Designing for Overheating in a Warming Climate

As Melbourne’s climate warms, overheating has become one of the most pressing challenges in housing. Rising summer temperatures, longer heatwaves and warmer nights are exposing the limits of traditional building design. For those of us in construction, it is no longer enough to rely on intuition or conventional detailing. Homes must be built to perform under far greater thermal stress.

At HONE, we approach overheating prevention as a construction challenge as much as a design one. Passivhaus provides a rigorous, evidence-based framework that allows builders and designers to work together to deliver homes that remain comfortable, healthy and efficient even as Melbourne’s climate continues to warm.

Melbourne’s Changing Climate

According to the CSIRO and Bureau of Meteorology, Melbourne’s climate is set for significant change over the coming decades:

• By 2030, average temperatures are expected to rise by 0.7°C to 1.2°C compared to 1995–2014 levels.

• By 2050, that increase is projected to reach 1.5°C to 2.5°C.

• The number of days over 35°C is likely to double from around 11 to 20–24 per year.

• Night-time temperatures are rising faster than daytime highs, limiting natural overnight cooling.

• Heatwaves are expected to become 50–100% more frequent by mid-century.

These conditions place new demands on the way we design and build. Conventional passive solar homes, even well-oriented ones, can easily overheat without careful modelling and construction control. Passivhaus addresses this through quantifiable design performance verified by what happens on site.

How Passivhaus Tackles Overheating

1. Predictive Modelling (PHPP)

At the design stage, Passivhaus projects are modelled using the Passive House Planning Package (PHPP), a detailed thermal performance tool that simulates the building’s year-round behaviour using Melbourne’s climate data.

Builders like HONE rely on these models not as abstract targets but as construction benchmarks. Airtightness targets, thermal bridge details and shading strategies are all derived from PHPP data and built accordingly.

By incorporating future climate files such as CSIRO’s NARCliM2.0 datasets for 2030 and 2050, we can stress test projects to ensure comfort even under hotter, more extreme conditions. That is how design intent becomes measurable performance.

2. Airtightness and Controlled Ventilation

From a builder’s standpoint, airtightness is one of the most critical and often underestimated components of overheating control. Gaps, penetrations and poorly sealed interfaces allow hot air infiltration and compromise ventilation balance.

In a certified Passivhaus, airtightness is paired with Mechanical Ventilation with Heat Recovery (MVHR) to maintain a constant supply of filtered, tempered air. During summer, MVHR systems use bypass mode to flush out warm indoor air. When installed with precision, this creates a stable, comfortable environment without mechanical cooling.

Execution matters. Airtight layers, service penetrations and sequencing on site all directly influence performance. It is a fine balance between precision and practicality, something we manage daily in the build process.

3. Solar Gain Management

In Melbourne’s climate, solar gain is both friend and foe. Passivhaus design quantifies the balance and the builder ensures it is delivered accurately:

• High-performance glazing must be installed to millimetre precision to achieve intended U-values and g-values.

• External shading systems need correct mounting, angles and clearances to perform as modelled.

• Thermal bridge control at window junctions is vital to prevent heat transfer into the envelope.

These details are where theory meets craftsmanship. We have found that working closely with architects during detailing, particularly around façade interfaces, is essential to maintaining comfort targets once the building is complete.

4. Internal Load Management

While much of overheating risk stems from solar exposure, internal heat gains can be equally significant. Modern households generate constant heat from lighting, appliances and occupants.

A well-built Passivhaus manages this through:

• Efficient services installation, ensuring exhausts, ducts and seals perform as modelled.

• Zoned ventilation setups, balancing extraction from kitchens and bathrooms with supply to living spaces.

• Commissioning checks, verifying airflows, filter integrity and summer bypass function before handover.

These final steps are often invisible to the eye but crucial for maintaining real-world comfort.

5. Thermal Envelope and Material Performance

Passivhaus is not about thermal mass alone, it is about thermal consistency. A continuous, well-insulated, airtight envelope is the foundation. From a construction perspective, this means:

• Coordinating airtight membranes, insulation and service cavities to prevent performance gaps.

• Avoiding thermal bridges at slab edges, roof junctions and penetrations.

• Using materials that deliver durability and predictable thermal behaviour under heat load.

The result is a home that stays cool and stable through heatwaves, not because of air conditioning but because every layer works together as a system.

Adapting Construction for Melbourne’s Future Climate

As our summers grow hotter, the way we build must adapt alongside design. Future-ready Passivhaus projects will increasingly include:

• Smart ventilation and automated shading that respond to real-time weather.

• Reflective and ventilated façade systems to minimise absorbed heat.

• On-site renewables with battery backup to maintain comfort during grid stress.

• Continuous verification during construction, ensuring design assumptions are achieved on site.

Conclusion

Overheating is no longer a future concern, it is a reality Melbourne builders must confront today. Passivhaus gives us the framework to do so with precision through airtight construction, quantified design targets and verifiable comfort outcomes.

At HONE, we see overheating prevention as a shared responsibility between architect, engineer and builder. Our role is to ensure that the intent behind every design decision, from shading to ventilation, is realised faithfully in construction. Resilience is not achieved on paper; it is built, detail by detail, on site.