Low-E Windows and Energy Efficiency Trade-Offs

An low-e window with curtains.

Green energy is not only about building sustainable clean energy products to protect the planet, but it is also about energy efficiency, and that impacts the homeowner right in the pocketbook. As with many industries in the 20 years since I co-founded our business, which became one of the largest replacement companies for vinyl framed windows and doors in the Los Angeles area, I have seen significant strides by most door and window manufacturers to provide more energy-efficient and economical products.

Windows & Doors

A large part of a home’s energy efficiency is a function of the degree to which the home is properly insulated, and having energy-efficient doors and windows is a giant step in the right direction. Changing doors and windows is not only a matter of putting in new ones. There are many things you will need to consider such as view obstruction, lighting, noise, maintenance, ventilation, and building code and safety requirements.

Glass Efficiency

A large window with an unobstructed view frequently serves as the centerpiece of a room. The glass can be as much as 90 percent or more of a picture window. Most municipalities have building codes that require energy calculations that must be met. These calculations take into consideration the square footage of glass in a window versus the cubic footage of living space in a particular room.

The more efficient a window, the less heat that will be transferred from the inside of the house to the outside during cold weather, and from the outside to the inside during warm weather. The U.S. Department of Energy has released an excellent publication on window glass efficiency.

The Tradeoff

The often not well-understood problem with glass efficiency is that the more efficient the glass, the more natural light it blocks. An inexpensive but limited way to improve the efficiency of single glazed windows is to add tinting film; the film blocks light, reduces glare, and some heat. But glass itself conducts heat and cold. One needs only to touch a window on a cold or hot day to know that story.

In order to make glass more energy efficient, glass manufacturers developed dual-pane or double-pane glass. This is two pieces of glass separated by a spacer bar, made of energy-efficient material, and hermetically sealed to prevent air or moisture between the pieces of glass. Heat and cold transfer is less efficient because of the dead space between the two pieces of glass, giving the glass greater insulating value.

Insulated glass does not protect from direct sunlight, so manufacturers coat the interior surface of the exterior piece of glass with a chemical called Low-E. Low-E coating is a thin, nearly invisible coating of a metallic substance that reflects ultraviolet light, the heat-producing rays of the sun. Unfortunately, some light in the visible light spectrum is also blocked, so the more efficient the glass the more light is blocked and the darker the room.

The Moral of the Story

There are different levels of protection and there is something called "spectrally selective glazing," which attempts to maximize the visible spectrum. Confusing. The moral of the story is to do your homework. Discuss the project with your building materials expert and your local department of building and safety, even if you intend to do the actual replacement work yourself.

If you still have more questions about energy-efficient homes you can check out this smart guide to geothermal.