BCA 2010 - ABCB - Australian Building Codes Board

BCA ENERGY & INDUSTRY EFFICIENCY NEWS IN THE BUILT ENVIRONMENT
Comparative Stringency
of Elemental Glazing
Provisions For BCA 2010
Volume Two
Written by Bruce Lightfoot, Consultant, Australian Building Codes Board
Key points:
1. Glazing Provisions for BCA
2010 use a somewhat different
approach than BCA 2009, which
prevents a direct comparison
of stringencies in simple terms
of percentage area change.
However, examining the types
of glazing needed at typical
glazing ratios (ie the proportion
of glazing to floor area) is a
practical means of comparing
outcomes.
2. The BCA 2010 provisions allow
for minimal changes of glazing
systems although glazing ratios
may be constrained in some
cases.
3. Glazing ratios under the BCA
2010 provisions, nevertheless,
remain comfortably within
the typical range (20-30%)
identified by an ABCB Office
survey in 2002 and found in
practice.
1. Elemental glazing
method for 2010
Provisions for BCA 2010 extend the BCA
2009 calculation method to produce
outcomes closer to those achieved by
simulation. In particular, the provisions
assess wintertime solar access in detail
to reward good orientation and glazing
selection.
Stringency has been increased by
changes to the glazing constants
(in Table 3.12.2.1) and flexibility has
been improved by adding winter
exposure factors for the calculation of
conductance requirements in climate
zones 2-8 (in Table 3.12.2.2a). The new
exposure factor is applied in a revised
conductance calculation for climate
zones 2-8 in Part 3.12.2.1(a)(ii)(B).
Although slowing heat conduction into
or out of a dwelling can be beneficial
year round, the stringency is set by
ensuring that the rate of heat loss in
winter does not exceed the rate of heat
gain that can be supplied by wintertime
solar radiation. Winter gains can be
at risk from shading or glass tinting
to control summer gains unless good
orientation and
beneficial shading
geometries are used.
The new winter
exposure factors
take account of
these opportunities
to balance the
competing
summer and winter
requirements.
2. Table of glazing types
needed for similar
dwellings
The extended calculation method for
BCA 2010 means that 2009 and 2010
stringencies cannot be compared
simply in terms of percentage changes.
Comparing requirements for similar
dwellings, however, can illustrate their
relative impacts. The table on the
following page shows how the 2009
provisions and the 2010 provisions
affect dwellings with similar glazing
layouts in each BCA climate zone. Two
configurations are compared.
The first configuration assumes that the
total glazing area is distributed equally
between the four faces of the dwelling
which are oriented to the North,
East, South and West. The dwelling is
therefore “orientation neutral”, meaning
that there is no advantage in rotating
it through any 90° step. A typical level
of shading by eaves is assumed for
each climate zone and the table shows
the glazing system needed if the same
system is used throughout the dwelling.
The glazing system needed may vary
20 • Australian Building Regulation Bulletin