Roof windows have become increasingly popular as homeowners and designers seek to better utilize space in smaller houses by creating more habitable rooms under sloping roofs. They are glazed with glass rather than plastic and are available with most of the glazing and solar control options of standard windows. Operable roof windows can be opened manually or by a motorized system. In addition, some manufacturers offer special venting mechanisms that allow some ventilation air flow without actually opening the window. Operable roof windows, or vents allow hot air that rises to the ceiling level to be effectively exhausted from the space. Roof windows have rotating sashes to allow cleaning of the exterior pane. Most roof windows can be equipped with operable interior shading systems to diffuse or reject intense sunlight.
- Flush Mount: A flush mount skylight is placed on the roof deck with no curb. It is overlapped by and sealed to the roofing material.
- Curb Mount: A curb mounted skylight is placed on a curb raised above the roof plane. This type of curb is not supplied with the skylight. The skylight may be fixed or operable.
- Integral Curb. In this case, the curb is supplied with the skylight as a complete unit. The skylight may also be fixed or operable.
Installing a skylight or roof window presents the same array of concerns found with windows in vertical walls. The roof window and skylight must maintain continuity with the roof/roof covering of the building envelope. However, they must also meet much more stringent conditions for shedding water.
Skylights and roof windows present special installation challenges because they are typically set into the thickest, most heavily insulated framing in the house. Sometimes an insulated well must be created when there is an attic space. In all cases, the integrity of the insulating envelope, air and vapor retarders, as well as the exterior water drainage plane must be maintained. Curb-mounted skylights, rising 6–12 inches above the roof, create additional heat loss surfaces right where the warmest air of the house tends to collect. Some manufacturers provide curbs prefabricated out of a rigid insulating foam, which can be further insulated at the site.
Light tubes are another method for providing overhead sunlight in a room. A clear plastic dome is mounted on the roof and is connected to a small light diffuser in the ceiling by a highly reflective tube. The tube is relatively small (10–14 inches in diameter) and is bendable making it easy to install in difficult places without reframing the roof and constructing a light well as required with a traditional skylight.
Condensation can cause problems in skylights. "Leaky" skylights are often misdiagnosed. What are perceived to be drops of water from a leak are more often drops of water condensing on the cold skylight surfaces. A skylight is usually the first place condensation will occur indicating too much moisture in the interior air. Insulating the skylight well and providing adequate air movement assists in reducing condensation. Also the use of high-insulating glazing with a well designed frame can help to solve condensation problems.