Understanding Window Glass and Energy Efficiency

By Christopher Wittmann

One of the questions I hear from homeowners looking for window replacement is “Are your windows energy efficient?”

Well, yes, they are.  I, and the other Callen exterior product specialists, wouldn’t sell you windows that weren’t energy efficient.  But since this question comes up often, I think it needs to be addressed.

First a bit of history on energy efficient windows: Energy efficient windows were developed during the energy crisis of the 1970s.  As then President Jimmy Carter was putting solar panels on the White House, the Department of Energy began a program at the Lawrence Berkeley National Laboratory to find additional ways to conserve energy; windows were among its targets.  Developments from that research included Low-E coatings, insulating argon glass between window panes, and the development of the National Fenestration Rating Council (NFRC).  The NFRC provides an independent verification of product performance through a rating and labeling system that is clearly visible for consumers to see when they go to purchase windows.

A Window’s Job

There are several components that make a window energy efficient.  In this blog, I will address the glass element because that is what people look at first when choosing windows.

It’s important to understand a window’s basic job, which is to control heat gain and loss, regulate air flow, and provide natural light.  These functions are rated by the NFRC so you, as the homeowner, are knowledgeable about the window’s performance.

Heat gain and loss occurs through conduction, convection, and solar radiation. Conduction is the transfer of heat through the windows to the exterior.  For example, if you touch a hot pan, you’ll feel heat conducted from the stove to the pan.  Heat flows through glass in a similar way.  Convection occurs when warm indoor air meets a cold window, loses its heat, and sinks toward the flow.  More warm air rushes to take its place and the cycle repeats.  You feel this movement as a cold draft.  Solar radiation moves the heat from a warmer body to a cooler one.  Sit by a window on a sunny day and you will feel like an ant trapped under a magnifying glass.

The NFRC measures heat gain and loss (or insulation) through the U-Factor and Solar Heat Gain Coefficient (SHGC).

The U-Factor measures insulation value – the heat from inside a room that can escape.  This rating takes into account all parts of the window – the glass, the frame, and the sash.  The lower the number – .30 or lower is recommended – the better insulation and greater performance.

Solar Heat Gain Coefficient (SHGC) measures the percentage of sun’s solar heat that passes through the window.  Higher numbers mean more heat gain, which you would not want during the summer, but it is desirable during the winter.

Windows also need to regulate air flow – they should be able to open to allow fresh air in and be airtight when closed.  The NFRC use an Air Leakage (AL) number measures air that passes through the window assembly.  The lower the number, the lower the potential for draft through the window.

Because windows are our opening to the world and contribute to how we feel inside our homes, they should provide natural light and frame views both near and far.  The NFRC’s Visible Transmittance (VT) measures the amount of physical light that passes through windows.  Values range from zero to one; a higher number equals more natural light.  Most ratings are between .3 and .8 because manufacturers take into account the light that is blocked from the frame, screens, and sash.

Double vs. Triple Pane

In Wisconsin, we live in a Zone 6 climate, so with our weather a double-pane, Low-E coating and argon gas window will give you the return on your investment.

Do we need triple-pane windows in Wisconsin?  Triple pane windows, from a numbers point of view, would be better than a double pane.  But a lot of it has to do with the environment it is in.  In our zone, a triple pane window’s return on investment is 12 to 15 years out.  A lot of manufacturers try to sell people on triple pane, but they are vinyl window companies.  The question is whether their vinyl will last as long as the window pane.

In urban areas, some may desire a triple-pane window as additional sound insulation.  For those who have a need for that, Marvin manufacturers a “sound transmittance glass” available for their double paned windows.

By choosing a window with a Low-E (emissivity) coating, the glass protects the house from heat coming in from sun in summer, but during winter it blocks heat loss to the outside while reflecting heat back in.  By reducing ultra violet rays, it helps keep upholstery, carpeting, and drapes from fading.

Glass Spacers

Spacers are also an important part of energy efficient window construction.  Spacers keep an appropriate distance between the panes of glass on an insulating unit.  Without a quality constructed spacer, the window can lose much of its energy efficiency.

I hope you found this blog helpful on the fundamentals of window glass.  Again, it’s only one element of having energy-efficient windows in the home.  For any questions on replacement windows, don’t hesitate to call us.  It’s our pleasure to provide you with all the information you need to make the window choice that’s right for you.