Clay helps passide design principles in Australia

Clay Technology magazine
7 Jun 2019

Clever use of clay products and passive design principles are helping drive energy efficiency in Australian homes. 

While cookie-cutter style homes by a property builder or developer are a popular means for Australian newbuilds, more and more people are considering a site’s natural qualities and designing houses to suit.

These houses are designed based not on their looks, but on how to maximise the thermal properties of the materials used to build them without using artificial heating and cooling to regulate the temperature inside.

Commonly, the houses – and their living spaces in particular – will be angled towards the north to capture the sun’s heat. The houses will have a central ‘heat bank’ wall made of a clay material, for example brick or rammed earth, which can absorb the sun’s heat in winter during the day and radiate it out at night. Meanwhile, cross ventilation will allow the house to keep cool in summer.

A study by clay industry body Think Brick Australia (TBA) and the University of Newcastle, Australia, has demonstrated not only brick’s performance as a material choice, but how their placement, along with elements including windows, can help improve a home’s thermal performance.

This manner of designing so the house and not a heater or air-conditioner regulates temperature is not new nor exclusive to Australia. Germany pioneered the Passivhaus standard, which sets particular requirements for energy consumption, and scrutinises how air travels around the home. But the heavy use of clay products and the way a floor plan is arranged seems to be quite innovative to the land Down Under.

Of course in general, Australian houses need to be kept cool rather than being heated, but passive design is aiding this in a way that seems original to the Australian landscape.

The case for brick

A 16-year study of different wall types that started in 2002 by TBA and the university found an insulated cavity brick wall delivered the best thermal performance compared to non-insulated cavity brick, and insulated and non-insulated brick veneer, and lightweight walls. Also, the insulated cavity brick wall exhibited superior thermal lag.

Tests were performed in two stages, starting outside using real-world buildings, and then in a laboratory. Results have been used by TBA in its guidance documents about best sustainable passive design practices.

For the first round of tests, four buildings were made for the experiment, each 6x6m, using cavity brick, brick veneer, lightweight and reverse brick veneer. The buildings were slab-on-ground and built with an independent roof system which allowed the walls to be changed over time. Tests were done with and without windows, and the results were published in the report Energy efficiency and the environment - the case for clay brick, edition four, in 2011 on the TBA website.

In the no-window cavity brick wall facing west during summer, the heat energy recorded at 700-900W/m2, which fell to about 200W/m2 entering the external wall. By the time the heat energy transferred to the internal space, the measurement had dropped to 5-6W/m2.

‘The study found that a large portion of the heat is reflected and radiated back to the external environment by the exterior surface of the brick and does not come into the internal space,’ the report stated.

Temperatures in the houses were also recorded. Analysing data from a summer heatwave in January 2006, where temperatures reached 46.7°C, the report found the internal temperature of an insulated cavity wall building never reached 30°C, but the lightweight building spent eight hours with a temperature of more than 30°C. The study also found that when the day’s external temperature peaked at 5.15pm, the temperature inside the lightweight building peaked 15 minutes later, while the insulated cavity building’s temperature reached its peak temperature of 29°C at 10pm.

‘This important observation confirmed that although insulated lightweight walls reduce the amplitude of heat entering the building, they do not provide any thermal lag. In comparison, the lag time provided by the cavity brick walls delays the maximum temperature to later in the evening.’

The lightweight module responds directly to the external environment with a rapid increase in reduction in temperature due to its low thermal mass. The lightweight walls immediately transmitted heat into the module when solar radiation was incident on the external surface.

Performance of the lightweight building improved when it was converted into a reverse brick veneer construction later in the study, with the brick layer on the internal walls.

Testing showed that measuring a wall’s thermal resistance value, or [R-value] – the rate at which heat travels through a wall – alone, is not an effective way to measure the thermal efficiency of a building. While the lightweight building had a better R-value, it did not result in a cooler building. Similar results were also found in a laboratory environment.

Passive design principles

Achieving environmentally sustainable homes through passive design principles is being championed through Australia. Groups including TBA, Master Builders Association (MBA) and the Royal Australian Institute of Architects (RAIA), have performed research, implemented policies or published papers advising their members of best practises to help reduce a home’s carbon footprint during its life.

All groups have advised of similar themes, including orientation, building envelope, glazing, ventilation, insulation and thermal mass, as key aspects of passive design. The core driver behind these principles is capturing the sun’s heat and distributing it best through the house to the occupier’s advantage, without the need for artificial heating and cooling.

Gareth Cole’s paper Residential Passive Design for Temperate Climates, published by RAIA in 1997 and updated in 2011, forms part of RAIA’s Environmental Design Guide series.

TBA’s report Sustainability and Energy Efficiency, last updated in December 2018, also covers the issue. TBA said it is estimated that up to 40% of end energy usage in domestic buildings is used for heating and cooling.

‘The design and construction of energy efficient buildings has the potential to substantially reduce the dependence on artificial heating, cooling, lighting and ventilation with consequential reduction in greenhouse gas emissions and energy expenditure,’ the report read. ‘To maximise sustainability in building design, it is imperative to understand the key factor is the use of the most appropriate material in all aspects of the design.’

External factors

The way the house is laid out, not only in position, but also the placement of different rooms, windows and walls of the building, and the materials used in those, have an effect on the way the home can naturally keep heat in during winter and repel it in summer. ‘The single most important aspect of passive design is facing the building towards the sun,’ Cole said.

