To view this page ensure that Adobe Flash Player version
11.1.0 or greater is installed.
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-
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
Co nstruction Canada
10/8/13 11:45 AM