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Class 2 Units: Building Average Calculation

Dr Anderson recently conducted some training Workshops for an AIBS Energy Efficiency Conference about calculating a Class 2 (Apartment units) Building Average. Since the workshop was well received with good positive feedback, we’ve decided to put the content on our website here for free. So if you’re having trouble getting your head around this sometimes difficult topic, this article is for you!

Class 2 Units

How do you calculate a Building Average?

First, we need to know what’s required. The Queensland Development Code, Mandatory Part MP4.1 – Sustainable Buildings has the answers we need, under the section labelled “Performance Requirement P2 – Energy Efficiency of Class 2 Buildings”. These are the requirements 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)

In addition, the Building Average can have two different credits, determined by the features of individual Units. Some say these adjustments are secret back-doors that let lesser-performing buildings comply, but really they’re ways to take things like lifestyle choices into account – something the Thermal Simulation software can’t calculate. Available Credits that apply only for NCC Climate Zones 1 & 2 are:

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

(It’s also worth mentioning here that any room adjacent to the Outdoor Living Area that has an air-conditioner must have an automatic shut-down if the door to the Outdoor Living Area is open for more than 1 minute – a minor but often forgotten thing)

Class 2 Units

A complying Outdoor Living Area must have a roof covering which has a total R-value of at least R1.5 for downwards heat flow. Also, the geometry of the Outdoor Living Area matters too – you can find the requirements in the QDC MP4.1 Definitions section. The ceiling fan in the Outdoor Living Area must be permanently fixed, have a speed controller, and a blade rotation diameter at least 900mm wide too.

Keep in mind that individual units are not automatically better by just having a complying Outdoor Living Area. Having one is what makes the unit applicable for the building average calculation.

This sort of stuff is often easiest explained with an example. Imagine a Class 2 building with 10 Units, and the individual units score as follows:

  • 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 × 5.0 + 4 × 4.0 + 6 × 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.

Don’t forget, having 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).


NatHERS, aka the Nationwide House Energy Rating Scheme, 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:

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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