Bricks, blocks and panels: What’s in a wall?
There are many different approaches used for building the walls of a home, but which one is ideal for your build? Lance Turner takes us on a quick tour of the different systems, materials and their sustainability credentials.
For those embarking on a sustainable building project, there can be almost too much information available, making it hard to quickly compare the possible building approaches. One important decision is the wall-building system to use in your build.
To help in that evaluation process, this article provides a quick guide to the different wall-building systems and materials available. For each system, we consider how the walls are constructed, their thermal performance and sustainability. A table at the end of the article summarises each approach in terms of a range of sustainability criteria. It’s intended as a quick guide; you’ll need more information before you start your build, but we hope to give you a head start on the different systems available.
So, what is in a wall? There are many different methods of wall building, but they all fall into four broad categories—stud frame with cladding, bricks/blocks, cast/poured materials and pre-fabricated panels.
Stud frame with cladding
Probably the most common wall system used in Australia is a structural timber frame with cladding, in either a single or double timber stud system.
Single stud walls have one layer of framing—the internal cladding (such as plasterboard) is attached to the inside of the frame and the external cladding (such as weatherboard, fibre-cement or brick, as used in brick veneer, see later) is attached to the outside. Bulk insulation is fitted into the spaces between the studs of the frame, and foil insulation can be added as an additional layer around the outside of the frame, allowing for R-values up to almost R 4 with the right material combination. For example, according to the TasTimber document R-values for timber framed building elements —walls, a 90 mm stud wall with R 1.5 batts, reflective foil layer and AAC external cladding can achieve an R-value of 3.9.
To enable even more insulation, a double stud wall can be used. Double stud walls are just like two single stud frames, built one beside the other with a small gap in between. The resulting walls can be 200 mm or more in depth, so a great deal of bulk insulation can be installed. Of course, a double stud wall costs more than a single stud wall, but its advantages may well offset the extra cost if you live in an alpine area or area with low average temperatures, such as north-west Tasmania.
For a truly thermally efficient home, thermal breaks (thin layers of insulation material) between the studs and external wall cladding should be considered, although the extra expense may not be justifiable in moderate climates where low levels of heating and cooling are required.
Pest damage must also be considered, and the requirements will vary from location to location. In most areas except Tasmania, some form of termite barrier will be required. In cold, damp climates cavity wall ventilation may also be necessary to prevent moisture buildup.
We are all familiar with brick veneer, it being the mainstay of the Australian building industry for many decades, but, as traditionally built, these houses were not very energy-efficient.
In a brick veneer build, a single-layer brick wall is built around the outside of the house framework, the brick walls being tied to the wooden frames. Apart from bricks having a high embodied energy, the main problem with brick veneer is that the thermal mass is on the outside of the home instead of on the inside, where it is needed.
Reverse brick veneer
This is similar to brick veneer but addresses the issue of the thermal mass being on the wrong side of the walls. With reverse brick veneer, the brickwork is on the inside of the external walls, while the outside is insulated and clad. This results in a home that performs better thermally than regular brick veneer, despite the same or similar materials being used.
One advantage is that the materials used are familiar to most builders, so a specialist builder may not be required.
Weatherboard and other cladding
A very common wall style in many parts of Australia is weatherboard. This uses a timber-frame with weatherboard cladding and plasterboard or fibre-cement sheet interior lining.
There’s a large range of weatherboard materials available, from plantation softwoods through to hardwoods and modified woods such as Weathertex boards (which are 3% wax for water resistance), fibre-cement materials such as BGC Duraplank and Ezylite board, composite boards such as PermaTimber Eco Cladding, and even steel, such as Lysaght Weatherboard.
Wooden weatherboards usually need regular maintenance, such as painting or oiling, and must be protected from pests. Modified woods such as Weathertex are supplied pre-primed and require minimal maintenance if a good outdoor paint is used initially. Fibre-cement boards are resistant to rot and pests, and require minimal maintenance once painted. Of course, Colorbond steel weatherboards require virtually no maintenance.
Not all weatherboards are suitable for all fire zones. Wood-based boards tend to be rated to BAL19 or so and other materials, such as fibre-cement or steel should be used in higher bushfire zone areas.
Thermal performance of all weatherboard systems is generally low due to the low thickness of the materials, so weatherboard walls should include bulk fill and reflective foil insulation.
While all of the above-mentioned materials are supplied as individual boards which are installed like regular weatherboards, there are many sheet products that have a weatherboard finish while being faster to install. These are usually made from wood composites or fibre-cement sheets. Urbanline Euro Clad is an example of a composite weatherboard-style cladding panel.
