Passive solar design versus Passive House: why we need both for healthy homes in Queensland

The binary article recently published by The Fifth Estate on passive solar design versus Passivhaus standard reignited, locally, a long-lasting debate around the “right way” of building comfortable homes that do not harm our environment. Having been in the design and construction industry for many years and in different countries, I welcome this debate beyond the nitty-gritty of the matter, as it catalyses the discipline around the real threat: the lack of quality and the poor performance of the housing stock in Australia. 

A worryingly large part of Australian homes, both old and newer, share indeed a sad record: they are uncomfortable or even unhealthy. National studies demonstrate that the Australian construction industry still fails to meet international best practice, or even to meet minimum compliance expectation, despite the large body of research linking housing conditions to occupant comfort and health. 

The threat that cold homes pose to occupant health, even in the mild-climate regions of Australia, has been neglected, while public health initiatives focusing on heat-stress-related health conditions and mortality, often have offered little remedies beyond retrofitting of airconditioning, a poor strategy in terms of operational cost, indoor air quality and affordability. 

This scenario is predicted to worsen due to climate change, with most existing dwellings in Australia unable to adequately protect people from severe heatwaves, and the number of airconditioning units sold in the country rapidly growing. Annual sales reported in Australia increased from 833,000 in 2012 to a record number of 1,356,000 in 2017, placing further stress on the national power system, as space cooling can account for a large share of peak demand, and impacting the most vulnerable. 

Minimum requirements of the National Construction Code (NCC)  only partially address buildings’ high energy consumption, and consequent high energy bills, while other invisible impacts, such as health conditions caused by extreme temperatures and mould growth, endemic in Australian houses, remain largely unresolved. 

Furthermore, energy efficiency provisions have dramatically altered the building fabric, with additional layers of insulation, weather-resistant barriers and new materials that substantially modify not only the thermal performance but also air and water permeability of the envelope of the traditional homes. 

Without evidence-based knowledge of the hygrothermal performance of the building envelope, new construction methods and materials may dramatically increase moisture management risks and produce negative impacts not only on building integrity but also on people’s health.

This is a hazardous path already known in other developed countries, especially Europe and North America, where alternative, more energy efficient yet healthier ways of building have been consequently developed and long monitored. 

In the past decade, some of the international voluntary standards regulating high performance construction have gained traction in Australia, trialled by the best construction practice sector, which advocates for radical changes in building regulations to increase housing quality and performance . 

The Passivhaus Standard, in particular, is currently at the centre of an animated debate between its local promoters, represented by the Australian Passive House Association (APHA) , and those who oppose it, allegedly in defence of tropical architecture’s consolidated principles. 

However, are Passivhaus standard and passive solar design actually expressions of two antithetical ways of designing sustainable buildings? If the true scope of both parties is to build better homes for all in Queensland, and Australia in general, we need to step back and embrace a design approach that coherently employs all beneficial principles, tools and technological advances beyond individual factions or creed.

Passive solar design principles should be common knowledge amongst building designers, but they are too often ignored or traded off in the current Australian housing market. 

The iconic Queenslander has long been the emblem of a climate-adapted living in subtropical Brisbane; light timber construction, core living area shaded by extensive verandas, and ventilated undercrofts are some of the features that have gained these traditional houses their iconic role not only for sustainable tropical architecture, but for the identity of the city.

Beyond the heritage value, does this typology, expression of the Colonial time and a different lifestyle, actually match Brisbanites’ current needs and comfort expectations, and should be used as a design model for comfortable and energy efficient homes? 

The reality is that the majority of houses built in the last two decades have little to share with the traditional Queenslander: they are rather expression of different lifestyle, market pressure, technological gadgets and policy requirements. 

While the detached family home still represents the “dream” and a pillar of financial independence for most Australians, new houses, in both infill and greenfield developments, are typically selected from a set of volume home builder’s pre-designed floorplans: they are oversized and constrained on small lots, often in close proximity to the neighbouring buildings, which makes airconditioning a straightforward choice – and passive design principles economically “unsustainable”.

