If you grew up in Perth like most of us in the office have, then you’ll have grown up assuming that the way to build a house is to use bricks. The Australian Bureau of Statistics tells us that in the last 100 years or so there has been a long-term shift away from from timber towards brick as the most popular material for outer walls in Australia. In 1911 25% of outer walls were made of brick, in 1981 58% of dwellings had outer walls made of brick and by 1994, 65% of dwellings has outer walls made of brick and 87% of new dwellings had outer walls of brick. In 1993-1994 Western Australia led the way with the highest percentage (87%) of new dwellings constructed with outer walls of double brick of all states and territories, compared to New South Wales’ 13% and Victoria’s 1%. Results from a recent survey conducted by the Department of Housing and Department of Planning and published in the 2013 The Housing we’d Choose: a study for Perth and Peel show that approximately half of respondents prefer double brick as a building material for their homes. This suggests that resident sandgropers have come to develop and still maintain a significant fondness for the brick!
These days people are a bit more savvy when it comes to deciding how they want to build their houses with respect to future energy usage, especially given that that heating and cooling can account for more than 40% of the average home’s energy. Advertising materials from brick manufacturers will tell you that bricks will increase your home’s energy efficiency, offer superior levels of thermal comfort, and that heavy weight construction materials with high thermal mass (such as concrete floors and insulated brick walls) can reduce total heating and cooling requirements compared to a home built of lightweight materials. But is this actually the case in Perth, Western Australia? And is thermal mass the only consideration?
Thermal mass is the ability of a material to store and re-release heat. It is not the same as insulation which stops heat from flowing into or out of a building. When used correctly, thermal mass is considered most useful in locations that have large swings in temperature between day and night because of the capacity to delay heat flow through a building by up to 12 hours (known as ‘thermal lag’). However, in climates that are constantly hot or constantly cold, housing materials with a high thermal mass can actually be detrimental because the materials will tend towards the average daily temperature, which during Perth’s summer can be around 30°C!
Generally speaking, a building of high thermal mass needs to gain or lose a large amount of energy to change its internal temperature, whereas a lightweight building requires only a small energy gain or loss to change its temperature. While double-brick has typically been promoted as useful for improved wall insulation, the high thermal mass of brick means that when these walls heat up, they hold their heat and release it slowly. While this is of obvious benefit in cooler climates, it is not necessarily a good outcome in Perth’s relatively hot climate when you’re trying to sleep during a summer night, particularly in the context of increasingly intense and frequent heatwaves (described in a recent post here). The Australian Government’s guide to environmentally sustainable homes suggests that lightweight, low mass construction is most suitable for warm/mild temperate climates, particularly in the upper levels of a building in multi-level design.
Perhaps the way we have been building in Perth in recent decades isn’t actually the most climatically-ideal and the notion that if a house is not made of brick then it is not sound needs to be challenged……So what are the alternatives and are they being used yet?
A number of wall construction materials with significantly high energy efficiency ratings that meet Building Code of Australia standards are now readily available as alternatives to brick. These materials are typically used with a lightweight frame (steel or timber) and some examples and their R-values include:
- Highgrade polystyrene (~R2.5)
- Straw bales (~R5.5 – 8.5)
- Brick veneer and batts (~R2.5, depending primarily on batts)
- Insulating concrete forms/cladding (~R0.9 – 3.35)
- Aerated concrete blocks (~R3.6 – 3.9)
The R-value or thermal conductivity of a material or an assembly of materials (such as insulation panels) is a measure of its thermal resistance and depends on the material’s resistance to conductive heat transfer, that is, its ability to insulate. The higher the R-value of a materials, the better its insulation properties. Quoted wall R-values are usually a combination of R-values associated with the elemental construction of a wall, and include values for air pockets, studs and frames, as well as the material itself. Different R-values are appropriate for different climates and are defined for different regions in the Building Code of Australia and Australian Standards AS 2627.1-1993. The R-value of uninsulated double brick is typically between R0.50-1.0 (as shown here and here).
Lathlain, Perth – a small-scale development featuring two townhouses achieved 10 and 9 star energy ratings using reverse brick veneer and strawbale construction.
North Shore, Sydney – townhouses were design to include wool polyester batts installed in the walls with a lightweight timber frame and plaster board. The overall design achieved a 6 star energy rating.
Of course the material used for wall construction is only one of a number of elements that combine to affect the comfort and energy efficiency of a house. Factors such as roofing and windows (through which a large proportion of house heat is transferred), as well as ventilation, positioning and shading, also need to be considered in the design of an energy efficient house. Other ways to improve the energy efficiency of new or existing homes include:
- Good solar passive design (positioning of windows and choice of flooring materials) which lets sun and heat in during winter and keeps it out in summer;
- Good insulation of the roof and ceiling
- Reduce air leakage through gaps in door and window frames, floors and ceilings
- Double glazing for windows
- Shade – planting deciduous trees along eastern and western walls to protect them from the summer sun
- Wide eaves or a verandah along the northern aspect of a house to block summer sun but let in winter sun
- Installation of curtains or pelmets to reduce heat movement through windows
Unfortunately in Perth, the perception still exists that if a house is not made of double brick it is not as structurally sound as a lightweight-framed house and this can even affect re-sale value. How we change this perception is a major challenge. A greater challenge is how to encourage Australians to improve the energy efficiency of existing homes given they represent the most significant proportion of housing stock. Do we introduce mandatory energy efficiency ratings such as they have done in parts of the UK and elsewhere in Europe for some time? Such strategies have the potential to get people to move away from double brick and embrace alternative building materials and techniques that allow for faster construction and better thermal performance, and thus lower energy bills. That issue will have to be explored in another article….