The MBA said north-facing facades receive the sun for the longest part of the day. Cole and the MBA said rooms with maximum daytime use, such as the living, dining and kitchen room, should face north so they are kept warm in winter. 

Flooring to northern rooms should be tiled or polished concrete for thermal mass, with an ideal floor to glass ratio of 2:1.

According to Cole, a passive house should have a ratio of 2:1 length to depth (east-west axis to north-south axis). ‘This provides the added advantage of assisting with cross ventilation, as the house will have minimal rooms in the north-south direction, allowing for better cross flow (especially as most cool summer breezes on the east coast of Australia come from the northeast, east to southeast).’

The landscaping, colour choices, house size and attachments to a property all influence its thermal performance.

Cole said evergreen trees planted to the east and west of the house can protect against cold winds in winter, and if placed correctly, will focus cool breezes into the house in summer. Deciduous trees planted on the northern side of the house can help keep the house cool in summer, while allowing the sun to permeate through the house in winter. Dark colours will tend to absorb light and in turn, are best used on internal floors and walls in north-facing rooms.

Thermal efficiency

Using materials with a high thermal mass on the inside of the house can help drastically reduce the reliance of artificial heating and cooling. These are most appropriate for the central wall running east-west, and for the floors. These two features can act as heat banks in winter, so once the sun’s heat has entered north-facing glass, the central wall and floor can absorb it throughout the day and release it at night.

TBA said a concrete slab is beneficial as it increases thermal mass significantly, while Cole believes concrete, mud or brick are good for floors, while brick, mud brick and rammed earth are better for walls. Windows work in tandem with a home’s thermal masses. Testing by TBA and the University of Newcastle, Australia, showed that windows can be used to help with energy efficiency.

Trials found that not using windows on the north side did not help thermal efficiency, as the wall that faced the sun radiated the heat back outside, and the thermal heating opportunity inside was reduced. When a single glazed window was introduced on the north side, any heat that entered the home escaped through the glass as it had no thermal lag. But when double glazing was introduced, the solar energy was able to enter the house and be absorbed by the thermal mass floor and wall and released into the home, without as much escaping through the glass.

Eaves had been identified by all groups as a great helper for regulating the sun’s effect inside the house, particularly on the northern side. The sun is higher to the horizon in summer than it is in winter, so eaves can help deflect the sun off the windows and in turn, reduce the amount of solar heat that enters the home. In winter, however, the sun sits much lower to the horizon line, so the solar energy is able to enter the home through the glass, be absorbed by the thermal mass floor and central wall, and then released through the home.

Ventilation and insulation

Allowing air to move around effectively will help internal surfaces keep cool in summer, retain heat in winter, prevent mould growth and remove air toxins and smells.

Windows should be placed so when they are open in summer, they follow a straight-line to allow a breeze to pass, and ideally a path that captures the natural cooling breezes to the house’s location. For winter, a house’s design should be zoned so the living areas are grouped together and are protected to help keep heat in, while wet areas and other rooms that do not need to be heated are kept away and do not waste energy through unnecessary heating.

Insulation should be installed in the walls, the roof and ceiling which can help reduce heat loss. Minimising heat loss through other areas including doors and windows also needs to be considered. ‘If they are not all insulated correctly and holistically, then the “insulation chain” is broken and the building’s thermal performance will be affected. It will fail at its weakest link. As water always flows downhill, so heat will always rise or flow towards the coldest leak point in a room: a fire place without dampers, an uninsulated cornice – even electrical power points,’ Cole said.

Insulation batts in walls should be installed without compressing them, as this can reduce their insulation value by preventing them from being breathable. Reflective foil should be installed to all external walls and all windows and doors should be fully taped to avoid any air loss.

The commercial reality

There are numerous property developments across Australia which encourage and even require these design principles to be incorporated into the final building.

Beyond Today is a sustainable housing development located in Victor Harbor, a seaside town about 90km south of Adelaide. Landscaped reserves, parks and wetlands make up 47% of the total development area. Design guidelines are provided by the developer to help people make decisions about their homes.

Among the specifications required for houses to be built at the site are:

  • Ventilation provided from the ceiling level upwards through the roof to the outside, or through high and low-level windows
  • External timber wall frames containing bulk insulation to a minimum value of R2.5, and
  • All roof spaces insulated to R4 rating and fitted with foil reflective sheets.

Similarly, Lochiel Park, about an 8km drive east from Adelaide’s city centre, is an award-winning green village with 70 private homes and 23 government-funded properties on-site, many of which incorporate passive design principles.

While the sites above are for newbuilds, passive design has also been incorporated into existing buildings, such as the Tiger Prawn in Fitzroy North, a suburb in Melbourne, by architecture firm Wowowa. The back extension to the house took inspiration from its brown and gold brick pattern brick façade, known colloquially as a tiger prawn, by following a scallop formation. Its passive design features include a double brick cavity, double glazing, cross ventilation, thermal floor mass and a rectangular footprint.

Coming to the UK

While this passive design approach requires a larger degree of planning and consideration during the design phase – and although the UK does not have the same climate – many of these principles and practices can be adopted in the UK.

Not only would passive design result in more comfortable and pleasant homes, but with the pressure to reach high housebuilding targets, incorporating just some of these methods could contribute to improved wellbeing from comfortable homes, and a reduction of energy consumption on a vast scale.