Skylights
Skylights are designed with one primary purpose: to bring warm, natural light into the home. Skylights not only add natural light but also a sense of spaciousness and value to any home. They can flood a room with up to five times more light than a traditional wall window of the same size. When fitted with a movable sash they function as a roof window providing effective, draft-free ventilation.
Skylights come in a variety of styles. Homeowners can chose skylights in varying sizes and shapes, with different types of glazing (glass, plastic, single glass or insulated glass) or with various accessories such as light diffusers and blinds.
A skylight is, in effect, both an opening in the roof and part of the roof covering. As an opening in the roof, the roof framing must be structured to provide support for the skylight and properly transfer roof loads. As a part of the roof covering, the skylight must be watertight.
Unfortunately, skylights are not trouble-free. Regardless of the type skylight and installation methods employed, skylights are prone to leakage. Improperly installed flashings are the primary causes of skylight leakage. Other conditions and concerns that can lead to skylight leakage or malfunction include:
- Use of the wrong skylight for the application
- Damaged flashing or glazing
- Lack of proper curbing
- Condensation problems
- Damage to the skylight frame
- Defective seals on insulated-glass units
- Ice damming around the skylight
- Tree branches, hail, pets and other external factors
A skylight can be installed on just about any roof regardless of the slope or roof covering. But it must be the proper type skylight for the application. Skylights of any design constitute an obstacle to water runoff. The use of the wrong skylight increases the potential for problems. At the very least the opening must be carefully flashed on all sides to divert water and prevent leaks.
Flashings will tend to lift and separate with age as membranes dry out or become distorted with thermal expansion or as a result of movement of the roof structure. Periodic repair and resealing of the flashings is a standard maintenance item. In some cases skylight joints, seams, and flashings are coated during installation as a precautionary measure. However, a properly installed skylight does not rely solely on cements and other added sealant to prevent leakage. The presence of surface sealants may be an indication of prior leakage. The only way to address a chronic leakage problem may be to install new flashing or replace the unit and flashing together.
Skylights are also subject to damage from falling tree branches and other debris. Poor quality plastic glazing can breakdown or become discolored from the ultraviolet rays of the sun. Plastic or glass surfaces can be scratched from squirrels and other pests. Heat can also distort the glazing. Fortunately many of these conditions do not prevent the skylight from serving its intended function.
To minimize the potential for injury to occupants from breakage, safety glass or plastic is now used in most skylights. Labeling in a corner of the glazing may provide confirmation that it is safety glass. Older skylight units may have been manufactured prior to the establishment of current safety standards. Many newer units have plastic glazing, especially if they have a curved shape such as dome or convex units.
Ideally skylights should be installed on raised curbing, which lifts the skylight above the roof surface and facilitates the installation of manufacturer-supplied trim and flashing to help ensure watertight conditions. Installing a diverter at the topside of the opening helps to move the roof runoff away from critical joints. Flush-mounted skylights or skylights with relatively low curbs are especially susceptible to seepage.
No matter what the style, skylights also breach the thermal envelope of the house – the insulation system that basically creates an envelope around the house. Like windows, skylights form a minimal barrier between the warm, moisture-laden air found in the living areas and the colder outdoor environment at certain times of the year. This temperature differential can lead to condensation of water vapor on the inside surfaces.
The amount of moisture from condensation can be substantial enough to cause staining of adjacent ceiling surfaces, creating the appearance that the unit is leaking. Maintaining proper indoor temperature and controlling humidity levels by ensuring adequate fresh-air exchange rates within the house (using ventilators if needed) will help keep indoor moisture levels at a reasonable point. The use of insulated glazing also helps to control condensation, as well as reduce the amount of heat loss.
The loss of heat through a skylight can contribute to the formation of an ice dam, usually below or downslope of the skylight. See the Homeowner Information Series on “Ice Dams” for more information.
Like insulated windows, insulated skylights are subject to failure from manufacturing defects, installation deficiencies, warping of the frame, damaged sealants and other factors. These failures are difficult to detect in many cases, as certain climatic conditions must be present. Eventually the seal failure may manifest as fogging or discoloration of the glazing. While the difference in heat loss may not be significant, the change in light transmission may be a factor.
Under direct sunlight some skylights can produce “hot spots” on interior surfaces. This can result in discoloration of the carpeting and other finishes from the ultraviolet rays of the sun.
All skylights should be inspected periodically for the presence of damage or other conditions that can cause or contribute to leakage and other detrimental situations. More specifically, all flashings, glazing, and related components should be checked for cracks, separation, deterioration and other damage.
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