The big picture on materials and high performance
Energy Star windows
While performance should be based on the entire window assembly, the consumer’s guide published by the Office of Energy Efficiency, Natural Resources Canada states that fiberglass, with foam insulation in the frame cavities, provides the best energy performance for window frames. The Mnjikaning first nation early childhood education centre by Teeple Architects. [ Courtesy Inline Fiberglass Ltd.]
by Jim Taggart .Over the last twenty years, ongoing research and development in all aspects of design and performance
has transformed the window from a simple product to a sophisticated environmental control system.
Depending on where you are in Canada, sealed double-or triple-glazed units are now the baseline specification. With new ‘warm edge spacer’ technology, low emissivity coatings and other innovations, the thermal performance of window assemblies continues to improve, translating into both energy savings and more comfortable living conditions.
With these innovations, the U-value [that measures the rate at which heat is transmitted through a window assembly] has improved by about 50% from 0.55 to 0.27 for double glazing, and from 0.33 to 0.17 for triple glazing.
For the purposes of this article, a high performance window is one that qualifies under the Energy Star program administered by Natural Resources Canada. Under this program, the country is divided into four climatic zones A-D [warmest-coldest], and the maximum qualifying U-values for window assemblies range from 0.35-0.25.
It should be noted that U-value [which is the inverse of the more familiar R-value] is not the only factor contributing to the thermal performance of window assemblies. The solar heat gain coefficient [SHGC], which measures the amount of solar radiation admitted through the glass, also contributes to overall thermal performance.
Frames
The two frame materials that once dominated the residential market, wood and aluminum have faced stiff competition from vinyl, fiberglass and composite systems. While manufacturers argue the environmental benefits of one framing system or another, Mark Lombard of the Lighthouse Sustainable Building Centre in Vancouver observes that, “with the reduction of greenhouse gas emissions being the main focus of global climate change strategy, it is the energy savings over the life of the window, rather than the environmental attributes of the materials themselves, that are the most important.”
Frames should be low-profile and thermally broken for best energy performance as Energy Star window. In fact, because larger windows reduce the ratio of frame to glazing, a few large windows maximize overall performance in comparison to many smaller windows.
Spacers, Sealers and Gas Fillers
Almost all glazing units are designed on the rainscreen principle, such that any moisture that makes its way into the channels of the frame is returned to the outside via drainage holes. The glazing unit is held off the frame by hard rubber spacers, and the seal between the glazing unit and the frame is usually achieved by double-sided adhesive foam tape. Some specifiers prefer a continuous gasket-like seal and/or a bead of clear silicone applied between glass and frame to reduce the possibility of water penetration particularly in severe exposure conditions.
Within the glazing unit itself, the perimeter spacer is now almost always made of dense, almost incompressible foam. Some manufacturers also offer composite metal and foam spacers that combine the strength and durability of traditional metal spacers with the thermal performance of warm edge technology.
Foam spacers should be supplied with an integral, impregnated desiccant that keeps moisture from forming inside the sealed unit. Look for a continuously formed spacer, as opposed to one that is made in four pieces and mitered at the corners. Continuous spacers always include a single joint, usually placed midway up one side, that compared to the mitered systems, greatly reduce the chances of air leakage or moisture infiltration.
The standard gas used for sealed units is argon. It performs best in a 1/2-in. air space. Because of frame depth limitations, some triple glazing manufacturers supply units with 1/4-in. air spaces. For maximum performance these narrower air spaces should be filled with krypton, a more expensive gas than argon.
Low Emissivity Coatings
Another innovation in glazing technology is the introduction of low emissivity [low-E] coatings that are applied to the interior surface of one or both panes of glass. Low-E coatings, usually take the form of microscopically thin metal foil that is almost invisible and which has virtually no impact on the transmittance of visible light through the glass.
Low-E coatings used to be readily available in two basic types: hard coat designed to maximize solar heat gain during the day and reduce heat loss at night, and soft coat designed primarily to reduce solar heat gain during the day and heat loss at night.
Although more suited to Canadian applications, hard coatings are less common than they used to be. This is due in part to new types of soft coatings whose properties more closely mimic those of hard coatings - but probably also in part due to the fact that most glass used in Canada is manufactured in the US, where the greatest demand is for soft coatings.
