Off-grid, sustainable farmhouse | Architecture Republic

Harcross House, an off-grid strawbale home on a cattle farm near Braidwood, NSW, achieved an extraordinary 9-star NatHERS certificate and BASIX compliance, redefining sustainable architecture. The client, Adrian, sought a comfortable, energy-efficient house to withstand the harsh alpine climate of the Great Dividing Range. Working together with Architecture Republic, we optimised the design for maximum energy efficiency, delivering a home showcased on Sustainable House Day 2025 that the client described as “a delight to live in.”

Challenge

Located on an alpine cattle farm, Harcross House faced extreme weather: up to 60 frosts annually, occasional snow, strong spring winds, and hot, dry summers. The client initially envisioned a Passive House-style design (well beyond the NatHERS and BASIX standards), but needed a cost-effective solution to avoid diminishing returns.

Solution

Powerhaus Engineering conducted a two-stage NatHERS assessment, integrated with BASIX compliance, to optimise the efficiency of the house. Rather than the full Passive House approach, we instead helped prioritise high-impact efficiency measures to maximise bang for buck. We modelled the strawbale walls, fully insulated concrete slab, and central rammed earth wall, optimising insulation and triple-glazed windows (some with heat-reflective films) to balance south-facing heat loss with northern solar gain.

A bold decision was discouraging in-slab heating (saving tens of thousands), relying instead on a 6 kW split-system air conditioner and a slow-combustion fireplace with external air supply for occasional use.

Results

Harcross House achieved a 9-star NatHERS certificate—an extremely high building energy rating—and full BASIX compliance, setting a benchmark for sustainable architecture.

The optimised strawbale design is energy self-sufficient with a modest solar array and minimal battery use. A 10 kVA diesel generator remains a rarely used backup.