Star Bands & Mega Joules (MJ)
Published: 19 December 2010 / Amendment1 / BCA 2011
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Star Rating
Under the BCA 2011 “6-Star” provisions, 3.12.0 Application of Part 3.12 includes the option to verify the thermal performance of the building by complying with:
(A) 3.12.0.1 for reducing the heating or cooling loads, and
(B) 3.12.1.1 for building fabric thermal insulation, and
(C) 3.12.1.2(c) and 3.12.1.4(b) thermal breaks, and
(D) 3.12.1.2(e) for compensating for loss of ceiling insulation, and
(E) 3.12.1.5(c) and 3.12.1.5(d) for floor edge insulation, and
(F) Part 3.12.3 for building sealing.
3.12.01 Heating and cooling loads (see (A) above) allows the use of AccuRate, FirstRate and BERS to verify the energy rating of the building (the ABLE project refers to this as Modelling).
Typically a 6-Star rating is required, but in BCA DTS Climate Zone 1 a lesser star rating is allowed when the nominated outdoor area is provided (refer to 3.12.01 (a)(ii) & (iii)).
NatHERS Climate Zones, Star Rating and Mega Joules
The Nationwide House Energy Rating Scheme (NatHERS) measures the energy rating of a building as required by 3.12.01 and expresses the outcomes using a 0 – 10 star rating.
The star rating is determined by the “area adjusted” total MJ load (MJ/m2.annum = annual heating and cooling energy requirement) for a building (see below).
The “area adjusted” total MJ load for a nominated star rating varies between Climate Zones (refer to the highlighted 6-Star band in Table 1). The variance in “area adjusted” total MJ load reflects differences in climatic conditions, the degree of difficulty in achieving an “equivalent” outcome across Climate Zones and States, and range of other issues.
Typically, a house with a higher star rating is considered more thermally comfortable than one with a lower star rating. A house with a 10 star rating is considered thermally comfortable without the need for artificial heating and cooling.
Area Adjustment
Modelling software reports two total MJ outcomes, an (unadjusted) total MJ load and an “area adjusted” total MJ load. As noted above, the star rating is determined by the area adjusted figure and therefore only the area adjusted MJ figure is used for Assessment Reporting.
The unadjusted total MJ load reports the actual thermal performance of the building. The NatHERS protocol then requires that, for houses with a Net Conditioned Floor Area (NCFA) greater than 200m2, the total MJ outcome is adjusted upwards, and for houses with a NCFA less 200m2, the total MJ outcome is adjusted downward.
The adjustment factor varies by NCFA and Climate Zone and is intended to off-set the tendency for larger houses to higher ratings. This comes about because the total MJ load that determines the star rating is actually the annual total MJ load divided by the NCFA (MJ/m2.annum).
5.5-Stars or 0.1MJ Away From 6-Stars?
MJ’s are a more accurate measure of house performance than the star rating. For example, in Climate Zone 13 (see Table 1) a 6-Star rating is achieved when there is less than or equal to 70 MJ total load. Therefore, a house with a 5.5-Star outcome and a 70.1 MJ total load only requires a 0.1MJ improvement to achieve a 6-Star outcome.
Total MJ load = Heating + Cooling MJ Loads
The total MJ load is the sum of the building heating and cooling loads. These figures profile house performance and inform effective and efficient design improvement. For example, a high heating load suggest a “cold” house (lack of winter sun “harvesting”), and a high cooling load suggests a “hot” house (excessive exposure to summer sun).
House Zone Heating + Cooling MJ Loads
Modelling software also reports internal zone (living, family-kitchen, bedrooms etc) heating and cooling loads. Again, this information informs design improvement because it allows targeted improvement rather than the use of simple (more expensive) whole of house upgrade solutions.
For example, the performance of a “cold” living area could be improved by shifting a Cabana area (shading) to the east or west to allow northern sun into the living area3.12.1b.
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