Feature Ensuring ENERGY EFFICIENCY By Steve Fronek, PE I n most parts of Canada, commercial building energy efficiency is significantly affected by conductive heat loss. In the winter, resistance to undesirable condensation on interior surfaces is important. New insulating glass and framing options are available to reduce heat loss and air leakage, control solar heat gain when appropriate, and minimize the potential for phase- change condensation. However, understanding how to best specify materials for a particular project involves first knowing the basic principles of heat transfer, which occurs from objects of higher temperature to those of lower through conduction, convection, or radiation. Conduction, convection, and radiation Heat transfer through solid materials takes place via conduction. While aluminum is widely used as an engineering material because of its strength, light weight, and ability to accept durable finishes, it exhibits a high thermal conductance. This attribute makes an aluminum surface ‘cold’ to the touch, even when its surface is quite close to ambient air temperature, since heat quickly transfers from the skin to the aluminum. In an aluminum window or curtain wall, the most common means of reducing conductive heat transfer through framing is by adding low-conductance ‘thermal break materials’ such as polyurethane, fibre-reinforced polyamide nylon, polyvinyl chloride (PVC), or flexible elastomers like silicone, neoprene, or ethylene propylene diene monomer (EPDM). All these thermal break materials exhibit unique design advantages and limitations. An effective thermal barrier may be either structural (i.e. designed to resist wind loads and dead loads as an integral part of the extruded aluminum assembly) or non-structural (i.e. designed to be supported by fasteners or interlocks). The National Building Code of Canada (NBC) requires thermal breaks in metal window and curtain wall framing. Non-conductive wood, cellulosic composite, vinyl, and fibreglass framing has each captured or retained significant market share in residential markets, but for the reasons mentioned, thermal barrier aluminum remains the material of choice for non-residential applications. Photo © Paul Crosby. Photo courtesy Wausau Windows and Wall Systems 18 Specifying windows and curtain walls for cold climates October 2013 CC_Oct13.indd 18 Co nstruction Canada 10/8/13 11:45 AM