Of course, there are other cladding options if you don’t want the weatherboard look. These include regular fibre-cement sheet, available smooth or with woodgrain finish, engineered timber such as SHADOWclad exterior structural ply from Carter Holt Harvey, lightweight cement sheets such as INEX Renderboard, which contains 60% post industrial recycled content, and many others.
The other common cladding option, although not to everyone’s taste, is steel sheet. Whether it be Colorbond or zinc finish, corrugated steel sheeting is becoming more popular as a robust, maintenance-free material that is quick to install.
The sustainability credentials of exterior cladding materials vary widely. Wood may be plantation-grown or from managed forests; composite materials often contain considerable recycled or waste material content and so reduce materials going to landfill. Fibre-cement materials are quite high in embodied energy, although some use recycled content or cementitious materials with lower manufacturing energy requirements such as magnesium oxide instead of Portland cement.
Steel-based products are light for the area they cover, reducing transport emissions, although they take considerable energy to manufacture. But when you factor in longevity and the elimination of the need for repainting, their environmental credentials are considerably improved.
There are so many external cladding options made from so many different materials that they can really only be evaluated individually—something potential home builders and renovators must do when researching materials. We’ve included a list of resources at the end of the article to assist with that process.
Of course, there are two sides to every wall, and so internal lining materials are also important. The most common options include plasterboard, magnesium oxide board, fibre-cement sheet and wood panelling. The same issues apply to interior materials as to exterior materials regarding sustainability and embodied energy.
Block-style wall systems
Now let’s look at systems that are similar to conventional ‘bricks and mortar’, starting with the most conventional.
As the name suggests, double-brick homes have external walls made of brick or other masonry blocks on both interior and exterior layers (called leaves), with an air gap (often 50 mm) in between. No timber or metal frame is normally used.
The exterior layer is usually conventional brick, while internal layers may be brick or concrete blocks or possibly other masonry blocks, depending on preference. The two layers of wall are connected together with brick ties for strength and stability.
The air gap between the two layers provides some level of insulation and further insulation can be added using rigid foam sheets or spray-in foams. The high thermal mass of the internal masonry helps stabilise indoor temperatures.
The internal masonry can be lined in plasterboard or another lining of choice, rendered or even left bare for interest. Of course, the choice of lining will determine how effective the thermal mass is, with high density materials such as cement render providing the best thermal coupling. Lighter materials such as plasterboard will introduce a thermal lag effect caused by the separate sheet and the small air gap inevitably created between lining and masonry.
While fired clay bricks are generally considered to have a high embodied energy, some manufacturers are making an effort to reduce the carbon footprint via reduced energy inputs for the firing kilns.
Autoclaved Aerated concrete (AAC)
As its name suggests, AAC is concrete that has been aerated—in effect, concrete foam. AAC is around 80% air (although this varies depending on the specific material and its intended use) and as such is much lighter than regular concrete. This gives it reasonable insulating ability while still retaining some thermal mass.
Autoclaved aerated concrete is available in numerous forms, from various sized bricks and blocks through to wall panels and even floor panels. It is easy to work with—blocks are simply cemented together with purpose-made cement. As the blocks are much larger than regular bricks, construction time can be quite rapid. The most popular AAC brand is Hebel; indeed, AAC is generally known by this name.
Because AAC comes in both blocks and panels, walls can be built in several ways. If built from blocks like other large-block materials, the blocks are the load-bearing components—no frame is needed.Alternatively, AAC panels can be used as cladding over a load-bearing frame, providing extra load-bearing support to the structure.
Because of the relatively high insulative value of AAC, especially the block form, no additional insulation is normally needed. For AAC-clad walls, additional insulation can be fitted into the frame like any other clad building.
Insulating concrete forms (ICF)
Moving into the more novel materials, we come across insulating concrete forms. These are large blocks, often made from polystyrene foam, which are built up like giant Lego bricks. Once the wall is complete, the forms are filled with concrete, which sets, forming a strong, insulated wall.
The outsides of the forms are then rendered or clad (more accurately, faced) to protect them from impact damage, as well as to provide fire protection.
Insulated concrete form walls can be good thermal performers and also provide high levels of acoustic attenuation, so are ideal in noisy environments, such as for homes near major roads, industrial areas or railway lines.
However, as they are filled with concrete, they can have a high embodied energy unless an eco cement is used. Also, polystyrene is not a very environmentally sound material (although production processes are slowly improving, such as with the elimination of CFCs as expanders) and it generally doesn’t biodegrade. It is also a large component of waste plastic found in the oceans. Also, because installed ICFs are a composite of foam and concrete they are not readily recyclable at the end of the building’s useful life.
