Location is crucial for skylight efficiency

September 16, 2007

I’m going to be remodeling my home and I’m considering adding a skylight. I like the idea of additional light in the house, but am worried that a skylight might increase my heating and cooling bills. What do you suggest?

The additional light provided by a skylight can make a home feel much more open and cheerful, especially on bright winter days. Even in cloudy weather the light is frequently enough to reduce the need for electric lighting.

A skylight can be a positive addition to a home if it is properly located, or a real nuisance if not. Skylights located on south or west roof slopes can let in too much sunlight in the summer, resulting in unwanted heat gain and potential fading of carpets and furniture due to ultraviolet light. Movable shades or heat reflective coating on the glass surfaces can help.

What makes a skylight more energy efficient?
You’re probably familiar with R-values of insulation, which is a measurement of resistance to heat loss. The efficiency of skylights and windows is measured in a similar fashion with a U-factor. This is just the opposite of an R-value.

U-factor measures the heat transfer through a skylight, window or door and tells you how well the product insulates. A lower U-factor means better insulation value, hence greater resistance to heat flow in and out of your home.

Understanding the ratings
A skylight with a U-factor of .50, which is the Energy Star® standard, converts to an R-value of 2. When you consider that homes today have R-38 insulation in the attic, R-30 in the floor and R-21 in the walls, R-2 is going to be the path of choice for heat to leave the home since heat seeks cold and follows the path of least resistance.

Solar Heat Gain Coefficient is the fraction of solar radiation admitted through a skylight, window or door. This tells you how well the product blocks heat caused by the sun. The lower SHGC, the less solar heat it transmits. Skylights are great for letting light into a home but energy wasteful especially in the winter.

A standard skylight isn’t much different from a window. In fact, the more efficient skylights use the same technologies as windows, but these technologies are even more valuable for skylights, which receive direct sun in summer and greater outside/inside temperature differences in winter.

Tube-type devices, which are much more energy efficient, gather sunlight at the roof and transmit it down to a diffusing lens mounted in an interior surface, usually a ceiling. This natural light can illuminate closets, bathrooms, hallways or other spaces that typically would not have access to sunlight, decreasing the need for electric lighting.

If you decide to buy a skylight or two, you can easily compare competing products. Manufacturers are required to provide both the U-factor and SHGC ratings on skylights, as well as windows and doors.

But the easiest way is to select a skylight with the Energy Star label. We like Energy Star products, because they have been certified to meet high standards for energy efficiency, regardless of whether you’re buying a refrigerator, computer or a skylight.

Tax credits
Energy Star skylights are eligible for a federal tax credit of 10 percent of the cost, up to $200 credit. You’ll want to consult your tax adviser for more details.

To get the most out of your new skylight, be certain that it is installed according to the manufacturer’s instructions.

The decision to purchase skylights or solar tubes comes down to this: while they do let in some natural light, you’ll generally end up with higher costs for heating and cooling. So you’ll have to decide whether the benefits outweigh the added costs.