Walls – Design Tips

magazine spread from original article
Note: This article appeared originally in Green Magazine as in the image above. The text below may differ slightly from the original.

The lightweight pine wall frame

Pine wall frames are nowadays factory-made and erected in a day or two on a building site. They are inexpensive, made of plantation timber, a renewable, bio-degradable and non-polluting material, put little weight on footings and over openings, and are generally a flexible material to use requiring little pre-construction planning. They can be clad in a myriad of ways, can accommodate decent amount of insulation within their cavity and the thin wall takes little precious room on tight blocks. Use it as a reference construction for other types of walling because it does the many jobs well: structure, insulation and aesthetic flexibility, and it does this while being low-tech, inexpensive and well-suited to the skills of residential builders in Australia. Teamed with an environmentally-sensitive insulation and cladding, it constitutes a most green wall. Indeed, if a wall touches the earth lightly, it would have to be the pine-framed wall! Steel wall frames are great in areas with high termite activity but are thermally conductive and do not offer the construction flexibility of timber. Also note that combined with a steel roof, they create an electro-magnetic cage, a potentially unhealthy though undocumented habitat.

Improvements on the common wall frame construction.

A French architect explained to me once how they do timber wall framing in their country. Issues of cold, heat and condensation being universal, there is a leaf we could take from their construction book! Studs are 140mm deep so that a thicker R3.0 batt can be accommodated in between. The outer face is entirely clad with plywood which acts as bracing and wind break, and the inner face is lined with a 40mm thick wood fibre board (a type of Masonite hardboard) to bulk up insulation further. The units or panels thus sealed are transported to the site and installed, never to be opened again. Once the units are in place, the outer face is battened and clad with the selected material, effectively delivering a ventilated façade that guarantees a dry wall. The inner face gets battened as well before fixing plasterboard, creating a 36mm cavity used for electrical and plumbing services. The end result is a thicker wall that is well insulated and sealed and yet well ventilated and lightweight. This certainly eliminates air movement in cavities as we are likely to have here as well as insulation gaps where services are installed. As this wall lacks thermal mass much like any lightweight walls, it is best teamed with a concrete floor slab.

The case of brick veneer

A paper by Austral Bricks claims that 90% of Australian homes today are built using bricks! Australia certainly has had a long love affair with brick veneer, with current fashion featuring dark or light-coloured bricks rather than the traditional reds, and often a mixture of claddings (particularly for the upper floor) rather than the all-over brick look. Compared to a simple pine-framed wall, a brick veneer wall offers a higher fire rating, a more solid feel, better acoustics, greater thermal mass and an extra durable substrate with no requirements for maintenance of a paint layer. Like concrete, bricks have no insulation value but significant thermal mass (about two thirds of the thermal mass of concrete). This energy-storing property is best utilised when the brickwork is located inside the insulated building envelope (read reverse brick veneer construction, internal walls or brick flooring) rather than outside as is the case with brick veneer. The reason might seem obscure until you realise that of course you put ice, a thermal mass, inside not outside an insulated esky! Where used in a conventional brick veneer situation, bricks make a more modest contribution by only providing a solid windbreak, thereby improving the air tightness of the structure. If you choose to build with brick veneer which will have bricks sitting in the summer sun, make sure at least that you select the lighter brick tones, they will store and radiate less Summer heat inside the house than mid-tones and dark colours. And wherever you use brick generally, beware of their great environmental downside: being kiln-fired, their CO2 emissions is substantial (around 6 tons for 10,000 bricks) which makes the prospect of using the great character-filled recycled brick a lot more attractive. A large proportion of our recycled brick stock comes from demolished solid brick buildings – featuring walls with two brick skins and central cavity – which was a popular wall type until the 1940s when they got replaced by the cheaper and lighter brick veneer. A visit to a recycled brick yard, with its many neatly sorted pallets of bricks, is rewarding and inspirational for the aspiring home builder. Should you use them on the inside of your insulated building envelope, you will be hard pressed to find a greener way to build.  

Earth walling

A stacked mud brick wall and a poured rammed earth wall are built quite differently but the raw materials used to make them are not dissimilar: soil (including sand, clay, gravel), additives like straw and stabilisers like cement, lime, fly ash or gypsum. The environmental footprint or embodied energy of the result is unanimously very small which is the materials’ greatest benefit, apart from its aesthetic appeal. Running costs however are greater as the large amount of thermal mass is not insulated – there is no cavity to house insulation and owners are not usually keen to hide the earth wall to install a batt. A research by RMIT Professor Ralph Horne has shown that the running costs of a house with earth walls over a 50 year life cycle are significantly greater than that of a house build with more conventional materials. As with brick, it is best to keep earth walls to internal feature walls or to insulate them from outside if they are used as external walls.

Masonry blocks and panels

There is a variety of load-bearing masonry blocks or cladding panels available on the residential market which offer fire-resistance, varying amounts of thermal mass and usually some insulation as well. The blocks or panels are usually made of a 10-20% cement base mixed with a raw material such as aggregates and/or  sand in the case of Hebel and besser blocks, clay in the case of Clinkabloks and Geobricks, recycled polystyrene for QT Eco Series, or an organic fibre for Timbercrete or Hempcrete. These products generally have a good fire resistance which makes them useful in fire-prone areas. From an environmental point of view, the best products are those with low cement content in view of the notoriously large carbon footprint of cement, good thermal insulation of R2.0+ especially when a block is used and no further insulation will be used, and if possible, a surface that does not require the finished wall to be further sealed, rendered or painted. While some of these products have become quite common in Australia, some of them are still quite experimental. With these, the designers or owners have to conduct their own research to ensure not only that the product is superior in the application envisaged to more common materials, but also that considerations such as installation method, termite protection, weatherproofing, structural strength, and final costs are well understood and addressed.