Timbercrete is a mixture of wood wastes (sawdust), cement and sand which is formed into bricks, wall panels and pavers. Timbercrete products also contain non-toxic additives to improve block strength and stop excessive water penetration.
Timbercrete comes in a range of block and panel sizes and finishes, such as cobblestone, smooth and textured finishes, to suit almost any building style. Because its main ingredient is recycled timber waste, no trees are cut down to make Timbercrete. The waste timber content reduces the carbon footprint by locking up carbon, and the material is termite resistant.
Timbercrete has better thermal performance than concrete, and there is a high thermal performance block called the Super Insulating Block which combines a polystyrene foam core with Timbercrete inner and outer facings to provide both good thermal mass and a wall insulation level of R4.
Highly compressed strawbales are laid like large bricks and tied together with wire ties. Strawbales may either form infill between structural posts or may be load-bearing with the right construction technique. Strawbale walls are rendered on both sides to exclude pests and seal against weather.
Strawbale walls have low thermal mass (although this depends on the thickness of the interior render), but have very high insulative levels and can produce very stable internal temperatures with minimal heating and cooling. Embodied energy is low if locally produced bales are used, higher if they have to be transported long distances. They also lock up carbon, at least until the end of the building’s life, which can be more than 100 years.
Despite being made from straw, strawbale buildings usually achieve high levels of fire resistance due to the sealed nature of the walls—there is very little air in the bales to allow combustion.
A mixture of soil, clay and water, and sometimes a reinforcing material such as straw, is poured or pressed into forms and allowed to dry. Mudbricks are used like conventional bricks, although they are usually larger and heavier. Mortar is usually a mixture similar to the brick composition. A small percentage of cement may be added to the brick and/or mortar mix for improved strength and weather resistance. Finished walls are usually sealed with a clear mudbrick sealer, or may be rendered or coated in another suitable material.
Mudbrick walls have high thermal mass and so provide stable internal temperatures, but insulation levels are not high and vary depending on the composition of the bricks. Fire-resistance levels are generally very high. Embodied energy is low if locally produced bricks are used, higher if they have to be transported.
Because stone is a naturally formed material, the embodied energy comes from quarrying/cutting/finishing of the stone, transport and the mortar used when walls are built.
There’s also the environmental damage caused by quarrying activities to consider, although some companies remediate their quarries once exhausted.
Some properties have abundant natural stone just below the surface which can be harvested, washed and used as-is without any cutting. This is usually more suited to owner-builders, given the labour-intensive process of collecting and cleaning the stone.
The main advantages of stone, apart from potential low embodied energy, are high thermal mass and extreme fire resistance. However, like concrete, stone is not a good insulator and so rendering externally with a thick insulating render is usually required. Alternatively, stone may be used just for thermal banks inside the home, such as feature walls and fireplaces.
Green energy bricks
This is a specific brand of unique building block made of high density polyisocyanurate (PIR) foam clad on the interior and exterior surfaces with 9 mm magnesium oxide (MgO) board.
The bricks include locating lugs which provide alignment of walls, a mastic groove for sealing between bricks, and central voids for running services such as pipes and cables.
The bricks measure 600 mm long x 300 mm high and 320 mm thick (including cladding), and are simply stacked up together, with no mortar or concrete fill required, making for a very fast build. There are eight different shapes of Green Energy Brick to cater for corners, T-sections and the like.
The MgO cladding is fireproof and the PIR foam is self extinguishing, giving Green Energy Brick (GEB) walls a high fire-resistance rating. Insulation rating of a standard GEB wall is a huge R8, making for very good thermal performance.
That takes care of the commonly available block-style building methods, but there are many other ways to build a home. Some materials are not built up using discrete blocks, but rather are poured on-site, making walls that are essentially one solid block. These are usually formed by pouring or packing materials into forms—temporary wood or plastic moulds which are removed once the wall material has sufficiently set to be self-supporting.
This material is composed of hemp hurd (the core of the stalks of the hemp plant), lime-based binder and water, although it can also include some sand for added strength and/or thermal mass. A wooden load-bearing frame is built and formwork installed on both sides. The hempcrete material is mixed on-site and packed into the forms in layers to produce solid walls that cover the frame. Finished walls can be sealed or rendered with breathable finishes; the material is interesting enough to leave as-is, although external sealing is generally required.
Hempcrete provides a good level of insulation and so thermal stability, and hempcrete walls are breathable and regulate indoor humidity, reducing the likelihood of damp and mould. The material is both fire- and termite-resistant. It also has a very low embodied energy, especially if the hemp and binder are produced locally.
