New Window Technology for Northern Housing
Window openings are one of the weakest links in the building envelope. Historically, windows have shown poor thermal and mechanical performance, a situation intensified in the climatic extremes of the Canadian North.
Recent innovations in window technology and manufacturing processes have the potential for greatly improving their performance, allowing improved energy efficiency and interior comfort.
This report outlines some recent advances in window technology and how these changes might be applied to fit into the northern building industry.
Problems and Modes of Failure
Many of the major problems associated with windows in the North occur because they are designed and built for the milder southern regions of Canada. The predominant causes of failure include:
Innovations have focused on solving these problems. Numerous systems have been developed using different materials and production technologies. None provides a single best solution. However, the designer/builder now has many options which can be explored to arrive at the best design for a specific application.
The on-going efforts to improve the thermal efficiency of windows include increased use of sealed glazing units, specialized coatings, inert gas fill between sealed glazing, plastic films and insulating edge spacers. Standard sealed double-glazing units have a through-the-glass heat flow resistance value of about RSI 0.36 (R2). In comparison, many manufacturers are claiming values of up to RSI 1.4 (R8) for the new configurations. It is estimated that space heating energy savings from 9 to 18% can be realized through the use of these new window components.
Heat Reflective Coatings
The main development of the past few years has been the incorporation of heat reflective (low-E) coatings to glass surfaces. These coatings are largely transparent to visible light but reflect much of the infrared (heat) wavelengths back into the building. Two types of low-E coatings are available, soft and hard. Soft coatings are more easily damaged and must be protected during window manufacture. They generally have lower emissivities and thus higher insulation values, but do not admit as much solar heat. Soft low-E coatings are the only types readily available from manufacturers of sealed window units, otherwise known as insulated glazing units (IGUs).
Filling the space between the glass on double- and triple-glazed windows with an inert gas also provides a greater resistance to heat loss. Inexpensive argon gas is commonly used, with the more expensive krypton available for specialized applications. Gas filling can be combined with low-E coated glazings.
Placing one or two reflective plastic films inside a double-glazed IGU gives low-E coated gas performance with additional air spaces, far greater R-value and less weight than a triple-glazed IGU.
Insulating Edge Spacers
The spacers used to separate the glass in a sealed unit act as significant thermal bridges. Originally, most spacers were made of aluminum, a poor insulator. New material and manufacturing technology has led to a successful butyl/aluminum combination spacer and there are promising developments with glass fibre and silicone foam units.
The domestic hardware typically used to assemble and operate windows and doors for the rest of the country has proven to be inadequate for the rigors of northern use. Some components imported from European manufacturers appear to have better quality, functionality and durability.
Rotary operating mechanisms are not as reliable as lever and scissor types. One high-tech development is the introduction of the European tilt-and-turn hardware (Figure 1). These windows operate as a hopper for winter ventilation and as an inward opening casement for summer ventilation and easy cleaning. They have multiple locking points and compression weatherstripping.
Framing Material Developments
While these improvements add significantly to window performance, their full potential cannot be realized until the window frame is better designed to reduce heat losses.
Polyvinylchloride (PVC) framing has been gaining acceptance in southern markets both for new construction and renovation. It has also seen some use in the North. There appear to be two grades of PVC window. In the North it is important to choose the higher grade of PVC frame. The less expensive type becomes brittle and does not offer good thermal performance under northern conditions. Properly compounded PVC and a careful approach to design, using metal reinforcing at pressure points, will produce windows suitable to the northern climate.
Glass fibre has high strength and thermal resistance, coupled with low weight and a similar coefficient of expansion to glass, which would appear to make it an ideal material for frames and sashes. Its use has been tried throughout the past 20 years but problems with ultraviolet degradation and inadequate protective coatings have hindered its acceptance. Continuing development has overcome these problems and new forming techniques are helping to lower costs and improve quality.
Aluminum, with its high thermal conductivity and coefficient of thermal expansion, is unsatisfactory for use as a framing material in northern climates.
Weatherstripping is subjected to severe physical stress during the cold months as ice build-up can cause the weatherstripping to be pulled out of place and torn when the window is operated. Replacement is generally required every one to two years.
Compression-type weatherstripping provides the best performance, while the spring-loaded types are susceptible to mechanical failure. EPDM, high quality thermoplastic, and neoprene rubber systems based on or treated with silicone, offer excellent low-temperature flexibility and resistance to moisture absorption.
Passive Metal Conducting Fins
A simple metal fin which can conduct heat from the warm side of the window to the weatherstripping will help keep it warm and ice-free. Mr. Bob Chill, formerly of the Department of Indian Affairs and Northern Development (DIAND), has built and tested this concept in a house in Iqaluit (Figure 2). The window has reportedly functioned well over a number of winters.
Another proposed solution is to install heating tape around the window with an electronic lock. The homeowner must turn on the tape and thaw the weatherstripping before operating the window. Design problems need to be resolved to prevent homeowner impatience and misuse.
Metal Clad Insulated Covers
Covering the weatherstripping and frame on the outside with an insulated cover has the added advantage of providing protection from physical damage.
One built-in-the-North development is the third pane window. The third pane system uses a standard double-glazed OGEE (it may be low-E coated and argon gas filled) with an additional single pane of glass attached to the outside. The third pane provides physical protection for the much more expensive sealed OGEE while also adding to the overall thermal resistance.
Another development is the In-Line concept produced by In-Line Ltd. where the window pulls back and slides to one side, much like a sliding van door. This design allows for simple hardware and uniform compression of weatherstripping.
A prototype design by G. K. Yuill and Associates takes the previously mentioned third pane system one step further. Fresh outside air is brought in between the outer single glazing and inner, double glazed OGEE and into the home (Figure 3). Theoretically, the heat lost through the OGEE is returned to the interior. See Research and Development Highlights 90-107: A Fresh Air Ventilation System for Energy Efficient Housing.
Combining fixed windows with passive or active ventilation will eliminate window operation problems while still providing fresh air to the home. Another approach is a separate ventilator built into the window frame. Two designs use a cylinder which, when rotated, provides a screened slot for air flow.
All of these advancements add to the cost of producing windows; however, mass production of the improved IGUs has substantially lowered their cost. In 1990, windows suited to the Arctic were as much as $108 to $215 per square metre ($10 to $20 per square foot) more expensive than conventional windows. The average window area for a typical Arctic house is about 10% of the floor area, so a house with 100 m2 (1,076 sq. ft.) of floor area would have a total window area of about 10 m2 (108 sq. ft.). This added cost may be justified when using life-cycle costing and taking into account the following:
For further information on the use of windows and doors in the North, please refer to Technical Series 94-207: Windows and Doors in Northern Housing.
Research Report: Development in Windows, Doors and Hardware for Northern Conditions, January 1990.
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