There is a wide range of information available to assessors on building products from manufacturers and suppliers, much of it now on the internet. This information is available to assessors, builders and clients and includes thermal performance data in many different and sometimes misleading ways.

    Q. How does an assessor know what or whose information to believe and how to model it in the rating tool??

    A. By querying the supplied information, understanding the proposed construction system, the individual material, air gap and air film R-Values, the total R-Value of the building element and then modelling it correctly in the rating tool.

    Assessors must ensure they understand what the information supplied by the client, builder or from the internet represents.

    Assessors need to know what the published R-Values represent. e.g. materials and building elements are often presented with “Up and Down” or “Winter and Summer” or “In and Out” values that are assumed by clients and builders to be the R-Value of the insulation material.

    This is not the case.

    These values are not a material’s R-Values but are the Total R-Value (sometimes referred to as system R-Values) of a complete building element such as a wall, roof or ceiling.

    It is important that assessors do not model the claimed R-Value of a building element as the R-Value of the material itself in the rating tool. You will be overstating the performance of the building element and the rating tool will generate an incorrect star rating.


    The assessor is instructed by the client to use 50mm building blanket with foil facing downwards under a metal roof with a flat plasterboard ceiling as the client has a brochure that says this product has a high R-Value of R2.6.

    The assessor does not accept this information and decides to investigate the claimed performance. Using the Insulation Handbook published by ICANZ as a guide, investigations show that 50mm blanket has a material R-Value of around R1.3 and that the Total R-value of the roof/ceiling system with a non ventilated roof space in a downwards direction i.e. as used in warm/hot climates is R2.6. This data is supported by checks at the manufacturer’s website.

    The blanket must be modelled in the rating tool with an R-Value of R1.3.


    Source : ICANZ Insulation Handbook - June 2009 - Part 1 : Thermal Performance.
    For underlying calculations and assumptions see Page 14 of the handbook or go to

    Both Accurate and Bers Pro allow assessors to build custom elements that reflect how the building element is constructed including each individual material, any added insulation and air gaps. BERS Pro also includes a suite of tools that enables assessors to calculate the R-Value of individual materials and air gaps that are not part of the default sets included in the software.

    Remember it is the assessor who is responsible for correctly entering data into the rating tool. Assessors must ensure the residence they are rating is modelled correctly in the rating tool in accordance with NatHERS Protocols and Assessor Procedures.

    Michael Plunkett is Principal of SmartRate based in Cairns and a practising ABSA accredited assessor.

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