Introduction

The Australian market for comfort and energy efficiency in the built environment can be difficult to understand for those inside the design and construction industry, let alone for those new to the industry.

What’s the difference between a Nationwide House Energy Rating Scheme (NatHERS) rating and a Certified Passive House construction for example? How do we bridge the ‘performance gap’ between what we design and what we build?

To address this potential for misunderstanding, APHA have created a process and ultimately an interactive user interface to contextualise and explore the relative benefit of Passive House design and construction in comparison with a 7 star NatHERS rated building.

We have chosen to “keep it simple” and have focused on 1) Occupant comfort 2) Energy efficiency and 3) Operational costs based on a single residential building model in key Australian climate regions.

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Project Background

Today, the Nationwide House Energy Rating Scheme (NatHERS) is the nationally adopted “Deemed-To-Satisfy” pathway for Residential compliance. Through the use of NatHERS, and 3 energy rating software packages accredited in accordance with the NatHERS Software protocols, designs are provided a star rating (out of ten). The minimum requirement varies from State to State.

While there are many critics of the NatHERS scheme amongst industry experts and more general construction practitioners, the greatest critique is arguably fact that NatHERS design ratings do not represent the built form. In the simplest terms, the NatHERS scheme can provide little assurance that comfort and energy efficiency are proven outcomes, as per its primary goals as it is based on design specifications and not the quality of the built form.

In this context, APHA members are increasingly required to assist in the justification of how or why the Passive House standard equals or surpasses the NatHERS program in the delivery of true, real world comfort and energy efficiency. While it is correct to state that Passive House construction techniques will always provide better outcomes in the built form, there is a major need to support our members and bridge the knowledge gap with calculated, quality data.

The statements above do not constitute a criticism of NatHERS by APHA, but rather simply reflect a position that states that relying on design values without reference to validated real world data, puts real world comfort and energy efficiency at risk in the built form.

Design for Place

Design For Place is an Australian Government initiative that aims to offer sustainable, energy-efficient housing designs that are free to download. The aim of the architect-designed suite of plans is:

  • for anyone planning a new home
  • to provide a set of floorplans and elevations for a single storey house in three different versions depending on block size. 
  • to showcase the use of sustainable design principles for domestic home design and construction.
  • to provide specifications and construction techniques for a range of climate zones across Australia designed to achieve a minimum 7 star NatHERS energy rating.

In order to compare a Passive House construction with a NatHERS 7 Star design, a comparison is required. As such, we have taken the opportunity and used the Design For Place floorplans and elevations for a single storey house to set a baseline for comparison as it is a verified and reputable source.

Of the various designs available, a 3 bedroom + study (Option 1A) single detached dwelling on a suburban block of just over 500m2 was chosen. Full plans and elevations can be found here.

Our Approach

Our approach to this project is simply to take the verified “Design For Place” 7 star specifications (floors, ceilings, roof, external and internal walls, draught sealing and windows) for 8 climate zones and model them in the Passive House Planning Package (PHPP) V9. PHPP is the most tested energy modelling tool globally, with long-term experiences and statistically verified measurement showing a very strong correlation between design and construction.

Specification (Design For Place)

The Design For Place initiative base specification include the following details:

For additional parameters not included in NatHERS modelling or for those that are not transferable into PHPP, we have used industry knowledge and research to stipulate what these are in the base specification.

  • Thermal bridging (ψ-value) – Via reference to the UK Standard Assessment assumptions on thermal bridge types, a generic value of 0.1W/mK has been nominated for the base specification for all linear thermal bridges.

Quality Bands

We have then sought to allow the user to improve the construction quality of the fabric and workmanship in the following quality bands. Anything pertaining to understanding engineering has been avoided in the primary interface to ensure the widest possible use.

  • Minimum = 7 star minimum fabric compliance
  • Improved = better than 7 star minimum fabric compliance
  • Best-practice = significantly better than 7 star minimum fabric compliance and possible Passive House Criteria
  • Excellent = equal to or greater than Passive House Criteria

Construction Types

The app allows the following construction types to be adjusted:

  • Floor (R-value)
  • Walls (R-value)
  • Glazing (combined U-value + SHGC)
  • Roof (R-value)
  • Thermal bridging (ψ-value)
  • Air-tightness (ACH)

Climates Zones

The following climate zones are available for assessment:

  • Adelaide
  • Brisbane
  • Canberra
  • Darwin
  • Hobart
  • Melbourne
  • Perth
  • Sydney

Results

Results display high-level results only, including:

  • Energy – A space heating and cooling (W/mand kWh/m2/annum) % improvement over the 7 Star NatHERS Design For Place initiative. For example, a 10% value display simply states a 10% improvement.
  • Occupant Comfort – Frequency of Overheating (25° C) % improvement over the 7 Star NatHERS Design For Place initiative. For example, a 20% value display simply states a 20% improvement in occupancy comfort has been achieved.
  • Operation Costs – Annualised space heating and cooling cost $$ % improvement over the 7 Star NatHERS Design For Place initiative. For example, the annualised cost of $1000 value for space heating and cooling will be displayed and reduced depending on improved fabric. Based on 25c/kwh elec.
Disclaimer

While the Australian Passive House Association has attempted to make the information on this ‘process and interactive tool’ as accurate as possible, the information is for personal and/or educational use only and is provided in good faith without any express or implied warranty. There is no guarantee given as to the accuracy of this ‘process and interactive tool’.

Persons accessing the ‘process and interactive tool’ who require confirmation of any information should refer to the Australian Passive House Association for more information.

The Australian Passive House Association does not accept responsibility for any loss or damage occasioned by use of the information contained within the ‘process and interactive tool’.

While the Australian Passive House Association will make every effort to ensure the availability and integrity of its ICT resources, it cannot guarantee that these will always be available, and/or free of any defects, including viruses.

Users should take this into account when accessing this resource. All access and use is at the risk of the user.

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