Gold Star: Summary of Thermal Performance Assessment


Andrew Kerrby author page

2 comments Design

Part of a team

Most projects will involve a number of consultants throughout the design and certification process: for 60k House this number has been seven (plus me). The following is an explanation of what consultants have been responsible for and when they have been involved. Cast in order of appearance:

Land Surveyor

The first consultant engaged was a Land Surveyor. As the name suggests they survey the land (not to be confused with a Building Surveyor). I wrote about the excitement of getting a detail site survey in ‘The Site’.


A site survey will accurately locate a variety of information (boundaries, trees, site levels, etc) and can be a valuable design tool that informs Site Analysis and Site Planning (see earlier posts).


I commissioned the site survey long before I had even decided I was going to design and build 60k House. A site survey is not dependent on a design. On the contrary: I believe any design should be responsive to the site. Having a survey of the site will help inform the design strategy.

Geo-tech Engineer

Most projects will require a Site Investigation Report that covers the Soil Classification, Wind Classification, Soil Profile and Site Classification.


A test of the soil is important so everyone knows what lies beneath the surface and what to expect when you start digging. A Geo-tech Engineer does this work, usually by boring a series of holes with a hand auger. Test holes need to be dug in the location of the proposed structure as soil profiles can vary significantly in different locations on sites in Tasmania due to our varied geological profiles. Typically each test hole will go down until rock or dense material is discovered. The soil profile will be analysed and recorded, detailing the various sediments at various depths.


Layered: the soil profile from test holes dug onsite (60k House is on the right listed as 'Shack')

Layered: the soil profile from test holes dug onsite (60k House is on the right listed as ‘Shack Site’)


You can commission a Site Investigation Report pre-design (the results may inform the design strategy from the outset) or wait until you know roughly where the building will go. I waited until I had completed Site Planning and then asked for test holes to be dug at each of the locations: 60k House site, a potential future house site and a possible shed (saving the engineer travel time from multiple trips, and saving me some money).


Other consultants will use the results from the Site Investigation Report – Structural Engineers and Hydraulic Engineers. The soil profile and classification will inform the design of the foundations or footings of the building while the wind classification will determine the bracing required; the absorption or dispersion capacity of the soil will determine the onsite wastewater options and design.

Bushfire Hazard Assessor

I explained the role of a Bushfire Hazard Assessor in a previous post, and a guest contributor has answered FAQs. Bushfire Hazard Assessors calculate the potential bushfire risk posed to the site, and make recommendations through a Bushfire Hazard Management Plan to mitigate these risks.


Only a few years ago it was acceptable to prepare your own Bushfire Hazard Assessment and have it signed off by the local fire service. However after recent fire disasters and legislation it is now a requirement that a suitably qualified professional undertakes this work.


Remember: any site within 100m of 1ha of unmanaged bush will require a Bushfire Hazard Management Plan – this includes most rural and even some suburban properties.


A site visit is usually required by the Bushfire Assessor to confirm the vegetation and slope of the site and the surrounding properties prior to preparing a Bushfire Hazard Management Plan, however the Bushfire Assessor isn’t required to inspect the site at the end of the project (that responsibility falls to the Building Surveyor). It was interesting to have the Bushfire Assessor visit 60k House recently and hear him say: yeah, that looks about right – you could defend this place if there was a fire.

Hydraulic Engineer

If your site is unable to be connected to a sewer main then you will need to deal with all of your waste water yourself. I talked about the main options for onsite wastewater treatment (septic or AWTS) in the previous post ‘Services – Septic’.


A Geo-tech or Hydraulic Engineer will prepare an on-site Wastewater Assessment for the proposed development. This includes the site conditions information gained from the Site Investigation Report such as slope, site drainage, vegetation, and the soil profile. The projected capacity for the house is calculated and the absorption or dispersion capacity of the soil determines the area required for the absorption trenches.


Some councils require a Wastewater Assessment to be submitted at the same time as a Development Application – this is usually dependent on the overall size of the site and to demonstrate capability of dealing with waste on the site. I had the Wastewater Assessment completed after council had granted Development Approval and I knew the project could proceed.


Whoever designs the Wastewater Assessment will need to conduct a site visit or two (or may be satisfied by photographic evidence) in order to certify that the onsite wastewater treatment system has been installed as per the approved design.

Structural Engineer

A Structural Engineer provides engineering design and certification for the structural components of the building. This includes the footings / foundations, the slab, the frame (lintels over windows and double studs or columns) and the bracing required to hold it all together and stop the building blowing over. The Structural Engineer might also provide design for the roof structure (if rafters are used) but in the case of 60k House the truss manufacturer provided this design and certification.


A meeting or two with the Structural Engineer will help to figure out the best methods for the structure. I didn’t want to use any structural steel in the project (posts or beams) but stick with timber – to keep the costs down and make it easier for Greg the builder. This strategy is feasible for a project of this scale but might not be for a larger or more ambitious project. Getting good, efficient engineering design can save a lot of time and money when it comes to construction.


