Wednesday, January 9, 2008

Energy Efficient Housing Construction: Windows

Window Design

Windows serve a variety of purposes, they are one of the most prominent architectural aspects, can provide ventilation and have a great impact on the energy efficiency and comfort levels of a home. Windows can account for 30 to 40% of the heat loss or heat gain in an energy efficient home.

Window in Winter

The overall energy performance of a window unit in a cold climate depends on the glazing (glass or sealed unit), window style or type, frame and sash materials, air leakage, installation and the use of interior coverings or exterior shading devices. Window orientation also plays a large roll in overall window performance due to the combined effects of solar gains, seasonal winds and shading factors. Views, ventilating, natural lighting and passive solar aspects as well as architectural and aesthetic values must be considered. Window types and placement depends on which combination of functions the window must satisfy.

Window selection and placement are key design considerations which effect home energy usage and lighting, as well as comfort and humidity levels. Successful designs usually exhibit a minimal total window area with the majority oriented south for passive solar gain. If possible plan spaces so that most windows face south, while few windows face east or west, and very few, if any at all, face north.

South-facing glass area should not exceed 8 to 12% of the total living area on an energy-efficient home unless new high performance units are used and precautions are taken to avoid potential overheating problems. Opening windows can help control overheating on sunny spring or fall days. If high performance window units are used the total glass area could be increased to 10% or 15% without increasing the overheating potential. Different window sizing rules need to be applied when dealing with increased internal mass, attached sunspaces or mass walls.

Fading, sun rot and damage to finishing materials are problems which can be caused by large areas of south or west facing windows. Low-E window units can reduce the UV portion of sunlight that causes the damage by 60 to 90% while still admitting visible light. One should also select materials such as wood, masonry or special fabrics which will not deteriorate from exposure to direct sunlight. Using a masonry material for floors or walls is an especially good choice since it provides some heat retaining thermal mass as well as being a durable interior finishing material. Framing members may have to be increased in size or number to carry the weight of a masonry floor or wall.

Although living spaces on the south with large windows capture valuable solar energy, there may be times when that heat and glare is undesirable. High performance Low-E units can be used to reduce solar gains and glare from large south or west facing windows.

Window Placement

Vertically designed windows can create a pleasing indoor feeling in terms of natural lighting, viewing and providing ventilation. In bedrooms, furniture placement is often improved with vertical windows. Vertical windows can simply be described as units which are taller than they are wide. On the other hand, it is often difficult to place furniture (like beds or seating units) under vertically designed windows. Because the sills are 32 inches (800 mm) or more above the floor, horizontal windows can be hard for children to open, view out of, or use as an emergency escape.

Adding windows to a side wall or using clerestory windows are two ways illustrated for balancing the natural light. In addition, clerestory windows can bring natural light deep into a building - north side rooms with no other windows, for example. Skylights work well for natural lighting but can cause problems in cold climates. Light pipes or tubes offer a new option for providing day lighting in cold climate homes.

Balancing Natural Light

Window Coverings

Interior and exterior window coverings can be used to provide control against overheating and night time heat loss. Louvred horizontal or vertical blinds, shutters or awnings are devices which can be utilized - either on the outside or inside to block the sun. Screening devices used on the outside are more efficient at blocking incoming energy but can be difficult to operate in the winter. Movable window insulation can also be used to help control heat losses. In addition to lowering heat losses, window insulation can function as the window covering (eliminating the need for drapes), control heat gain in the summer, provide privacy and protection and reduce convective drafts near windows. Swinging or rolling shutters, thermal curtains or shades and between-the-glazing insulations are some of the types commercially available.

Window Shading Devices

Window Units

When shopping for window units look for high performance windows which have high tested unit R (RSI) values. Units must offer good durability and materials, while meeting your design and budget requirements. Current window units offer a variety of new technologies and thermal improvements to reduce heat loss and condensation problems.

High Performance Windows

  • Low-emissivity - Low-E coatings applied to interior (or exterior) glazing surfaces which reduce the radiant heat losses and can be used to control solar gains.
  • Insulating, inert gases (like Argon or Krypton) between the window panes reduce convection heat losses.
  • New insulating spacers and low-profile insulating frames combined with better air sealing on opening units, have improved solar gain while reducing air leakage and conduction heat losses.
  • Low-E coated films made of thin polyester or plastic between two panes of glass provide lighter weight, high performance, multiple-glazing units.

Window Types

Fixed window units are large expanses of glazing primarily for viewing through. The frame and the sash are both fixed in place, do not open and may have multiple glazings. Fixed units are the most energy efficient.

