Passive Design
Oct 2024
Part 3 of a series exploring passive building design.
Theory's done. This piece is the doing — the design moves that take a Brisbane house from minimum-compliance to genuinely comfortable, expressed as numbers a builder can quote off.
Orientation: north matters, but not the way you think
The Brisbane sun is high. At summer solstice it tops out around 79° above the horizon at noon; at winter solstice it's still hitting 38°. That asymmetry is the lever passive design pulls. Orient your main living spaces and primary glazing within 20° of true north. With a 600 mm horizontal eave above the head of a north-facing window, you'll fully shade that window from October through February while still admitting the lower winter sun for free heat from May through August. YourHome publishes the calculator if you want to model it yourself.
East and west glazing is harder. The sun is low and the eaves above it don't shade much. Treat east and west like liabilities: keep glazing small, use external operable shading (battens, screens, deciduous planting), and specify low-SHGC glass.
Ventilation: where Brisbane wins
In a humid subtropical climate, moving air does more cooling work than thermal mass ever can. Design for prevailing breezes — Brisbane's are predominantly north-east in summer (the bay breeze) — and ensure every habitable room has cross-flow paths: openings on opposing walls, ideally with one high and one low for stack effect on still nights. Ceiling fans on every bed and living space. Aim for a ventilated home that genuinely runs without mechanical cooling for 60–70% of the year. The remaining 30–40% — the muggy March nights — is what air-conditioning is for.
The fabric: where the stars come from
For a Brisbane home to hit 7-star NatHERS comfortably:
Ceiling insulation: R5.0 minimum; R6.0 for a flat or shallow-pitched roof. Light-coloured roof or solar-reflective roof coating.
Wall insulation: R2.5–R3.0 in cavity or stud bays, with a reflective sarking layer where possible.
Floor: R2.0 underfloor batts on elevated timber floors. On slab-on-ground, R1.0 edge insulation if your design is otherwise lean.
Glazing: aluminium-framed standard glass struggles past 6 stars. Move to thermally broken frames or uPVC with double glazing on all primary openings. SHGC below 0.45 on east/west glazing, 0.5–0.6 on shaded north glazing where you want winter solar gain.
Air leakage: below 5 air changes per hour at 50 Pa pressure. Standard Brisbane construction is closer to 15. Sealing junctions and penetrations is the cheapest star-rating bump available — usually $1,500–$3,500 of labour on a single-storey build.
Thermal mass: less, and only where it works
Conventional wisdom says concrete slabs and brick walls store heat and stabilise temperature. In the Brisbane subtropics that's only true if the mass is shaded from summer sun and exposed to winter sun — and even then, ventilation typically does more work for less cost. A useful rule: mass on the north (winter solar exposure, summer shading), light-weight on the east/west. Don't put a slab-on-ground floor in a poorly-oriented west-facing room and expect anything but a brick that holds heat into the night.
Whole-of-Home: the appliance ledger
NatHERS thermal stars only cover the building fabric. The Whole-of-Home rating covers your appliances, hot water, cooktop, solar, and battery. To hit Whole-of-Home 60 (the NCC 2022 minimum), you'll typically need: a heat-pump hot water unit, induction cooktop, efficient split-system air-con, and rooftop solar sized to at least 5 kW. Add a battery and you can push Whole-of-Home above 90 — and substantially insulate the household from grid pricing.
The five-year horizon
If you're building now, build past 7 stars. The federal Trajectory for Low Energy Buildings sets a path toward 8 stars and tighter Whole-of-Home thresholds in NCC 2028. Mandatory disclosure of energy ratings at sale is on the horizon in most states. The home you certify at exactly 7 stars in 2026 will look subnormal in 2030. The home you certify at 8 stars will sell at a premium.
If you'd like us to walk your block before you commit, get in touch.
By Ashton Genrich — Building Design — Open, QBCC 15387939