a window. The higher the value, the more daylight is let into your home so choosing a window that has a higher VT value while reducing excessive solar heat gain is the goal.

Now that we know how we want our portals onto the world to perform, we need to find out from what materials they are made.  Window construction affects energy efficiency, durability, aesthetics, maintenance needs and, probably most importantly, cost. Wood windows are paintable, stainable, and a good insulator but, unfortunately, wood rots if not painted. Steel and aluminum are durable windows but they easily conduct heat and cold. Vinyl is inexpensive, durable, and can be energy efficient. While not paintable, it can be ordered in various colors. Fiberglass windows, while expensive, are durable, paintable, and strong. Wood windows clad or faced in vinyl or aluminum is another option. The cladding enhances the durability without compromising performance while providing the design flexibility of wood.

Windows are a huge investment – whether building a new home or replacing windows in an existing home.  There are many manufacturers and each offer various construction types and efficiency options.  Despite the dollar and the BTU, the design and style is critical to the aesthetics of your home.  Hopefully, this has helped make the selection process somewhat simplified but do take the time before you buy to select the model that is right for your home.

Matthew Peterson is the Principal of Element Design Group in Lewes. He can be reached at

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