Getting to Know Your Windows: Energy Efficiency and Durability

Glass and windows have changed a lot since the early days. Leaky, warping pieces of glass are a thing of the past. Today, you can find a number of energy efficient options and styles in almost any shape and style. Before we get too far, however, we need to look at where they've come from and what's available now.

Windows Weren't Always Efficient

Prior to the 1970s, the only thing windows were really good at, aside from letting you see outside, was letting all of the warm air out of your house (or letting the summer heat in). And it wasn't until energy became an issue that homeowners really started to worry about energy efficiency.

Of course, people have used storm windows for a number of years, but they were nothing like today's storm windows. In fact, they were highly inadequate. They usually didn't fit the window all that well. And to make matters worse, they were a pain to install and use. The installation of storm windows also meant you usually had to suffer with a lack of ventilation in your home because you couldn't open the windows more than an inch or two.

Today, however, there are a number of different options available, and none of them generally involve having to hammer another window in front of the ones already installed in your home. Here are a few of them:

Storm Panels

Not to be confused with storm windows, storm panels are tight-fitting pieces of glass semi-permanently attached to a window's sash. They're not hermetically sealed to the original glass, and therefore are not considered to be insulating glass. Instead, a storm panel mounts over top of insulating glass.

And while they can look like triple pane glass, don't get the two confused. Triple pane glass is true insulating glass, meaning it has three panes sealed around two spacers.

Insulating Glass

Invented during the first quarter of the century, insulating windows weren't used much until the 1970s. And back then, they consisted of hermetically sealed glass, separated by a space. The idea was that air is poor conductor of heat and cold. So, having dead air space between two panes of glass will improve the window's efficiency. So, the wider the space was between the panes (standard is 1/2" to 1"), the higher the insulating value of the glass.

Of course, it didn't take long for the experts to figure out that, if they filled the space between the panes with a gas that's less conductive than air (such as argon), the insulation value of the window increased.

Low-E Glass

Today, all window manufacturers offer glass with a low-E coating mostly due to its versatility. To understand how this type of glass works, think of the heat generated by a light bulb.

When you touch it, you feel the heat through conduction because the heat passes directly from the bulb to your skin. Now, if you hold your hand about an inch away from the bulb, you'll still feel heat, but it's radiant heat.

What's the difference? Conducted heat can be slowed through the use of insulation, like fiberglass batting in the wall or a dead-air space between panes of glass. Radiant heat, on the other hand, needs to be reflected away from the thing being warmed. This is how low-E glass works.

Low-E glass works in one of two ways, depending on which way it's facing in the sash. In a hot environment where air conditioning is used to keep the house cool, low-E glass will reflect the sun away from the house.

In areas that suffer cold winter weather, low-E coating will reflect the heat from your home's heating back into the house, so it can't pass through the windows. And how it works depends on which direction the low-E coating is facing and which glass surface the coating is applied to.


Similar to insulated and low-E glass, SunStop is a low-E film that's suspended between the panes in an insulated window. What makes this type of glass treatment popular is that its chemical makeup can be altered and customized to perform different functions.

For example, SunStop can be created specifically to block the UV light that fades the furniture and carpets in your home. In fact, insulated glass with SunStop stops 86% of the UV light that hits the glass, while low-E glass only stops around 72%.

The Race to Offer the Best

With the invention and discovery of insulating glass, triple-pane glass, low-E coating, SunStop, and argon gases, it didn't take long for manufacturers to begin experimenting. They worked hard to research and combine the various technologies in an attempt to create the most efficient windows on the market.

This resulted in a range of windows with varying degrees of energy efficiency. And this range varies greatly, since each window component can have a significant impact on a window's energy efficiency (R-value).

Metal spacers, for example, are often used between panes of insulating glass. But, they also conduct the temperatures they're meant to keep out. This means the center of a window might have extremely high R-value, while the R-value of the window's perimeter where the spacers sit can be relatively low.

Some of today's newer windows, however, eliminate this problem by using spacers made of non-conductive materials, or by preventing the metal spacers from touching the window.

Advancements in technology and the drive to provide the best product have resulted in several styles of windows with varying R-values. But be prepared for a full range of prices, as well. Remember: high quality is an investment, but one you can't exactly afford not to make.