Pros And Cons Of Skylights

Q: Adding skylights to our rambler sounds like a good idea. What are the advantages and disadvantages of skylights? Are they expensive?

A: Adding skylights to an existing home can be a big step. They're a fairly expensive improvement. They require good advance planning and high-quality installation. Check with a building contractor about factors affecting installation - not every home, or every room in a home, is a good candidate for lights. Also, consider the following:

-- Choose the right location. Carefully planning the location of skylights will help achieve an outcome you appreciate. A room or an area with a roof that faces two compass directions allows more choices than one with a single roof. The compass direction determines how much direct light enters through the skylight. South-facing skylights receive the most direct light. North-facing skylights receive little or no direct light. East-facing skylights will bring in morning light, west-facing the afternoon light.

-- Select the right skylight. Evaluate size, glazing material and energy performance of different skylights.

-- Size: A small, well-positioned skylight can illuminate a much larger floor area. The surface area should be big enough to bring light indoors and to open up a view, if desired. It should not be so large that it brings in a lot of glaring light during warm weather or allows significant heat loss in cooler weather.

-- Glazing material: The type of glazing you choose will also affect heat gain, glare and heat loss.

Both clear and diffusing skylights can be made of glass or plastic. Both come in flat sheets. In addition, plastics can be shaped into bubbles or ridges. Because a flat surface has a smaller glazing area, it has less surface through which to lose or gain heat. It also has a lower profile on the roof.

-- Energy performance: Unfortunately, both glass and plastic have relatively low R-values. A higher R-value indicates a material that resists heat loss. U-value is the inverse of R-value. It measures the rate of heat flow through a material. The lower the U-value, the less heat will flow through it. When choosing windows, you want a low U-value or a high R-value. An R-value of 2.5 is considered high for windows.

You should also consider whether the stated R-value refers to the entire skylight package or only the glazed portion. The R-values of the frame also have to be considered. Insulated Fiberglas window frames perform slightly better than wood or vinyl frames. Thermal breaks improve the energy performance of aluminum frames, which have the lowest R-values.

In addition, glass can be coated with Low-E substances. Low-E refers to low emissivity, or the ability of a material to absorb or emit radiant energy. The coating limits transfer of radiant heat between panes by reflecting heat back into the home in cold weather and back to the outdoors in warm weather. If heat gain may be a problem, it is better to use a selective or high performance low-E coating rather than tinted glass as the tint reduces the amount of visible light.

-- Follow code requirements: The Washington State Energy Code requires glass to be double-paned or triple-paned in heated spaces. The space between the double panes contains a pocket of still air that helps reduce conductive heat losses, thereby increasing insulation value. You might also choose to exceed code requirements to obtain even better insulation. For example, the R-value further increases when a third pane is added. Using argon or krypton gas between each pane raises the R-value even more.

If you have questions about skylights, contact the Washington State Energy Office's Energy Hotline at (800) 962-9731, or the Lighting Design Lab at (206) 325-9711.

Prepared by the Washington State Energy Office. People with disabilities who need this information in alternate formats should call 1-800-962-9731 (voice or TDD).