The four rules of Melbourne passive solar

Passive solar design in Melbourne is governed by four rules that flow directly from the latitude. Get them right and the mechanical HVAC load drops 40-60% against a naive design. Get them wrong and you either overheat every summer or underperform every winter.

• Rule 1 - Orient the primary living zone within 20° of true north. Beyond 20° the winter solar gain drops sharply.
• Rule 2 - Size north glazing at 30-40% of the living-zone floor area. Less and you leave heat on the table; more and you overheat.
• Rule 3 - Overhang the north glass by 600-900mm. This admits the low winter sun (22° altitude at midday June) and blocks the high summer sun (76° altitude at midday December).
• Rule 4 - Give the north-facing living zone something to store heat in. 100-150mm exposed concrete slab, honed and sealed, is the cheapest thermal mass. Alternatives: brick internal walls, water walls, phase-change plasterboards.

Why west-facing glass is the number-one Melbourne overheating cause

The Melbourne summer sun sets on the west-southwest in a shallow arc. Any unshaded west-facing glass admits solar gain from 3pm through sunset - the exact window when the internal air temperature is already peaking. External shading (fixed louvre, retractable awning, deciduous tree) is mandatory for west-facing glazing over 3m² if you care about summer performance.

Internal blinds do not solve this - by the time the sun hits an internal blind, 87% of the solar heat is already inside the room. Only external shading works.

How NatHERS actually credits passive solar

A NatHERS energy rating is a whole-house computer simulation using local climate data (Melbourne uses climate file zone 21). It runs 8,760 hourly simulations across a full year, tracking heating and cooling loads under a standard occupancy profile. A passive-solar-oriented Melbourne house typically returns 7.2-8.5 stars on the base plan. A poorly oriented one struggles to reach 6.5 stars without expensive glazing upgrades.