Properly sized and positioned skylights can help save energy by providing daylighting , consequently reducing the need for artificial lighting, and by providing warmth through passive solar heating in winter. Like windows, skylights have NFRC ratings for U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage and Condensation Resistance. The Department of Energy recommends that as a rule of thumb the skylight size should never be more than 5% of the floor area in rooms with many windows and no more than 15% of the room's total floor area for spaces with few windows. Skylights that face south should be fitted with a covering or shade for use in the summer months to prevent solar heat gain. The LEED for Homes Guidelines that apply to windows should apply to skylights.

In addition to glass glazing, skylights are available in various plastics such as acrylics and polycarbonates, but these can scratch and yellow over time and they may allow UV light to penetrate causing upholstery and carpet fading. Glass skylights are usually more expensive but they do not discolor or scratch, and with glass you have the same glazing options that apply to windows in order to maximize energy efficiency and minimize winter heat loss – multi-panes, inert gas injection (gas fills), low-e coatings, and heat-absorbing tints if you are more concerned about passive solar summer heat.

In very cold weather, skylights are often prone to form condensation which, if significant enough, may cause water to drip into the room. Look for skylights that have an interior channel to collect the condensate so that it can evaporate later. As with windows, the better the thermal efficiency and U-Factor the less prone to condensation problems the skylight will be. You can also look for skylights that have a NFRC rating for condensation resistance.

Standard skylights, but not tubular skylights or “light tubes” described below, can also provide ventilation.

Daylighting is a primary advantage of skylights. In adition to standard window-style skylights, advances in the design of tubular skylights, or light tubes, allow sunlight to be channeled to rooms all over the house, and some manufacturers claim that they can carry light to stories below the top floor but the amount of light that is transmitted will diffuse the further it travels. Tests performed at the Alberta Research Council, Canada, indicated that one 13-inch tubular skylight had equivalent light output in December of up to one 700-watt incandescent bulb, and in June of one 1,200-watt bulb.

Tubular skylights consist of a dome-shaped light collector mounted on the roof that is designed to direct sunlight down a metal tube that is lined with a highly reflective coating. At the end of the tube the light passes through a diffuser lens that is mounted on the interior ceiling surface; this diffuser lens spreads sunlight evenly throughout the room. The tube is relatively easy to install between the roof rafters. Some manufacturers have designs that include electric lighting as well so there is no need for different ceiling fixtures. Tubular skylights do not result in unwanted solar heat gain. They can be much less expensive than standard skylights, but they do not provide exterior views, ventilation, or passive solar heat gain which may be desired in the winter. Given their relative discrete size and light weight they do not require structural changes, they can be used in situations where other skylights may not be appropriate and do not have a big impact on the exterior aesthetic appearance of the home.