Energy-Efficient Residential Windows, Doors and Skylights
The basics
Here are some basic tips about buying a new energy-efficient window, door or skylight for your home.
Types and styles
Residential windows and skylights come in two basic types: operable and non-operable (or "fixed"). Non-operable products are more airtight, but it is often desirable to be able to open a window or skylight for ventilation or to provide an escape route in case of emergency.
Fixed
Operable
Among operable products, hinged windows (casement, awning, hopper, tilt and turn) are generally more airtight than sliding-style windows, because the closing mechanism pulls them tight against the frame. Skylights should have a low curb (the part that holds the glass at the roof line) for better energy efficiency.
A tubular skylight can be used instead of the traditional flat-glazed or large domed skylight. This product consists of a small-domed lens on the roof and a flat lens inserted into the ceiling. A flexible or rigid tube that has a reflective interior surface is connected between the two lenses, and natural light is transmitted into the home. These are more energy efficient than traditional skylights and easier to install, although they do not provide as much natural daylight or any ventilation. Some tubular skylights also have integral lighting fixtures. There are also "roof windows" that operate like "hung-style" windows but are designed to be installed at an angle.
Awning
Casement
Hopper
Vertical slider (single-hung)
Vertical slider (double-hung)
Horizontal slider (single)
Horizontal slider (double)
Tilt and Turn
Skylight (flat-glazed)
Skylight (dome)
Doors come in two operable styles: swinging (hinged) and sliding. They can range from having no glass to being full-glass models with optional windows beside the door (sidelight) or above the door (transom). Doors can be made from one material, such as wood (flush door) or constructed with a wood or metal "skeleton" that is covered with a "skin" on both sides, protecting an inner core of insulating foam (stile and rail door). Doors made from one material conduct heat (see sidebar) more readily than insulated core doors. However, the more glass there is, the less resistance there is to heat loss. Depending on where the door is located, one with more glass can also allow the sun's heat into the home.
Sources of heat loss
Heat energy naturally moves from warmer areas to colder areas. In the case of windows, doors and skylights, this process occurs in a number of ways:
- Radiation - Heat energy is absorbed by the window glass, then moves (radiates) toward the cooler side.
- Conduction - Heat energy moves through solid materials such as the frame, sash and spacer bar materials.
- Convection - Heat is lost through the movement of air near and in the space between the glass.
- Air Leakage - Heat is lost when the air moves through the seals or gaps in the frame. Air leakage may also occur around the frame due to poor installation.
While there is no way to completely eliminate heat loss through windows, doors and skylights, good-quality construction and the use of appropriate materials can minimize it.
Entry door system
Sliding patio door
Swinging (hinged) patio or garden door
French door
Material choices
Windows, doors and skylights installed in houses and low-rise buildings can be manufactured from a variety of materials, such as metal (aluminum, steel), fibreglass, vinyl and wood, or a combination of these materials. As a rule, metal tends to conduct heat more than non-metals. However, the overall energy efficiency depends on the product's design and is reflected in its energy-performance ratings. (See "Ratings and certification.") Most windows, doors and skylights installed in high-rise and multi-residential dwellings and commercial buildings are made primarily from metal because of the material's reduced-flame-spread rating and resistance to high winds, and for security considerations.
Glazing
Glazing is the generic term for the transparent material - usually glass - in a window, door or skylight. Ordinary glass is called annealed glass. Glass can also be tempered (heat-strengthened) or laminated, to make it more shatter- resistant. Laminated glass is a combination of two or more glass sheets with one or more interlayers of clear plastic or resin.
Glazing layers: A single-glazed product has one pane of glass. A double-glazed product has two panes; a triple-glazed three; and a quad-glazed four. One of the panes inside a triple- or quad-glazed product may be substituted for a thin layer of suspended polyester film to reduce the overall weight. In Canada, all windows, skylights and doors with glazing should be at least double-glazed.
Insulating glazing (IG) unit: The IG unit consists of at least two panes of glass that have been sealed around the edges to make the window airtight. A spacer bar is placed around the edge of the glass in an IG unit to hold the panes apart. The spacer bar also has desiccant in it to absorb all the moisture remaining inside the glazing unit after it had been sealed. This prevents fogging in-between the panes.
IG Unit
Frosted, patterned and tinted glass: Frosted translucent glass and patterned artistic glass do not significantly reduce the amount of the sun's heat and visible light coming into the home. Tinted glass, however, is made to reduce both. In summer, cooling costs are lowered, but in winter heating costs may rise because of the reduction of the sun's heat.
Low-E: Low-E glass has a fine coating of metal to reduce heat loss in the winter and heat gain in the summer, through the glass, by up to 30 percent. There are two types of low-E coatings: hard coat and soft coat. Both increase energy efficiency; however, some types of soft-coat, low-E glass can dramatically reduce the amount of the sun's heat coming into the home.
Gas fill: The inside of the IG unit is filled with an odourless, colourless, harmless inert gas, such as argon or krypton, to reduce heat transfer through the glass. For maximum efficiency, argon is normally put into double-glazed units and krypton is put into triple- or quad-glazed units.
Spacer bars: Traditional aluminum-box spacer bars in IG units contribute to heat loss and condensation problems. Spacer bars designed with thermal breaks that use materials such as foam, plastic or glass or that are made with other types of metal, such as stainless steel, will reduce heat loss.
Grills and dividers: Metal or plastic bars called grills are often placed inside the IG unit to give the effect of many individual panes of glass. Dividers may be placed over a single IG unit and attached to the sash or frame, or a number of small IG units may be placed inside the dividers to achieve the same effect. While grills and dividers are popular choices to enhance the look of windows and doors, they also reduce the amount of the sun's heat coming into the home.
Vacuum, aerogel and electrochromic glazing: These technologies are new to the market and are not widely available. A vacuum IG unit has no air or inert gas inside, dramatically reducing heat transfer. The unit is tightly sealed around the edges, and the panes of glass are held apart with tiny, invisible pillars. Aerogel glazing has a highly insulating, low-density silica-based solid called aerogel between the glass panes. Aerogel eliminates the need for low-E coatings and inert gas fills. Electrochromic or "smart" glazing (also known as switchable or active glazing) can save energy by reducing solar gain only when it's necessary. All these developing technologies promise significant improvements in energy efficiency.
Weatherstripping and hardware
Weatherstripping is used to create a seal between the frame and door or window sash to block rain and dust and to minimize air leakage. There are three basic types of seals: compression, brush and sweep. A fourth type, called a magnetic seal, may be found on some door systems.
Compression and magnetic seals provide superior airtightness. Casement, awning, hopper and tilt-turn-style windows, most operable skylights and swinging doors have compression seals to provide the primary air barrier. Sliding and hung windows have brush seals. Swinging doors also have a sweep seal between the bottom of the door slab and the sill, which tends to be the area with the most air leakage in a door system. High-quality hardware, such as cranks, handles, latches and locksets, also help to provide a good seal. If possible, they should not conduct heat through the product.
Compression and sweep weatherstripping
Brush weatherstripping
