Skylights brighten up dark rooms. Even small units make a big difference in the way rooms look and feel. There are many brands and several styles to choose from. But I advise my clients to stay away from inexpensive units with plastic glazing. Although they are cheaper initially, the bottom line is they don’t last and many of them leak. The better choice is a high-quality curbed skylight with energy-efficient glazing—and the factory flashing kit made for it. The kit won’t add much to the cost, but it will mean a leak-free installation. Skylights come as venting units, which open, or as fixed units, which don’t. A venting unit might seem like a good idea, but it is inconvenient to open and close when it’s out of reach. In fact, many customers who order them later tell me they rarely bother to track down the special rod needed to operate their windows. So unless the skylight will be easily accessible, buy the fixed unit and save yourself some money. This even applies to putting a skylight in a bathroom. But it’s essential to make sure the room has adequate ventilation so condensation doesn’t become a problem. A vent fan for a small bathroom should provide 1 cubic foot per minute of air circulation per square foot of floor area, or about eight air changes per hour. For more on sizing a vent fan, go to the Web site of the Home Ventilating Institute. Old-fashioned skylights were simply a single thickness of glass in a frame, but today they come with laminated or tempered glass, and low-e and tinted coatings to control heat transmission and UV radiation. Just like windows, skylights are rated for their thermal efficiency by the National Fenestration Rating Council. You can compare the U-values as well as heat and light transmission rates of various skylights. Even though tinted glass is available, I generally recommend that you stick with clear glass because it lets in more light. If intense sunlight does cause too much heat buildup or begins to fade carpeting and furniture inside, add a shade or screen. Skylights are available in sizes that fit standard 16- or 24-inch framing. Adding a large skylight means that the installer will cut one or more rafters (see illustration). This is not difficult when the roof is conventionally framed—rafters on the sides of the rough opening are doubled up and headers are added at the top and bottom of the opening. But a truss roof is different. Trusses are carefully engineered to carry roof loads, and modifying them in the field is a bad idea. If your home has trusses, make sure the installer sticks with units made to fit between roof members, or gangs several smaller skylights together to create a larger window.