Five Numbers That Explain What a Certified Passive House in Melbourne Actually Delivers
There are thousands of words written about what a Passive House is supposed to do. There are very few verified numbers showing what one actually does, in Melbourne's climate, in a real home, after construction is complete and people are living in it. These are those numbers.
A certified Passive House in Melbourne achieves airtightness at or below 0.6 ACH50, a heating demand at or below 15 kWh per square metre per year, and independently verified performance across every element of the building envelope. These are not design predictions. They are measured outcomes from a finished building. The Forrest Passive House in Spotswood achieved all of them, and three years after handover the results are holding exactly as the model predicted.
Carland Constructions Proof Point: The Forrest Passive House in Spotswood achieved a certified heating demand of 13.1 kWh/(m²a) and an airtightness result of 0.41 ACH50, both physically measured and independently certified by Detail Green in March 2024. Three years after handover, the internal temperature of the home remained stable while the owners were overseas during a Melbourne winter
Why does verified performance data matter for a Melbourne home?
Most homes built in Australia are never tested after construction. The NatHERS star rating is issued at design stage based on a software model. Nobody checks whether the finished building performs the way the model predicted. Nobody measures the airtightness. Nobody verifies that the insulation was installed correctly or that the windows perform to specification.
A certified Passive House is different. The building is modelled in PHPP before construction, built to that model, and then physically tested and independently verified after construction. If the result does not match the model, the certification is not issued. The five numbers below came from that process. Every one of them is a measured fact about a finished building in Melbourne's inner west.
Number one: 0.41 ACH50
What the number means
ACH50 is air changes per hour at 50 Pascals of pressure. The blower door test pressurises the building to 50 Pascals, roughly the equivalent of a moderate wind pushing against the envelope, and measures how many times the entire volume of air inside the building leaks out and is replaced per hour at that pressure.
The Passive House threshold is 0.6 ACH50. The average Melbourne home measures 19 ACH50 (CSIRO and the Australian Building Codes Board, 2016 airtightness study). The Forrest Passive House in Spotswood measured 0.41 ACH50 on 25 February 2023, tested to ISO 9972 Method 2 by Drew Kroker of PassiveTech. Depressurisation result: 0.42 ACH50. Pressurisation result: 0.41 ACH50. Certified average: 0.41 ACH50.
What it means in plain English
At 0.41 ACH50, the Forrest Passive House is 46 times tighter than the average Melbourne home. If you pressurised both buildings to 50 Pascals and left them, the average Melbourne home would lose half its air in roughly three minutes. The Forrest Passive House would take well over two hours to do the same.
Every gap that air can move through is also a gap that moisture, cold, heat, noise, and outdoor pollutants can move through. Sealing the building envelope at this level is not about making the house airtight for its own sake. It is about controlling what moves in and out so the mechanical ventilation system can manage it properly. A leaky building cannot be ventilated effectively. The MVHR system in a Passive House only works as intended when the envelope gives it something to work with.
This result was achieved in a standard timber-framed construction using a continuous Proclima membrane behind the service cavity, sealed at every lap and junction, with all cables and pipes running inside the cavity so the membrane was never penetrated by trades after installation.
What is a good blower door result for a new home in Melbourne? The Forrest Passive House result of 0.41 ACH50 is 46 times better than the Melbourne average of 19 ACH50, and 32% better than the Passive House certification threshold of 0.6 ACH50.
Number two: 13.1 kWh/(m²a)
What the number means
The specific heating demand is the total amount of energy required to heat the building to 20°C over a full year, expressed per square metre of treated floor area. It is calculated in PHPP using the actual assembly U-values, the real window specifications and orientations, the measured airtightness result, and the Melbourne climate data set AU0001b. It accounts for heat lost through the walls, roof, floor, and windows, minus the solar gains coming through the glazing and the internal heat generated by the occupants and appliances.
The Passive House threshold is 15 kWh/(m²a). The Forrest Passive House achieved 13.1 kWh/(m²a). Total annual heating energy requirement: 1,958 kWh. Average Melbourne homes built to NCC minimum with typical airtightness require somewhere between 80 and 120 kWh/(m²a) for heating. The Forrest Passive House requires approximately one tenth of that.