Let’s talk facts. With comfort, health and energy efficiency considerations in mind, what is the best way to build in Brisbane today? Does a “well-designed” house in Brisbane require any airconditioning for “standard-defined” (ASHRAE 55 and ISO 7730) thermal comfort or, rather, for satisfying people’s expectations? What should the performance of a “well-designed” house be and how do we measure it?

Brisbane’s hot and humid climate presents both challenges and opportunities for housing design. With an annual mean maximum and minimum temperature of 25.5°C and 15.7°C respectively, the climate is often deemed ideal to build energy efficient, naturally ventilated homes, with no need for either heating or cooling – providing good passive design principles, such as shading and cross ventilation, are implemented, together with good insulation and glazing. 

However, extreme temperature and humidity conditions, too hot for 20 per cent of the time and too humid for at least 30 per cent of the year, may be difficult to manage effectively without mechanical systems, and could affect the functional integrity of the building envelop as a consequence of incorrect detailing.  

A residence in Herston, A39 House, which was completed in 2016, can help cast some light on what a “well-designed” house may mean in Brisbane. 

It was conceived as a test ground for an integrated approach to design, including passive control of radiant heat, an air-tight envelope with phase change materials, enthalpy recovery ventilation, air velocity and humidity control. 

Brett Beeson, a Brisbane based engineer, tested key Passivhaus principles (without meeting the specific standard requirements) on a climate sensitive yet affordable home for his family: he designed, built, occupied and monitored the house for a few years, with the “bipartisan” goal of informing his own practice. 

For the first two years, the house had no air-conditioning: despite its good passive design features (insulation, glazing and shading), which provided the easiest improvements when simulated during design, the indoor spaces were found to not achieve adequate thermal comfort for several days in summer. 

The “uncomfortable” fraction of the year was quantified using dynamic modelling and found to be limited to 5 per cent according to the adaptive comfort model – which, however, stretches the comfortable indoor temperature to 28°C for Brisbane. 

Interestingly, data analysis showed that the peak load could be halved by controlling outside air movement and temperature with the heat exchanger, and by reducing uncontrolled infiltration. The reduced indoor humidity, also owed to the mechanical system installed, was reported as the key to comfort.

While the A39 House demonstrates that Passivhaus strategies are beneficial to achieve comfortable and healthy homes in Brisbane, are the performance-based requirements of the standards achievable in this challenging climate? 

In a nutshell, the Passivhaus is an ultra-low energy building standard that provides comfortable indoor conditions with extremely low heating and cooling loads . 

Originally developed in Germany, this performance-based standard has been already mandated by some countries and councils around the world to achieve a step-change towards net-zero emission buildings. 

High energy and thermal performance are achieved by implementation of simple design principles: high envelope thermal resistance, including glazing, mechanical ventilation with heat-recovery, to reuse thermal energy of exhaust air for heating or cooling supplied fresh air, and airtightness of the building envelope. 

In mild climates, it is entirely possible to build a Passivhaus that achieved thermal comfort solely by post-heating or post-cooling of the fresh air mass, which is anyway required for sufficient indoor air quality conditions, while the mechanical ventilation is still required for filtered fresh air supply. 

Active heating and cooling systems using additional recirculation of air are typically added where temperature and humidity fall outside human comfort conditions, like in Brisbane or tropical Queensland.

The implementation of the Passivhaus standard is certainly more challenging in hot and humid climates, where the risk of overheating due to unwanted solar gains increases, and dehumidification requires additional energy. 

However, there are successful certified examples built in the tropical monsoon climate of Sri Lanka (Star Garments Innovation Center), tropical Jakarta (Austrian Embassy), sub-tropical Chanxing, China (Bruck Building), and the UAE’s Autonomous House in hot Dubai. 

In subtropical Brisbane, where peak temperatures alternate with nocturnal or seasonal comfortable conditions, Passivhaus buildings may adopt a more “climate responsive” strategy, with passive shading to reduce unwanted solar gains and variable ventilation settings to suit the climate. 

