Forrest Passive House, Spotswood

The Forrest Passive House started as a $550,000 renovation. Altereco Design had a retrofit project in Spotswood and brought Carland Constructions in early, as we prefer to work. After pricing the renovation and running the numbers, we told the clients it was not good enough. The performance gains were not there. The comfort levels would have been like any other renovation. We asked for time to think.

What came back was a different proposal entirely. Knock down the existing home. Build a certified Passive House from scratch. We presented the idea, could not read the reaction, and assumed the project was dead.

A day later the clients said yes.

We relocated the existing California bungalow rather than demolishing it, finding it a new life rather than sending it to landfill. Then we built from the ground up, with Passive House certification as the non-negotiable outcome.

It was Carland Constructions' first certified Passive House. We failed forward through a lot of it. We missed labour in the costing, learned detailing on the fly, and leant heavily on Drew from PassiveTech whose knowledge of airtightness and insulation detailing was critical to getting the result across the line.

The blower door test was on the same day we had accidentally scheduled a client meeting. When the result came in at 0.41 ACH50, the clients looked at us and asked: is that good?

It was very good.

How we built it

The Forrest Passive House is a timber-framed construction on a suspended floor over a ventilated crawl space. The key envelope assemblies and their certified PHPP performance values are as follows.

Floor assembly: R2.5 insulation batts between 180mm joists, 19mm particle board, membrane. U-value 0.259 W/(m²K). Annual ground heat losses 728 kWh.

External walls: 140mm MGP10 timber stud frame at 600mm centres, R4.0 insulation batts in the primary frame, 45mm service cavity with R1.0 batts, Pro Clima vapour control membrane, 10mm plasterboard internally. U-value 0.258 W/(m²K). Total wall area 200.2 m². Annual transmission losses 1,240 kWh.

Roof: Five assemblies across the building geometry. Primary truss assemblies achieve between 0.105 W/(m²K) and 0.128 W/(m²K). Rafter sections achieve 0.157 W/(m²K). Total roof area 175.7 m². Annual transmission losses 510 kWh.

Windows: Viridian Lightbridge triple glazing, Koemmerling C70 PVC frames. Glazing Ug-value 0.749 W/(m²K), g-value 0.57. Overall installed Uw 1.275 W/(m²K). Total window area 37.84 m². Heating season transmission losses 1,157 kWh. Heating season solar gains 1,162 kWh.

Airtightness: Pro Clima membranes and tapes throughout. Service cavity protecting the airtight layer from trade penetrations. Intermediate blower door tests during construction. Final certified result 0.41 ACH50.

Mechanical ventilation: MVHR system with 80% heat recovery efficiency. Supply air change rate 0.375 ACH. Annual ventilation heat losses 320 kWh (2.1 kWh/m²a).

Why mechanical ventilation matters in Spotswood

Spotswood sits within Hobsons Bay, Melbourne's inner west. Yarraville and Brooklyn, both within a few kilometres, rank seventh and eighth in Australia for air pollution concentrations. Hospital admissions for asthma are around 20% higher in Hobsons Bay than the Australian average (Inner West Air Quality Community Reference Group, 2020). The EPA Victoria study examining PM2.5 particle sources across the inner west between 2021 and 2022 found that fine particles from trucks, major roads, industry, and the Port of Melbourne penetrate deep into the lungs and enter the bloodstream, contributing to respiratory and cardiovascular disease over time.

Most homes in this area ventilate by accident, through gaps in the structure. Whatever is in the air outside comes straight in.

The MVHR system in the Forrest Passive House draws fresh air in from outside, passes it through filters, and delivers it to every living space and bedroom continuously. Stale air is extracted from wet rooms and exhausted outside. The two streams pass through a heat exchanger, transferring 80% of the heat from the outgoing air to the incoming air without the streams ever mixing. The result is continuous filtered fresh air at minimal energy cost. For a house in Spotswood, that filtration is a genuine health decision, not a feature upgrade.

Three years on

The Forrest Passive House is completely comfortable. The clients do not think about heating. They do not think about cooling. The building manages itself.

The clients went overseas for a few weeks during winter. When they came back, the temperature inside had not moved. Melbourne had done its worst. The house had not noticed.

That is what 13.1 kWh/(m²a) looks like in practice. Not a number on a certificate. A home that holds its temperature through a Melbourne winter on almost no energy input, even with nobody in it.

The Performance

Airtightness: 0.41 ACH50

Heating demand: 13.1 kWh/(m²a)

Heating load: 11.0 W/m²

Cooling demand: 3.3 kWh/(m²a)

Primary Energy Renewable demand: 37.4 kWh/(m²a)

Renewable energy generation relative to footprint: 56 kWh/(m²a)

Frequency of excessive humidity: 0%

Passive House Classic: Yes