Most BASIX thermal failures aren’t about insulation. They come from design decisions made early, often before anyone has thought about energy. Here are the seven we see most, roughly in order of how much trouble they cause.
1. Large west-facing glazing
West glass gets low afternoon sun in summer, which eaves can’t block. It’s the single biggest driver of cooling load in most Sydney climate zones. A wall of west-facing sliders or bi-folds to a view or a backyard can sink an otherwise good design.
Fix: reduce the window size, move glazing to north where you can, use external vertical shading or an awning, or use a lower solar heat gain glass on those windows only.
2. No eaves on north-facing windows
North glazing is the best glazing, because it lets in winter sun and is easy to shade in summer. That only works with the right overhang. Flat-roofed contemporary designs with no eaves lose the benefit and overheat.
Fix: an eave or pergola sized to the window height. The model credits it immediately.
3. Raked ceilings and double-height voids
Big volumes are harder to heat and cool, and they usually come with highlight windows. Heat rises into the void in winter and builds up under the roof in summer.
Fix: higher insulation in raked ceilings, ceiling fans, and shading or better glass for highlight windows. Or consider a smaller void.
4. Uninsulated suspended timber floors
A suspended floor over an open, ventilated subfloor loses a lot of heat in winter. It’s common on sloping sites and in renovations of older houses.
Fix: insulation under the floor, and enclosing the subfloor where practical. It’s cheap to do during construction and very hard to add later.
5. Too much glass overall
Even well-oriented glazing loses more heat than an insulated wall. Once the glass is a large share of the floor area, the model struggles in both winter and summer whatever the orientation.
Fix: look at the glass-to-floor ratio room by room. Taking a metre off a few oversized windows is often enough to avoid upgrading every window in the house.
6. Skylights
Skylights add a lot of heat in summer for the light they bring in, and standard units perform poorly. Several large ones can move the result noticeably.
Fix: fewer and smaller skylights, better-performing units, or tubular skylights where the aim is just daylight.
7. Dark roofs in hot climate zones
Dark roofs absorb more summer heat. In Western Sydney climate zones, where cooling loads are higher, roof colour makes a real difference to the result.
Fix: a lighter roof colour, or better roof insulation and foil if the dark roof stays.
The DIY method treats these issues with blanket rules. A NatHERS simulation weighs them against each other, for example good eaves against a bigger window. That’s why simulation usually finds a cheaper way through for new homes.
How to avoid the problem
- Model at concept stage. A quick preliminary model before the DA drawings are finished shows whether the design is close.
- Fix design first, specification second. Moving or shrinking a window is free on paper. Upgrading glass is paid for in every window.
- Ask for options, not a single answer. There is almost always more than one combination of changes that passes. Pick the cheapest one to build.
- Keep the drawings and certificate in step. Every change after the certificate is issued means a revised certificate.
This is why it pays to involve the assessor early. A change tested in the model before the DA drawings are finished costs far less than the same change made after approval.
Check your design before the DA goes in
Assessments prepared from your drawings, anywhere in NSW. Turnaround is confirmed in your quote and starts once all required information is received.
General information only, current at the date above. Requirements change and every project is different, so confirm the rules that apply to your project with your certifier or council.