Horizontal Sliders come in several combinations. Choose ones with a fixed window on one side and a sliding window on the other, much like a patio door. The window segments may have double, triple or high performance glazing units incorporated into the design. Units are difficult to weatherstrip effectively, subject to air leakage and are not recommended for energy efficient homes.

Casement windows operate much like a door. They have side mounted hinges, a hand crank which opens the window and pivot on a vertical axis. Some units have a hand crank that swings the window open and then slides the window to the centre of the opening. Two hatch-levers on the sash lock the window to the frame, pulling it tight against the weatherstripping and provide good security. These windows are the easiest to weatherstrip effectively and are consequently the most draft free of opening windows.

Awning windows are very similar to casement windows except they open to the outside from a hinge along the top. They are very weather tight, provide good security and can be compared to a casement in overall energy efficiency.

Tilt and turn windows have special hardware that allows the window to tilt in at the top or to open like a door, toward the inside. These windows are also very weather tight, comparable to awning or casement in energy efficiency with a locking type handle and good security.

Pivoting windows are common in Europe. They pivot in the centre of the frame in a vertical or horizontal axis depending on the model. Moderately airtight, this window type is not a good choice in 'buggy' climates as it is difficult to screen effectively.

Bay or Bow windows are extremely popular. They are windows that jut out on a cantilever floor section, with a series of fixed or opening units joined together in a 3 window or 5 window configuration. Care must be taken to ensure that proper insulation and vapour barrier techniques are applied to the floor area or condensation, drafts and cold floors may occur.

Combination windows are simply an amalgamation of several different units such as fixed units and casement windows. These usually come pre-assembled from the factory ready for installation.


Good quality skylights can be an asset to any home during a long indoor winter. In cold climates, choice and placement of skylights has to be done carefully in order to avoid overheating and sun damage during the summer and excessive heat loss with dripping condensation in the winter. Skylights can on the other hand provide a view of cloud scapes and sky, while allowing light deeper into the home than wall mounted windows can, especially on cloudy days.

Glazings for skylights are available in acrylic, polycarbonate, polystyrene and glass. The requirements for a skylight unit should be at least the same as those for a high performance window unit or better.

The deeper the well of the skylight, the less air circulation and the greater the potential for condensation. Flaring the well at the bottom of the shaft will increase air circulation and the amount of light being delivered by the skylight. Sealing a piece of glazing at the ceiling opening of the skylight well can also help. The light well that frames the skylight should be finished in a light colored paint or mirror to allow the well to reflect the maximum amount of light.

When choosing a skylight consider the slope of the roof in relation to the shape of the skylight. Flat skylights on a low slope roof tend to collect snow and dust more readily than dome or pyramidal shaped skylights. Opening skylights can vent hot air out of a house rapidly but may need regular maintenance in order to seal effectively when they are closed. Also, opening skylights should be equipped with a screen.

When placing a skylight on your home, southern or western exposures should use glazing which is tinted, or has a Low-E coating that blocks at least 50% of the solar gain and 90% of the ultra violet light. Consider the percentage of roof area that skylights will cover in any one room. Skylights are usually poor insulators and large areas of roof glazing can be a source of cold drafts and condensation problems on long January days and nights.

Exterior Doors

Energy efficient exterior doors should have an insulated core bonded to the inside and outside skins of steel alloy, aluminum, fibreglass or wood composite. For cold climates insulated doors are a good choice. With much higher insulating values (R-10, RSI 1.8) insulated doors are less prone to warping and are easier to weatherstrip effectively. A door may also have one or two 'side lites' of glass which should be high performance glazing units. Metal doors should have good quality compressible, magnetic or adjustable weatherstripping to reduce air leakage.

Patio Doors

Patio doors can be the largest window in your house. All the components that make a good high performance window also make a good patio door. Triple glazing is very seldom found in a patio door as it is heavy and requires a very thick unit. Some patio doors have two sliding panels while others have one panel fixed and one panel that slides. Sliding doors are very difficult to weatherstrip. Friction and foot traffic wear the weatherstripping out in short order. Rollers can also wear out, requiring replacement. A better type of sliding patio door is available that operates like an airplane door - popping in and sliding away from the weatherstrip with a latching type handle.

Garden, Terrace or French doors are a better choice for energy efficient homes than traditional sliding patio doors. These are similar to double entry doors, with a large glazing area, and one or both opening inwards. Units are available with insulated cores, high performance glazings and can be weatherstripped very effectively. Screens attach to the inside or are mounted on a track on the outside.

When designing a new home take into consideration the location of patio doors. Avoid northern exposures and prevailing winds. Good installation is also critical. Poor installation can cause poorly operating doors, drafts and increased condensation problems.

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