Are skylights energy efficient?
Short answer. Modern ENERGY STAR skylights with low-e, insulated glass are much more efficient than older single-pane domes, but any skylight still lets in more summer heat and loses more winter heat than the insulated roof it replaces. In Georgia's cooling-heavy climate, a low solar heat gain rating, careful placement and shading make the difference.
Read the label
Skylights carry an NFRC label with two key numbers.
- U-factor. How well it insulates. Lower is better.
- Solar heat gain coefficient (SHGC). How much of the sun's heat comes through. Lower is better in Georgia, where cooling dominates.
ENERGY STAR sets limits for these numbers by climate region, so check that the model qualifies for yours.
What makes a skylight efficient in Georgia
- Low-e coatings and insulated glass that block much of the heat and ultraviolet light.
- North or east orientation. Steady light with less afternoon heat than south or west.
- Modest size. A common rule of thumb from the Department of Energy is to keep skylight area to about 5 percent of floor area in rooms with many windows and no more than 15 percent in rooms with few.
- Shades. Built-in blinds or solar shades cut summer gain.
- Venting. An operable skylight lets hot air escape at the ceiling.
The hidden loss: the light shaft
The shaft between the roof and the ceiling passes through a hot attic. Insulate its walls to the same level as the attic and seal air leaks around it, or the shaft becomes a heater in July.
Tubular skylights
Small tubes with reflective interiors bring light into closets, halls and baths with far less heat gain than a full skylight.
Older skylights
Plastic domes and early units with failed seals are often the least efficient part of the roof. Replacing them during a roof replacement improves comfort and removes a likely leak point.
See replacing skylights with a new roof and attic heat and shingles.