Commercial glazing systems
A summary of recent advances
Glazing considerations such as window area, elevation and orientation, thermal performance and solar shading to optimize natural daylighting and passive solar heat gain are very important to the envelope performance and energy consumption of buildings [1]. [BC Cancer Agency Research Centre, IBI Group and Henriquez Partners Architects, Photo: Nic Lehoux]
by Hugh Perry
Many of Canada’s commercial buildings were built over 40 years ago when there were few, if any, worries about energy performance and environmental responsibility. In an era of cheap and abundant energy, heating and cooling loads were of little concern; buildings were often clad entirely in glass with no differentiation between facades having different orientations.
Today, with energy efficiency a high priority, designers concern themselves with window area, elevation and orientation, thermal performance and solar shading in order to optimize the benefits of natural daylighting and passive solar heat gain, in the context of total envelope performance.
Even though the in-service energy savings for high performance glazing systems far exceed the energy used to produce them, manufacturers, specifiers and increasingly consumers, are concerned with life cycle performance that takes into consideration the energy and environmental impacts of raw material extraction and production, fabrication and transportation, installation and maintenance, demolition and reclamation.
The ultimate goal must be a cradle-to-cradle process which conserves raw materials and in which all products are not only recyclable but actually recycled. This involves not only manufacturers, but also contractors and municipalities in the creation and implementation of comprehensive waste management programs.
To its credit, the window industry has made considerable progress in these areas, in addition to the succession of innovations that have continued to improve in-service environmental performance.
Material Reuse
In commercial glazing systems, aluminum frames continue to dominate the market, followed by steel with other materials occasionally used for smaller installations. In British Columbia, wood is increasing in popularity in small commercial buildings, chosen for its inherent sustainable properties and as an expression of regional materials and technology [See Sidebar]. In residential construction metal has largely disappeared, being replaced by vinyl and more recently fiberglass, which have also taken market share from wood.
Whatever the frame type, the first challenge is sourcing recycled materials, which in turn must rely on a comprehensive system to separate and reclaim components at the time of demolition. Framing materials are more easily removed from window units than other components like the sealants from the thermopanes themselves. Recycling depots are often great distances from building sites and so require effort and cost to use, and waste management plans are not yet universally included in construction specifications.
Provincial authorities in Alberta are in talks with construction associations to determine practical ways to direct reclaimed materials back into new products. All sectors of the window industry have been asked to examine their in-house management of resources and seek new methods to decrease their carbon footprint and dependency on virgin resources. Among leading glass manufacturers, PPG is the first to receive Cradle to Cradle certification for its entire range of architectural glass products.
Most often, construction glass that is collected at depots is processed into fiberglass insulation, highway markers or crushed for clean fill. Coatings contaminate the glass preventing it from being reused for clear glass applications. Most architectural glass has about 10% recycled content, aluminum frames vary between 30 to 35%, and fiberglass approximately 10%, according to Marvin Windows.
The fiberglass industry is researching ways to grind scrap, as well as mixing ratios of scrap with raw materials. PVC products can be recycled back into windows, however, the sealants used make this more problematic, and much of this material still ends up in landfills. [For two perspectives on this issue, and PVC in general, see Green Building with Plastics in SABMag Nov/Dec Issue 14 available at www.sabmagazine.com, and this issue’s Viewpoint.] A new frame process known as ‘Composite Frames’ combines plastic with recycled wood.
Solar Control
New glazing technologies enable architects and engineers to take advantage of free solar heat and incorporate shading devices to reduce cooling costs. These may take the form of fritting – a ceramic dot pattern of calculated size and spacing that is fused onto the glass units during manufacture. Patterning may vary across the glass surface, and in full height glazing systems may occur only at the top and bottom leaving a clear ‘vision panel’ at eye level. Fritting can significantly reduce heat gain with minimal effect on light transmission.
Exterior shading devices are now being supplied by window manufacturers as a fully integrated option to their standard glazing systems.
Commonly, these shading devices are applied horizontally to restrict solar rays reaching heat adsorbing surfaces such as floors, desks or window sills. Vertical shading devices on east and west exposures accomplish the same purpose. Manufacturers identify their product with a variety of titles, for example, Solar Control and Solar Screen.