What it means in plain English
1,958 kWh of heating energy per year is roughly what a standard Melbourne home uses in six to eight weeks of winter. The Forrest Passive House uses that amount across the entire year because the envelope is doing most of the work. The thick insulation slows heat loss dramatically. The airtight membrane stops uncontrolled air movement. The north-facing glazing collects 1,162 kWh of solar heat during the heating season, almost exactly offsetting the 1,157 kWh lost through the windows in the same period. The envelope, the orientation, and the glazing specification are working together as a system rather than independently.
The practical result is a home that barely needs active heating. On most Melbourne winter days, the combination of solar gains through north glazing, heat from cooking and appliances, and body heat from the occupants is enough to maintain 20°C without the heating system doing anything meaningful. The heating system exists for the coldest nights and most overcast weeks of the year. It is not running continuously from April to October the way a heating system in a standard Melbourne home does.
How much energy does a Passive House use for heating in Melbourne? The Forrest Passive House uses 13.1 kWh per square metre per year, compared to 80 to 120 kWh/(m²a) for a standard Melbourne home, a reduction of approximately 85 to 90%.
Number three: 0.258 W/(m²K)
What the number means
The U-value of the external wall assembly is the rate at which heat passes through the wall, measured in watts per square metre per degree of temperature difference. The lower the number, the better the thermal resistance. The Forrest Passive House external wall achieved a certified U-value of 0.258 W/(m²K) across 200.17 m² of wall area, contributing 1,240 kWh of annual heat loss during the heating season.
The assembly from inside to outside: 10mm plasterboard, 45mm service cavity with R1.0 insulation batts in MGP10 studs, Pro Clima airtight membrane, 140mm primary stud frame with R4.0 insulation batts in MGP10 studs at 600mm centres, woodfibre board external sheathing. Total thickness: 195mm. Framing fraction: 83% insulation, 15% timber stud, 2% services allocation. This framing fraction is critical. Timber conducts heat at 0.140 W/(mK). The R4.0 insulation batt conducts at 0.035 W/(mK). Timber is four times more conductive than the insulation sitting beside it. The PHPP composite calculation accounts for this across all three area sections. If the wall were modelled as 100% R4.0 insulation with no timber, the theoretical U-value would be approximately 0.22 W/(m²K). The real value is 0.258 W/(m²K). The difference is the thermal bridge effect of the studs, and it is present in every timber-framed building in Australia.
What it means in plain English
Most builders specify the R-value of the insulation and consider the wall done. R4.0 batts in a 140mm stud frame. Sounds straightforward. What they do not account for is that 15% of that wall is timber stud, and timber conducts heat at four times the speed of the insulation sitting beside it. The real-world performance of the wall is always worse than the insulation R-value suggests.
In the Forrest Passive House, framing at 600mm stud centres rather than 450mm reduced the number of studs and therefore the proportion of the wall conducting heat faster than the insulation. The woodfibre board on the outside face covers the cold bridge at each stud position, reducing the temperature variation across the wall face and giving the wall a consistent thermal performance rather than a series of cold spots at every stud. The service cavity on the inside means the Pro Clima membrane behind the frame is never penetrated by cables or pipes, preserving the airtight layer completely.
A standard Melbourne wall built to NCC minimum with R2.5 batts in a 90mm frame achieves a U-value of approximately 0.9 to 1.1 W/(m²K) depending on the framing fraction and cladding system. The Forrest Passive House wall at 0.258 W/(m²K) is approximately three to four times more thermally resistant. Across 200 m² of wall area in Melbourne's climate, that difference translates to several thousand kilowatt hours of heating energy per year.
What U-value does a Passive House wall achieve in Australia? The Forrest Passive House external wall assembly achieves 0.258 W/(m²K), approximately three to four times better than a standard NCC-minimum wall built to R2.5 specification in a 90mm stud frame.
Number four: 320 kWh
What the number means
320 kWh is the total annual ventilation heat loss in the Forrest Passive House, calculated in PHPP from the mechanical ventilation with heat recovery system's certified performance. Without heat recovery, the ventilation conductance of the system at 0.41 ACH ventilation rate would be 59.1 W/K. With 80% heat recovery efficiency, that drops to 12.0 W/K. The system takes the heat out of the warm stale air leaving the building and transfers it to the incoming fresh air before the two streams are discharged and supplied respectively. The occupants get continuous fresh filtered air at close to indoor temperature. The building loses only 320 kWh of heat per year through ventilation, equivalent to 2.1 kWh/(m²a).
