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Fenestration Products –
Types of Windows and Skylights

Understanding the Terminology – Windows

Parts of a Window

Windows consist of a glazed unit, a frame and a sash. In Canada, the most common type of glazed unit is the insulated glazed unit, which consists of at least two panes of glass, sealed airtight with a spacer in between to separate the panes. The glazed unit fits into the sash, which in turn is held in place by the frame.

There are two types of windows: those that open (called operable), and those that don't (called fixed). Fixed windows tend to be more energy efficient because they are sealed on all four sides and have no moving parts. However, it is often desirable to have some operable windows for ventilation or for use as emergency exits.

Figure 4

These may also be required by building codes. Operable windows come in several styles. Those that swing inward and lock shut tend to be more airtight than sliding windows.

Figure 5

Understanding the Terminology – Skylights

Figure 46

Skylights consist of a glazed unit connected to a frame by a "curb." Like windows, they may be operable or fixed. (Operable skylights also have a sash, which is held in place by the curb). Skylights come in domed or flat-glazed styles, with the latter tending to be more energy efficient. Of the flat-glazed skylights, those with low curbs are more energy efficient


Skylights should be as resistant to heat loss as possible. This means avoiding high, poorly insulated curbs. Glazing should be as energy efficient as possible. Low-emissivity (low-e), gas-filled, insulated glazed units are a good choice. You may also want to consider light-reflective glass to reduce overheating, although this may also reduce the amount of daylight. To reduce overheating in summer, apply an exterior awning, reflective film, or even whitewash to your skylight. An extra layer of glazing on the inside of a skylight may also reduce condensation and heat loss.

Skylights that are not installed properly will leak far more heat than a standard roof and window. They can also cause special problems, including water leakage, condensation and summer overheating. A skylight's thermal performance is normally indicated by its U-value.

Tubular Daylighting Devices (TDDs)

This is an alternative to a skylight. A round lens is installed on the roof along with another similar lens in the interior where additional natural lighting is desired. The two lenses are connected by a special rigid or flexible pipe that reflects light from the outside into the interior. To help with a TDD's energy efficiency, the pipe should be insulated. Some TDDs also have a compact fluorescent light near the interior lens that turn on manually or by a light sensor.

Components of Windows and Skylights