What Does a Certified Passive House Actually Cost to Run? The Forrest Passive House Numbers.
Anyone can claim a home is high-performance. Very few builders have ever tested one. Here is every number from the Forrest Passive House in Spotswood, pulled directly from the certified PHPP post-construction file and the blower door test report.
The Forrest Passive House in Spotswood achieved a heating demand of 13.1 kWh per square metre per year, a cooling demand of 3.3 kWh per square metre per year, and an airtightness result of 0.41 ACH50, certified by Detail Green in March 2024. Every figure was physically tested and independently verified. None of it is a modelled assumption.
At 0.41 ACH50, the Forrest Passive House is roughly 46 times tighter than the average Melbourne home at 19 ACH50 (CSIRO / Australian Building Codes Board, 2016).
What does airtightness actually mean, and why does it matter in a Melbourne home?
ACH50 stands for Air Changes per Hour at 50 Pascals. The blower door test pressurises the building to 50 Pascals, roughly the pressure of a moderate wind, and measures how much air leaks through the envelope per hour relative to the building's internal volume.
The Passive House threshold is 0.6 ACH50. The Melbourne average is 19 ACH50. The Forrest Passive House measured 0.41 ACH50 on 25 February 2023, tested to ISO 9972 Method 2. Depressurisation: 0.42 ACH50. Pressurisation: 0.41 ACH50.
In plain terms: if you pressurised this house to 50 Pascals and left it, it would take over two and a half hours to lose half its air. A standard Melbourne home would do the same in about three minutes. Every minute of that difference is energy escaping through gaps you cannot see and cannot fix without knowing where they are.
How did we achieve 0.41 ACH50 in a standard timber frame?
The airtight layer in the Forrest Passive House runs continuously behind the service cavity. Proclima membranes were used at all junctions, laps, and penetrations. The service cavity exists to protect this layer. Cables and pipes run inside the cavity so the membrane is never punctured by trades.
We also conducted intermediate blower door tests during construction, before internal linings went on, so we could find and fix any leaks before they were buried behind plasterboard. By the time we did the final certification test, we already knew the result would be strong.
What do the wall, roof, and floor assemblies achieve in real numbers?
The external wall is a 140mm timber stud frame at 600mm centres. R4.0 Fletcher's insulation batts in the primary frame, R1.0 batts in a 45mm service cavity, Pro Clima building wrap, 10mm plasterboard internally. Wall U-value: 0.258 W/(m²K). Total wall area: 200.2 m². Annual transmission losses: 1,240 kWh.
The primary roof truss assemblies achieve between 0.105 W/(m²K) and 0.128 W/(m²K), with R7 Bradford Gold batts in the truss depth plus R2.5 in the bottom chord. Total roof area: 175.7 m². Annual roof transmission losses: 510 kWh.
The suspended floor over the ventilated crawl space uses R2.5 insulation batts in 180mm joists with a membrane below. Floor U-value: 0.259 W/(m²K). Annual ground heat losses: 728 kWh.
What do the windows contribute to heating and cooling demand?
Viridian Lightbridge triple glazing, Koemmerling C70 PVC frames. Overall installed window U-value: 1.275 W/(m²K). Glazing g-value: 0.57. Total window area: 37.84 m².
Heating season transmission losses through windows: 1,157 kWh. Heating season solar gains through windows: 1,162 kWh. Those two numbers are almost perfectly balanced. The windows are simultaneously losing heat and collecting it, and across the full heating season they essentially break even. That is not a coincidence. That is what PHPP modelling before design is finalised looks like.
What does the mechanical ventilation system contribute to the energy performance?
The MVHR system recovers 80% of the heat in the outgoing air stream and transfers it to the incoming fresh air without the two streams ever mixing. Without heat recovery, the ventilation conductance would be 59.1 W/K. With it: 12.0 W/K. Annual ventilation heat losses: 320 kWh (2.1 kWh/m²a).
In summer, the system runs an automatic bypass. When outdoor air is cooler than indoor air, the heat exchanger steps aside and cool fresh air flows directly into the house, contributing to the cooling demand of just 3.3 kWh/(m²a).
What are the final certified results for the Forrest Passive House?
| Metric | Certified Result | Passive House Maximum Allowed |
|---|---|---|
| Airtightness | 0.41 ACH50 | 0.60 ACH50 |
| Heating Demand | 13.1 kWh/(m²a) | 15.0 kWh/(m²a) |
| Cooling Demand | 3.3 kWh/(m²a) | 15.0 kWh/(m²a) |
| Excessive Humidity | 0% | < 10% |
| Certification Status | Certified Classic (March 2024) | — |
Three years after handover, the clients went overseas for a few weeks during winter. When they came back, the temperature inside the house had not moved. Melbourne had done its worst. The house had not noticed. That is what 13.1 kWh/(m²a) looks like in practice.
If your builder has finished your home but cannot hand you a certified blower door result and a post-construction PHPP file, what exactly do you know about how your home performs?