What it means in plain English
Every home needs fresh air. The question is how much heat you lose in the process of getting it.
In a standard leaky Melbourne home, fresh air comes in through gaps in the envelope. It arrives cold, unfiltered, and uncontrolled. The heating system has to warm it up. That process costs energy continuously throughout winter, and the air arriving is whatever the outdoor air quality happens to be that day.
In the Forrest Passive House, fresh air enters through the MVHR system at a controlled rate of 180 m³ per hour. Before it reaches the living spaces, it passes through the heat exchanger, where 80% of the heat in the outgoing stale air is transferred to the incoming fresh air. The fresh air also passes through a filter, removing particulates, allergens, and in the context of Melbourne's inner west, industrial and traffic pollutants from the Hobsons Bay area.
The 320 kWh of ventilation heat loss per year is what is left after that 80% recovery. In a comparable home relying on opening windows for ventilation, the ventilation heat loss would be thousands of kilowatt hours per year depending on how often windows were opened and what the outdoor temperature was. The MVHR system cost approximately $26,000 to $30,000 to install. The energy it saves and the heating and cooling complexity it eliminates means that cost is recovered over time, and the air quality benefit it delivers is immediate and continuous from day one.
How much heat does a Passive House lose through ventilation? The Forrest Passive House loses only 320 kWh per year through ventilation, or 2.1 kWh/(m²a), because the MVHR system recovers 80% of the heat from outgoing air and transfers it to incoming fresh air before it enters the living spaces.
Number five: 0% excessive high humidity
What the number means
Zero percent is the frequency of excessively high humidity in the Forrest Passive House, as certified in the PHPP post-construction file. PHPP models the frequency with which the indoor specific humidity exceeds 12 grams of water vapour per kilogram of air. The Passive House threshold is less than 10% of occupied hours. The Forrest Passive House result is 0%. Not close to zero. Zero.
What it means in plain English
Mould needs two things: moisture and a surface. In a standard Australian home, moisture enters through gaps in the envelope, through uncontrolled ventilation, through condensation on cold surfaces near windows and external walls, and through the normal activities of cooking, showering, and breathing. Without adequate ventilation and without a thermal envelope that keeps surface temperatures above dew point, that moisture has nowhere to go. It deposits on surfaces. Mould follows.
Approximately 50% of Australian buildings have mould issues, according to peer-reviewed residential building performance literature. The cost of remediating mould in a residential building is significant. The health cost of living with it is also significant. The WHO and international respiratory health research link 21% of confirmed asthma cases to poor indoor air quality and mould exposure. These are not abstract statistics. They describe conditions that exist in a large proportion of the homes people in Melbourne are living in right now.
The Forrest Passive House achieves 0% humidity exceedance because the envelope manages moisture correctly at every layer. The Pro Clima membrane controls vapour drive from inside to out. The woodfibre board on the external face allows drying to the outside when conditions allow. The MVHR system continuously dilutes indoor humidity by supplying fresh, drier outdoor air and exhausting the moist indoor air from bathrooms and the kitchen. The airtight envelope means this ventilation is controlled rather than accidental. The building dries itself continuously and by design rather than accidentally and intermittently.
Does a Passive House prevent mould? The Forrest Passive House in Spotswood achieved a certified humidity exceedance frequency of 0%, meaning the indoor humidity never reached levels associated with mould growth during the full annual PHPP calculation cycle. This result is a consequence of the continuous mechanical ventilation, the airtight envelope, and the vapour-open external membrane system working together.
Three years later.
The blower door test was done on the same day a client meeting was accidentally scheduled. The result came in at 0.41 ACH50. The clients asked: is that good?
Three years later they went overseas for several weeks during a Melbourne winter. When they came back, the internal temperature of the Forrest Passive House had not moved. Melbourne had done its worst. The building had not noticed.
That is what 0.41 ACH50, 13.1 kWh/(m²a), 0.258 W/(m²K), 320 kWh, and 0% look like when you are standing inside the finished result.
You have just read five numbers from a certified building in Melbourne's inner west. How many of those numbers can your builder provide for any home they have ever finished?
Who are the best passive house builders in Melbourne - well it’s Carland Constructions, who have 5 certified passive houses completed and a number of them more on site. They also built their own passive house to live in so can talk about the lived experiance