All of the structural information needs to be integrated into the drawing set – either as stand alone documentation from the Structural Engineer or incorporated by the Building Designer. The Structural Engineer will then stamp each page, and issue a certificate stating the structural design is adequate as documented.

Building Surveyor

Unlike a Land Surveyor who surveys existing land, the role of a Building Surveyor also includes the assessment of buildings that are yet to be built. Yes Building Surveyors do inspect already constructed buildings but they also spend probably just as much time looking at drawings of buildings on paper as they do walking through actual buildings.


Building Surveyors inspect drawings of proposed buildings and assess them against building codes to ensure the building will comply (if built as it has been drawn). Building codes don’t just include the National Construction Code or Building Code of Australia, but also Australian Standards, and regulations prescribed by other consultants and authorities such as bushfire codes.


If the Building Surveyor is satisfied the design will satisfy all of the relevant codes then they will issue a document called a Certificate of Likely Compliance. This is exactly what it says on the box – a certificate stating the building will likely comply. There may be some additional conditions attached to the CLC.


Either the Building Surveyor or the Structural Engineer must inspect prior to pouring concrete and again before enclosing the structural frame. The Building Surveyor also needs to inspect before occupancy (to check that the building is safe for inhabitants) and upon completion to ensure the building meets all of the requirements. The Building Surveyor inspects the work during construction and then signs it off once all work is complete and likely to comply with the loads of legislation that they have to get their head around!

Thermal Performance Assessor

A recent amendment to the Building Code of Australia addresses Energy Efficiency. It is by no means comprehensive but means essential elements need to be met, such as basic levels of insulation dependent on which climate zone you are located in (Tasmania is zone 7 out of 8). The building element that requires some modeling is glazing. How much glazing you are allowed to have depends on the floor area and which climate zone you are in. For Tasmania the basic premise is that north facing glazing is good (for heat gain) and south facing glazing is bad (heat loss). There are lots of variables including the thermal values of the glass, the frame and any shade protection.


There are two methods to assess the glazing for energy efficiency: the National Construction Code Glazing Calculator or a Thermal Performance Assessment. The Glazing Calculator is much simpler – each window is entered into an Excel spreadsheet listing the size, orientation, U value (the coefficient that describes how much heat is transferred through the window) and Solar Heat Gain Coefficient (how much solar radiation passes through the window).


The lower the U value the better the insulation properties are at keeping the heat or cold in or out. Windows with a higher SHGC allow more solar radiation into the building, providing free solar heating. Both the glass AND the frame is considered for a window’s U value and SHGC.


The Building Designer can usually complete the Glazing Calculator; a Thermal Performance Assessment requires an accredited specialist. After you have entered all of the values for each of the windows using the Glazing Calculator you will either receive a big red X or a big green tick. Despite good building orientation and double glazed window assemblies I couldn’t quite get the green tick without specifying expensive glass for some of the windows. Instead I opted to engage a Thermal Performance Assessor to evaluate the projected energy use for 60k House.


Big green tick: Argon filled double glazing would have been more expensive than commissioning a Thermal Performance Assessment, for minimal performance improvement

Big green tick: Argon filled double glazing would have been more expensive than commissioning a Thermal Performance Assessment, for minimal performance improvement


Design tip: generally a larger house will perform better if the Glazing Calculator is used (because of the large floor area); a smaller house will perform better if you use a Thermal Performance Assessment.


A Thermal Performance Assessment not only assess the glazing component of the building but ALL other elements, including insulation, wall materials, flooring, even ceiling penetrations. Because the other sustainable features were also considered in this method (such as the in-slab insulation) the project easily passed. By calculating the expected energy use of the building you also receive an energy efficiency star rating – just like when you buy a new electrical appliance. 60k House – 7.1 stars!


Good design and good consultants save you money. The cost of commissioning an Thermal Performance Assessment can save you heaps of money when it comes to windows for your house.


An upcoming post will talk about all of the approvals required before building even begins.


Costs: site survey – $880; site investigation report – $550; bushfire hazard assessment – $600; wastewater design – $495; structural design – $792; building surveyor – $1320; thermal performance assessment – $306


Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.


  1. Andrew – Can I ask – How come the soil report only gives a bearing capacity down to the 3 stratum (which has a bearing capacity of 60-100 kPa) – is is because your slab only needs a bearing capacity of min 50 kPa? Also, the image of the soil report you gave doesn’t give a Site Classification (A, S, M, up to P(roblem) type soil). Is it just not included on the image you included? Could I ask what Site Classification you received for your site (according to NCC 2015 – old BCA)?


    1. Andrew Kerr

      Thanks Hamlet.
      The soil bearing capacity is listed for the horizon’s where it is under 100 kPa. In the other horizons it exceeded 100 kPa. For AS 2870 deemed to comply solutions footings require a min. bearing capacity of 100kPa and slabs 50kPa.
      The image only showed part of the report (the interesting looking part). The soil classification for the site is ‘M’.

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