Mechanical ventilation with active cooling is effective to temper peak temperatures and for safe night purging, as a constant flow of fresh air can be supplied simply bypassing the heat recovery system. When outdoor conditions are within the comfort range of temperature and humidity, natural ventilation through openable windows may be the most suitable strategy, subject to outdoor air quality and noise pollution.     

A successful Passivhaus design process actually starts from the same key passive solar design principles, which are needed to reduce energy needs in prevalent cooling climates. 

However, it also includes mandatory considerations for occupant comfort , energy efficiency, envelope airtightness and moisture management, verified through a set of measurable requirements during both, the design and construction phases. Therefore, its rigorous design approach fills the gaps still existing in the Australian NCC, in particular concerning indoor air quality, air tightness and mould risk.

Understanding airtightness: “Buildings that breathe” principle is not uncontrolled air infiltration. 

Ventilation practice in contemporary Brisbane’s houses still relies on a certain amount of air infiltration through the envelope, feeding public misconception about air tightness, and misunderstanding of what a “building that breathes” may be. 

Air infiltration through the building fabric is an uncontrolled process that allows external moist air a direct path into the construction assembly, potentially compromising its functional integrity. 

In hot and humid climate, with typically air-conditioned home interiors, internally driven moist air may reach the dewpoint within the building envelope, leading to interstitial condensation, which can cause mould growth and even structural deterioration. 

The 2015 Condensation Stakeholder Survey conducted by the Australian Building Codes Board should have been a wake-up call for the construction industry: it showed that approximately a third of new and existing Australian buildings suffer from condensation problems and moisture defects. 

As indoor air quality of houses has been associated to incidence of asthma and respiratory diseases, it is not a surprise that Australia is the country with the highest prevalence of clinical asthma (21.5 per cent), according to data collected by the World Health Organization on the prevalence of several medical conditions in adults aged 18–45 years from multiple countries around the world. 

While “breathability” is one of the key principles of tropical architecture, healthy buildings must exchange fresh air only through purposely designed openings or mechanical systems. 

In summary, while there is no single design response to build comfortable and energy efficient houses in Brisbane, as each project responds to its specific social, economic and environmental context, there are many lessons to be learned from good examples of both passive solar designed and Passivhaus buildings:

• In Australia’s hot and humid climates, passive design strategies, such as orientation and shading, are essential to control overheating, reducing reliance on air-conditioning
• Insulation is not only useful for cold climates but also helps keep cool, conditioned air in
• A mix of natural and mechanical ventilation with cooling and dehumidification may assist in achieving indoor thermal comfort in variable to extreme climatic conditions
• Synergistic use of active cooling systems, sufficient ventilation and airtightness helps avoid interstitial condensation and poor indoor air quality linked to air-conditioning with insufficient ventilation practices
• Mechanical ventilation for continuous filtered air supply may help improve indoor air quality and occupant well-being, especially in the context of high pollution and bushfire smoke
• An energy recovery unit allows for the heat or “coolth” contained in the exhaust air to be re-used, while also reducing dehumidification loads and aiding humidity management
• Finally, performance-based assessment during design, construction and post occupancy may secure housing quality for all, while ensuring critical indoor conditions for occupant health and well-being  

The first certified Passivhaus building in Brisbane is a two-storey single family home in Auchenflower, Vanquish [25], by developer Solaire Properties. The house has recently received certification under the PHI Low Energy Building Standard [26], one of the categories of Passivhaus certification awarded by the Passive House Institute, depending on the renewable primary energy demand and generation of renewable energy, as well as performance targets. This house will be critical to assess the Passivhaus Standard in Australia’s warm and humid climate: results of an ongoing monitoring campaign carried out by the University of Queensland are expected to help develop an evidence-based understanding of how both passive and active systems may work together in a subtropical Passivhaus building. 

This is still a work in progress but a big step in the right direction. Ultimately, innovation requires brave attempts and the ability of learning from both successes and failures within a constant feedback loop. There has never been a one path fit all successful story in housing design.