The optimal specification of Solar Heat Gain Coefficient [SHGC] and hence type of coating, will vary according to latitude and window orientation. South-facing elevations benefit from high solar gain coatings in winter - assuming the house is designed to store and distribute the heat, and there is shading provided to block summer sun. East and west elevations more typically need coatings that block the sun, so it can happen that two or more types of coating are used on even a small residential project.
UV Ray Blocking
An important aspect of glazing specification is UV ray blocking. Clear, double-pane glass blocks about 44% of incident UV, while standard low-E softcoat with argon blocks 84%. The latest generation of low-E coatings blocks over 90% of incident UV.
Double versus Triple Glazing
Double-glazed units with low-E coatings can achieve the same U-value as uncoated triple-glazed units. When high performance frames, low emissivity coatings and gas fill are combined, double-glazed units can now achieve U-values of about 0.27, and triple-glazed units can achieve U-values of about 0.17. It is important to compare U-values for complete window assemblies using a standard testing methodology. The National Fenestration Research Association carries out standardized testing, and all major manufacturers can provide NFRA figures for their products.
In the less extreme climates of Canada, it is difficult to justify the cost of triple glazing through energy savings alone. However, there are other benefits including: improved thermal comfort, better sound reduction, less risk of condensation and the opportunity to simplify the building’s heating system.
In cold weather the inside surface of a triple-glazed window may be three or four degrees warmer than that of a double-glazed window. This apparently small difference can greatly reduce the discomfort of radiative heat loss experienced by those seated next to a cold window surface. The usual reaction to this kind of thermal discomfort is to raise the ambient temperature by turning up the thermostat, an outcome not considered by most energy modelling software.
With improved thermal comfort, it may no longer be necessary to position radiators or heating vents on exterior walls, and consequently the cost of pipe or duct runs may be reduced.
Installation
The use of energy star windows should be accompanied by a more sophisticated ventilation system, such as a Heat Recovery Ventilator, and strategic natural ventilation. Also essential is a wall system with proper insulation and vapour/air barriers. In short, high performance windows must be part of a high performance envelope for their advantages to be realized.
Vinyl
Hollow vinyl [UPVC] frames have the advantages of high durability and low maintenance. Sealed at the corners, they encapsulate 
insulating cells of air. Some of the initial drawbacks of UPVC frames, including span and colour limitations have recently been overcome.
Some companies now offer UPVC frames made of recycled material and vinyl-compatible low toxicity, water-based paints are now available. Frames can also be supplied with integral metal channels to compensate for differential thermal expansion. Spans of 9ft. or more are now achievable in vinyl with no warping. [Courtesy Vinyltek Windows]
Fiberglass
Fiberglass, a relative newcomer to the market, is durable, inert and has thermal expansion characteristics closely matched to 
those of glass. Fiberglass frames are pulled [rather than extruded] into profiles that are then mechanically fastened and sealed at the corners. Frames may be ‘open-backed’ or ‘closed’ - the latter resembling sealed vinyl frames - and either type can be filled with laser-cut rigid insulation. They can also be fitted with an interior frame of wood.
The Consumer’s Guide published by the Office of Energy Efficiency, Natural Resources Canada states that fiberglass, with foam insulation in the frame cavities, provides the best energy performance for window frames earning it the Energy Star window rating. [Courtesy Inline Fiberglass inc.]
Wood
Clear finished, solid wood frames still offer visual warmth and beauty unmatched by synthetics, but they are less durable and 
require regular maintenance. Dense wood species like Douglas fir are to be preferred, and are available from sustainably managed forests.
For most manufacturers, obtaining wood from FSC-certified forests, as required by LEED, is still a special order. Some manufacturers use cross lamination of smaller sections to make up their frames. This improves dimensional stability, minimizes defects and enables wood to be sourced from smaller trees. Some manufacturers also offer composite wood and aluminum frames where the exterior of the wood sections is protected by an aluminum extrusion. This should be a factory fitted option, with an air space left between metal and wood to allow for drainage of moisture. [Courtesy Loewen Windows]