1.1
The angle from horizontal at which windows are installed has some effect on
thermal properties. The Standard specifies the determination of reference
values for vertical installation, as most windows are installed at this angle.
These values of F and U are generally appropriate for comparing the performance
of windows installed at other angles. The ER value, however, is based on solar
radiation incident on vertical surfaces and therefore does not apply directly
to windows installed at other angles. 1.3
The energy rating in the Standard is not currently relevant for energy use
required for mechanical cooling of residences. The effect of windows on energy
requirements for cooling of a residence can be calculated, but a simple
comparative rating procedure has not yet been standardized. Similarly, the
energy rating is not directly applicable to heating or cooling applications in
multistorey apartments or commercial building applications, where the
relationship of heating and cooling loads to weather conditions is more
complex. In developing the energy rating, each of the weather factors in 13 Canadian
cities with more than 4000 heating degree-days were averaged, including solar
radiation incident on windows facing the four cardinal compass directions, to
produce a single set of average weather conditions. Rating values based on
these average conditions therefore do not apply to any specific location. The
ranking of the windows, however, is the same, for the most part, as that which
would be obtained using local weather conditions. The ranking of windows would, however, likely be affected if an ER value were
computed for a specific orientation. A method for obtaining ERS values for a
specific location and orientation is provided in Part 2 of this User Guide, for
those who may wish to select or market windows on this basis. The ERS values
so obtained can be used to make a comparative estimate of the effect on annual
heating energy requirements of alternate window installations, and the User
Guide provides a method for making such estimates. See Clause 8 of this User Guide regarding the effect of window size on Er
values. 1.5
Thermal properties of windows may change with time. For example, there may be
some uncertainty about the long-term service life of some sealed glazing
systems. Similarly, air leakage rates for some air-sealing systems change more
rapidly than others. Standard methods for establishing the ability of windows
to retain certain thermal properties have not been established.