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New BASIX Thermal Comfort Caps, July 2017

From 1 July 2017, the NSW Building and Sustainability Index for Thermal Comfort (better known as BASIX) has increased in stringency. As a result, buildings that used to comply under the old requirements may not comply under the new ones. In particular, this means that building designs which are currently under way can have extra hurdles to cross which may not have been anticipated during their early design phase. This may sound like a bad thing to some or a source of more headaches, but it’s actually a positive step forward – buildings that meet the new requirements will be more comfortable to live in. The higher up-front construction costs are more than made up in the ongoing expenses to keep a building cool in Summer and warm in Winter.

The new BASIX requirements mean more comfortable, more efficient buildings.

A thermal camera pointed at a house

Because this can mean some extra effort required to find a complying solution for your building, we thought we’d provide some hints on how to get your building over the line under the BASIX system. But before we start, this is all “generic” information, some ideas listed below may not work well (or at all) for your specific building. BASIX’s Thermal Comfort simulation results for heating and cooling (often abbreviated to just TC) are calculated on the location & orientation of each project so what works in one area may not work in another. And as always, contact us if you’d like an obligation-free fee proposal for advice for your specific building. But before we get to the good part, we need to re-state some of our standard advice & warnings.

Firstly, as always the full TC Simulation method will be the most flexible. Money spent on having an Accredited Energy Assessor perform a full Simulation will represent good value for money to optimise your building for both lower initial construction cost and higher energy efficiency.

Secondly, the TC DIY method is somewhat, well, “clunky”. It may seem attractive and a potential way to save money, but it doesn’t give you many options or selections, and therefore it can’t represent the complete range of available material properties (e.g. all the different types of glazing). Only a full TC Simulation method can give you the full picture, and let you fine-tune the building to optimise the result to achieve lower initial construction cost and higher energy efficiency.

Thirdly, the old adage Garbage In, Garbage Out applies here too. If you don’t select your Energy Assessor carefully, you may not get the result your building deserves. Some Assessors may have learnt poor data entry habits that leads to them arriving at incorrect results. When they’re investigated by their Assessor Accrediting Organisation, they may fail their Quality Assurance audit. Because of the delay between your report and these investigations, this loophole means sub-standard buildings may be built. Your building deserves to be thermally comfortable, so we recommend you stay clear of “dodgy” assessors.

So how does the new BASIX DIY tool work?

According to a Report from the NSW Department of Planning & Environment, “The DIY method does however still predict heating and cooling loads within a certain degree of agreement with full simulations”.

Simply put, the new DIY tool is based on the BCA2010 Glazing Calculator with minimum insulation R-values set between BCA2009 & BCA2010. The old (pre-July 2017) DIY tool resulted in only 9% of test houses passing the BCA2010 Glazing Calculator.

The glazing scores are converted into heating and cooling loads through the “DIY algorithm”. From the BCA Glazing Calculator, the Aggregate Conductance (heat loss) is converted to the glazing heating score, and the Aggregate Solar Heat Gain is converted to the glazing cooling score.

The DIY method adds a “construction load adjustment” to these scores, based on the minimum BASIX insulation and different generic construction materials. The DIY TC loads are then calculated based on these approximations and assumptions.

The Report states that of the DIY buildings in 2009/2010:
  • 84% had concrete slab on ground floors
  • 47% had framed suspended floors (including 2 storey)
  • 68% had brick veneer external walls
  • 10% had cavity brick external walls
  • 34% had weatherboard external walls
  • 89% had flat ceilings with pitched roof

There’s a mismatch between the DIY calculation and the Thermal Simulation averages. We figure this is because the DIY method doesn’t take into account any of the following:

  • Area of external walls
  • Area of ceiling
  • Cross-flow ventilation
  • Roof type, or
  • Zoning of different rooms (Living and Sleeping)

Because of this, much of the rest of the DIY adjustment factors seek to remedy this difference, by fitting adjustments to the data.

While the average comparison of DIY method versus Simulation looks reasonable, the deviation from the average between the two is alarming. The discrepancy between the two methods varies from up 2.29 Stars to down 2.04 Stars. Only 34.7% of designs analysed in the Report had the DIY result within 5% of the Thermal Simulation total loads. This also means the new DIY method is, on average, about 6% more stringent than the Simulation Method.

In short, the new DIY doesn’t completely match the results of the Simulation Method. Clients and Energy Assessors should remain wary of the DIY tool which is alleged to deliver something equivalent to the Simulation Method in BASIX Thermal Comfort for residential buildings, but regularly varies quite a bit.

All that being said, the DIY method still has a place in reducing the prevalence of buildings that may be thermally poor, so long as you keep in mind that it is not a design guide for good practice. For this, use the Simulation Method instead.

