Lighting with Skylights
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Sunlight from above makes a house feel more open and lively and can reduce the need for electric lighting. But skylights can bring headaches if you aren't careful about your choices. They can cause condensation and draftiness in winter, and overheating in summer.
Purchase skylights with the lowest U-factor you can find, and be particularly picky about the frame and spacers. Reducing solar heat gain with a low SHGC is also very important for skylights, because they are more exposed to the sun in hot weather. Get a skylight with a low infiltration rating, and make sure it is tightly installed and caulked. For ventilation and summer cooling, look for a skylight that can be opened to allow warm air rising toward the ceiling to escape.
Choosing and Installing Skylights
Many skylights are made of plastic domes, ridges, bubbles, or other shapes, instead of flat glass. Because of the way they are molded and sealed, these plastic glazings cannot incorporate low-e coatings and gas fills, and they generally don't insulate as well as flat glass. In addition, shaped skylights have more surface area than flat glass in the same size opening. This is more area for heat to escape or for unwanted heat to enter.
Even with double-pane flat glass, skylights have higher U-factors (by about 20%) than similar vertical windows, simply due to the angle of the skylight. The air or gas between the panes doesn't remain as still when the unit is not vertical, lessening its insulating value. Narrower air spaces help to reduce this effect, and many double pane skylights now have air spaces of about 1/4 inch.
Skylights do not need to be large to provide substantial daylight. In fact, a skylight can illuminate a room with a floor area 20 times the area of the skylight. If the skylight will have a light well through the attic, splay the walls to admit more sunlight and distribute it more broadly into the room. Make sure the light well is insulated where it passes through the attic and that it is well sealed from the attic and outside.
The supporting frame for the skylight, called the curb, also adds area for increased heat loss. The curb fits on top of or between roof rafters, and could be made of lumber as large as 2 x 6. To prevent air leakage and heat loss, make sure the curb fits snugly, insulate it, and caulk the joints.
Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)