Environmental modelling software may also confirm the efficacy of other shading configurations such as the diagonal louvers installed on Busby Perkins + Will’s Normand Maurice building in Montreal.
Thermal Performance
The low-E coatings that are now standard on most commercial double and triple glazed units can be specified to minimize heat gain or heat loss according to the orientation of the elevation on which they are installed. [See SABMag March/April, 2007 Issue 4 on www.sabmagazine.com for a more detailed discussion of this topic.] One innovation in low-E films, the heat mirror, has recently arrived in Canada.
Heat Mirror was developed by Southwall Technologies based in Palo Alto, CA, with manufacturing operations in Germany. The company launched heat mirror insulating glass units in 2008. Canadian manufacturers use this method of introducing a free standing film between the exterior and interior panes of glass, thus creating two air spaces. The same approach can be applied using two free standing films in a triple glazed unit.
Perimeter spacers, the size of the spaces between the panes, and the choice of the inert gas used to fill them, all affect the overall performance of the glazing unit. Argon-filled units with foam spacers and a 1/2-in. spacing between panes are the norm. Variations that improve overall performance are discussed in the SABMag March/April, 2007 issue.
Currently, typical U values of high performance windows are: 0.27/R-3.7 for double glazing, 0.17/R-5.88 for triple glazing, 0.11/R-9.09 for single layer of Heat Mirror, and 0.07/R-14.28 with two layers of Heat Mirror.
Indoor Environmental Quality
Green building rating systems encourage the use of operable widows to promote occupant health and well-being by providing on-demand natural ventilation. A European alternative to operable windows, now available in Canada from Oldcastle Glass and Titon, is known as trickle venting. The trickle vent is built into the window frame, and brings fresh, outside air into the building, and allows stale indoor air to escape without the use of chemicals or electricity. The trickle vent can operate 24/7 whether the building is occupied or not, without any security risk. An adjustable, diffusing head with permanent polypropylene filter allows the air to flow in and out gradually to eliminate drafts, and can be closed to an airtight seal when not in use.
Bringing natural light deeper into the interior of spaces further reduces the lighting-related use of energy. Horizontal shelves placed below a top portion of window and sufficiently high to not restrict outside views accomplish this. Windows manufacturers now supply these as well as exterior shading to meet window specifications. Consideration is required when locating perimeter ceiling diffusers, not to prevent obstruction to air flow across glazing.
Higher performance windows offer increased flexibility in window placement, and in some areas of Canada the energy savings provided by glare-free daylight may more than compensate for the energy lost through north-facing glass.
The other advantage of higher performance windows is increased thermal comfort for building occupants. Higher thermal resistance brings the interior surface temperature of the glass closer to that of the adjacent wall surfaces, reducing the discomfort of the radiant heat effect heat loss from the body to cooler surfaces, and heat gain from warmer surfaces.
Conclusion
As the baseline energy performance of green buildings has improved incrementally, the window industry has certainly played its part. The energy efficiency of window assemblies has improved by up to 50% as a result of recent technical improvements; and on the environmental front, PPG’s Cradle-to-Cradle certification has demonstrated to others what can be achieved.
Hugh Perry is contributing editor of SABMag.
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Squamish Adventure Centre
Architect: The Iredale Group; Photographer: Andrew Doran
A showcase for the community of Squamish, BC,the self-proclaimed ‘Outdoor Recreation Capital of Canada’, the Squamish Adventure Centre is a 10,000sf heavy timber building of complex elliptical geometry.
To enhance the visual connection with the stunning landscape, the architects wanted to conceal the aluminum channel frame by rebating it into the structural timber members, creating a clean appearance without cover plates or intermediate mullions. Each 12ft bay has three glass panels, the two side panels having channels top and bottom and on one side only, the centre panel having top and bottom frames only, with the vertical joints between the faceted panels being made of structural silicone.
To create this level of precision, sophisticated CAD detailing software was employed to generate three-dimensional computer models of each unique timber connection and timber member. The models guide-precision milling that include crosscutting of rebated and mitered edges along the length of each beam and column to receive the exterior glass curtain wall.