Thermal Comfort maximum loads, or caps

The Thermal Comfort is based on a maximum “allowable” heating load and separate cooling load. These are measured in specific energy units of MJ/m² of the conditioned floor area (area-adjusted, living and sleeping rooms). These numbers are commonly called the Heating and Cooling Caps, and they vary according to:

  • Climate Region (determined by Postcode)
  • Project Type (single dwellings or average of multi dwelling Units, or individual Units in multi dwellings developments)
  • Construction Style (slab on ground, suspended floor, mud brick walls, etc)

Additionally, the caps for slab on ground for single dwellings and individual Units are the same as the average for Units. The caps for slab on ground are lower than for suspended floors (whether open or enclosed sub-floor) and mud brick walls, because the thermal mass of slab on ground generally makes a building more energy efficient. The caps for individual Units are higher than the average for the multi-residential development, meaning there needs to be higher-performing Units in the project to balance out the “just complying” Units.

Compared to the former DIY tool, all new DIY projects will most likely need more wall and/or ceiling insulation. From the report’s analysis of 50 current DIY-compliant projects, in order to comply with the new DIY method, approximately 32% of projects will need significant changes to glazing, shading or their building design. This makes the Simulation Method even more attractive over the DIY method than before – the cost of the simulation is often less than the cost of expensive improved glazing for just one single window, let alone for a whole building.

What can I do if I’m over the Heating cap?

If you’re over the heating cap, the building is getting too cold in winter. There are two related remedies: reduce unwanted heat loss & capture more free solar heat.

Solutions for NSW climates may include:
  • Reduce the amount of glazing facing South or where there is a covered verandah (too much shade)
  • Increase glazing facing North (move East & West windows to the North side of the building, if possible)
  • Reduce shade to the glass (e.g. shallower eaves)
  • Avoid adjacent structures blocking the sun to living rooms
  • Remove visible tinting from glass (higher Solar Heat Gain Coefficient)
  • Switch to low-E glass and/or thermally-improved frames instead of Aluminium (thermally broken, timber, uPVC, fibreglass, etc)
  • Double glazing, optionally with argon gas fill (for windows or views you just can’t live without)
  • Reduce the total amount of glazing (walls are cheaper than glazing too, so you’ll save money)
  • Use a darker colour on the roof & external walls (higher solar absorptance)
  • Increase insulation to the ceiling, walls & suspended floor
  • Enclose & insulate the sub-floor perimeter walls (don’t forget there’s a minimum ventilation amount required)
  • Increase insulation to any internal walls separating habitable rooms from other rooms, e.g. the Garage, Laundry & Bathroom (especially if these rooms are facing South)
  • If possible with the design, move the living rooms so they face North (with a glazed door) or East
  • Employ beneficial internal thermal mass that absorbs solar heat in winter & is shaded in summer

What can I do if I’m over the Cooling cap?

Going over the cooling cap means the building is getting too hot in summer. There are two main remedies: reducing unwanted heat gain, and minimising solar penetration.

You may notice many of the possible solutions for over-heating are actually the opposite to what’s suggested for over-cooling. It’s very possible to over-compensate and turn an over-heating problem into an over-cooling problem, and vice-versa. Therefore, the appropriate options for your project may need to be a blend of these two lists. That is why we recommend seeking the advice of a professional Energy Consultant, who can work out what options will work best for your specific building.

Solutions for NSW climates may include:
  • Increase the amount of glazing facing South or where there is a covered verandah (better shade)
  • Reduce glazing facing North, East & West of a building, if possible
  • Increase external shade to glass (blinds or screens) and to walls (wider eaves)
  • Employ adjacent structures to block the sun to living rooms
  • Increase visible tinting to glass (lower Solar Heat Gain Coefficient)
  • Switch to low-E glass and/or thermally-improved frames instead of Aluminium (thermally broken, timber, uPVC, fibreglass, etc)
  • Reduce the total amount of glazing (walls are cheaper than glazing too so you’ll save money)
  • Lighter colours to roof & external walls (brilliant “cool” white is coolest)
  • Optimise insulation to the ceiling, roof & walls (more may not always help)
  • Reduce insulation to a suspended floor (the sub-floor is normally cooler)
  • Open up the sub-floor perimeter walls to allow more breezes underneath the building
  • Increase insulation to internal walls separating habitable rooms from other rooms, e.g. Garage, Laundry & Bathroom (especially if these rooms are facing North & East)
  • If possible with the design, move the living rooms to South (with glazed door)
  • Employ beneficial (shaded) internal thermal mass that absorbs daytime heat & releases this after dark with ventilation
  • Improve ventilation of all openings & the building’s cross-flow ventilation (e.g. swap sliding windows to louvres or casements). Outside NSW, add ceiling fans

Aside from both construction and ongoing operating costs (although some of these options can have no or a negative cost), other considerations to keep in mind include:

  • Orientation of the block of land
  • Access road & slope – the position of any garage/carport
  • Scenery views that you want to keep
  • Existing services, easements
  • Neighbouring & overshadowing structures
  • Yard layout (detached garage, pool, shed)
  • Boundary clearances (build-to-boundary rules)

For a free, no-obligation Fee Proposal for your BASIX Thermal Comfort Simulation method assessment, Contact Us!

We are Accredited with ABSA and have an ISO9001 Quality Management System, so you can expect professional, consistent results.

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