Lighting

By Craig DiLouie, LC

Well-designed daylighting features can deliver many benefits to the home, such as high light levels, potential for energy savings, a view and connection to the outdoors, and light wavelengths that help maintain human circadian rhythms and immune system health.

One effective way to bring daylight into the home is through the use of skylights, which are windows installed in roofs or ceilings. Skylights provide daylight and, like windows, opportunities for ventilation.

The U.S. Department of Energy (DOE) encourages use of skylights as a way to reduce electric lighting use. At its website Energy.gov, it offers a number of tips about good daylighting. This Light Logic post summarizes these recommendations for choosing, operating and installing skylights.

Shapes

Skylights come in many shapes, with rectangles, circles, ovals, diamonds, triangles, multi-sided and tubular being most common. Tubular skylights feature a roof-mounted light collector that feeds daylight into a mirrored pipe to distribute daylight through a small aperture in the ceiling. This skylight option minimizes heat gain during the day and heat loss during the night but does not offer a view or ventilation.

Skylight size

The size of the skylight is directly related to daylight levels and solar heat gain. The DOE suggests a rule of thumb that the skylight should not be more than 5% of the floor area in rooms with many windows and no more than 15% in rooms with few windows.

The glazing may be flat, domed, arched, pyramidal or “warped plane” (flat on the low side and concave on the high side). The slope or curvature of the glazing may aid in shedding leaves and water.Glazing: Skylight glazing is typically plastic or glass. Plastic is cheaper and less likely to break than glass and is common with non-rectangular skylights, though it scratches easily and can become discolored and brittle over time. Unless the plastic is coated with a special film, it will allow most of the ultraviolet content in the sunlight to pass through, which can increase fade damage to fabrics, furnishings and artwork. Glass skylights, which use tempered glass for safety, are more expensive but are more durable and don’t discolor over time.

Orientation

Skylights on north-facing roofs provide relatively constant illumination with a cool light appearance, while skylights on south-facing roofs provide constant, high illumination levels and the greatest potential for heat gain. East-facing skylights provide morning daylight and solar heat gain, while west-facing skylights provide afternoon light and heat gain.

Energy

Choose a skylight with an energy performance rating that is appropriate for your climate and home design. Various metrics describe how much visible light passes through the glazing, heat gain and loss, air leakage and light-to-solar gain, which indicates the relative efficiency of glazing in transmitting daylight while blocking heat gains. Look for the ENERGY STAR label. Some skylights feature special glazing (e.g., heat-absorbing tints, low-emissivity coatings, insulated glazing) that controls solar heat gain. Other options for reducing heat gain include movable window coverings on the inside or outside of the skylight, and installing the skylight in the shade of leafy trees. Another factor in solar heat gain is the slope or tilt of the skylight. The DOE recommends a slope equal to the home’s geographical latitude plus 5-15 degrees.

Ventilation

Some skylights offer the ability to ventilate hot air that gathers near the ceiling. The skylight may open manually or automatically. Some feature sensors that automatically close the unit when it rains. Larger skylights, installed within a few feet of the floor, can be used as emergency exits.

Moisture control

Poorly installed skylights can be prone to water leaks. Skylights are best installed by professionals in accordance with manufacturer instructions. The skylight should be mounted above the roof surface and feature a curb, flashing and thoroughly sealing joints.   A layer of sheet waterproofing may be applied over the flanges/flashing for protection against ice dams. The DOE recommends avoiding water diversion devices such as roof crickets and diverter strips.

Properly installed skylights can provide the benefits of daylight in many spaces in the home.

What you need to know:

• Daylight provides desirable benefits in a home
• Skylights provide a way to let daylight into the home while permitting outdoor views
• Selection of daylight shape, size, glazing, orientation and energy performance rating will all impact daylight levels and solar heat gain
• Some skylights provide opportunities for ventilation
• Skylights should be properly installed to avoid water leakage