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Class 2 Units: Building Average Calculation

In this article, we’d like go in depth about calculating a Class 2 (Unit’s) Building Average.
This came about due to feedback received from the AIBS Energy Efficiency training Workshops conducted by Dr Anderson.

Class 2 Units

How do you calculate a Building Average?

First, we need to know what’s required. This is found through the Queensland Development Code, Mandatory Part MP4.1 – Sustainable Buildings. It includes: Performance Requirement P2 – Energy Efficiency of Class 2 Buildings.

The Performance Requirement is what you need to know for calculating the thermal performance of sole occupancy units. In order to be a NatHERS Star Rating it needs to have:

  • each sole occupancy unit of no less than 4 Stars, and
  • a Building Average of not less than 5 Stars (lower with credits)

The Building Average can have two different credits, determined by the features of individual Units. Credits that apply only for NCC Climate Zones 1 and 2 are:

  • ½ Star for a complying Outdoor Living Area, and
  • 1 Star for a complying Outdoor Living Area with at least one ceiling fan.

(This is assuming that any room adjacent to the Outdoor Living Area that has an air-conditioner with an automatic shut-down; if the adjoining external door is open for more than 1 minute.)

Class 2 Units

The complying Outdoor Living Area has a roof covering which achieves a total R-value of R1.5 (for downwards heat flow). The geometry of the Outdoor Living Area that complies is explained in the QDC MP4.1 Definitions. The ceiling fan must be permanently fixed, have a speed controller, and a blade rotation diameter of no less than 900mm.

Individual units are not automatically better by just having a complying Outdoor Living Area. However having one makes the unit applicable for the building average calculation.

For example, consider a Class 2 building with 10 Units:

  • 6 Units score 5.0 Stars,
  • 4 Units score 4.0 Stars,
  • 6 Units have a complying Outdoor Living Area with a ceiling fan, providing 1 credit each. The credit does not apply to the individual unit.

The Building Average Star Rating is therefore (6 x 5.0 + 4 x 4.0 + 6 x 1) / 10 = 52/10 = 5.2 Stars.

This is more than the Required Building Average of 5 Stars, so the building complies with the QDC MP4.1 Performance Requirement A2(1).

The Class 2 Building Energy Efficiency Report should list the Energy Rating for each Unit (including Cooling & Heating). It should also show the calculation for Building Average including applicable credits. The Units that qualify for Star credits should be identified to assist checking.

Calculating the Building Average by averaging individual unit Cooling & Heating Energy numbers and converting this to a Star Rating is not valid. This is because the unit floor areas would be different and the Cooling and Heating numbers are in MJ/m².

Star Credits

Star credits apply only to the building average calculation and do not change the result for individual Units. For example, a Unit that scores 3½ Stars, that also has a Complying Outdoor Living Area which qualifies for a ½ Star credit, still only gets 3½ Stars and does not comply with the per-Unit Requirement.

Having a star credits does not reduce the stringency of the Required per-Unit Star Rating (of no less than 4 Stars) or the Building Average Star Rating (of no less than 5 Stars).

References

NatHERS: Nationwide House Energy Rating Scheme (www.nathers.gov.au) is an Australian framework to enable regulating how homes are rated for thermal performance – Reference: Queensland Building and Plumbing Newsflash #548.

Assessment of a building’s thermal performance at the design stage needs four components:

  • Approved Procedures with Accredited Software – the NatHERS Technical Note
  • Comprehensive Training – Certificate IV NatHERS Assessment (CPP41212), and
  • Accredited Assessors – from an Assessor Accredited Organisation (www.absa.net.au or www.bdav.org.au)
  • Standard reporting with the NatHERS Universal Certificate

If this all sounding a little too complicated, don’t worry about it. That’s why we’re the experts in Energy Efficiency. We’d love to hear from you if you have any queries or concerns about Energy Efficiency code assessment requirements.

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