Rammed earth (pisé)
A mixture of clay, silt, sand and gravel is placed into wall forms and compressed using mechanical methods, such as a pneumatic hammer. Sometimes a small amount of cement is added to increase strength (known as stabilised rammed earth), and/or internal reinforcements are used. Walls may be left natural or sealed with a clear sealer, rendered or clad.
Thermal mass is high but insulation is relatively low. To improve performance, we recommend walls be built with insulation inserted in the centre of the wall, or existing walls be clad externally with insulated boards. Rammed earth walls have high fire resistance, good acoustic properties and low embodied energy (the more locally the earth is sourced, the lower the embodied energy). They are fully load bearing (up to 12 storeys high). An interesting possibility with rammed earth is embedding layers of different coloured soils in the walls to make impressive feature walls.
The main drawback with rammed earth is that it can be labour-intensive to build, depending on the method of compaction used (manual or mechanical). However, if there is a suitable source of soil on or near to the site, material costs can be relatively low.
One interesting variation on rammed earth wall building is used in an earthship. Old car tyres are stacked in layers and filled with earth that is then compacted. The external irregularities formed by the round shape of the tyres are then filled and rendered, resulting in walls with huge thermal mass and strength.
Earthships are usually made using locally sourced soil, so the embodied energy is low, and they often incorporate other waste materials such as glass bottles into walls for natural lighting. However, they are labour-intensive, so labour costs can be high unless you owner-build, and building regulations can be tricky. There are some examples of earthships in Australia; for example, see www.earthshipironbank.com.au.
A mixture of cement, aggregates (gravel and sand) and water, concrete is the mainstay of the building industry. It can be poured on-site or bought in block form or even as pre-cast wall panels which are simply assembled on-site.
Its main advantages are speed of installation, high thermal mass and strength (concrete slabs and panels contain steel reinforcing which increases strength enormously), but it has a high embodied energy.
There are more eco-friendly versions of concrete (such as TecEco’s reactive magnesia cements, and Wagner’s EFC) that replace the cement with waste materials such as blast furnace slag and fly-ash from sources such as coal-fired power stations, while the aggregates can be replaced with crushed recycled concrete. This can reduce the embodied energy of concrete, while having no effect on the quality.
Structural insulated panels consist of an expanded foam core (usually polystyrene, polyisocyanurate or polyurethane foam) clad with various materials, often oriented strand board (a type of wood-based board), but also other materials including metal (steel or aluminium), magnesium oxide board and fibre-reinforced cement composites. They are provided in large panels that simply lock together to form walls. This results in a very fast build.
The main advantages are rapid construction, high strength and high insulative values, but there is little thermal mass in most SIPs, so if that is required then it needs to be provided through a concrete slab, internal masonry features or the like.
SIPs are generally termite- and fire-resistant and are often cyclone-rated, making them ideal for use in the northern part of the country, on coastlines or wherever weather extremes are experienced. It should be noted that the foam used in some SIPs may off-gas, at least when they are new, but most claim to use low VOC/non-toxic foams.
Prefab wall panels
While SIPs are a form of prefabricated wall panel, the more traditional types are usually made of reinforced concrete. They are trucked on-site and assembled using a crane to lift them into place (they are much heavier than SIPs). Prefab wall panels can be painted, rendered or clad with sheet materials or facings.
The high thermal mass makes them suited to homes located where there are large diurnal temperature ranges, but insulation may need to be added externally—insulated cladding, such as RMAX ThermaWallPlus, is a simple method of achieving this.
One advantage is fast building times, but the large quantity of concrete means that they have a high embodied energy, unless eco concretes are used.
Not all prefab wall panels are concrete. Rapidwall is made from gypsum plaster with fibreglass reinforcement. There is no sand, gravel or cement used and the gypsum can be sourced from industrial processes where it is a waste product. Rapidwall panels are hollow and can be filled with concrete (for a high fire rating) or other materials such as insulation for improved thermal performance.
There are other variations on prefab panels which use various materials, but they are all based on some form of concrete, plaster or similar material.
While we have talked about timber-framed homes, you can eliminate timber from the framing system completely and use steel framing. The advantages of steel framing include higher strength to weight ratio and complete resistance to termites. However, steel framing is highly thermally conductive and so you must include thermal breaks (insulation) between the framing and the exterior wall material. See ‘Emerging materials’ in Renew 132 for a longer discussion of the advantages and disadvantages of steel and timber.
Other building methods
Like any industry, the building industry evolves over time, even if it is at a snail’s pace. While things change slowly here in Australia, around the world there are many interesting building methods being explored, with some making it off the drawing board and into the real world.
An example of this is 3D printing. Winsun of China has been manufacturing wall sections using a huge 3D printer which extrudes a fibre-reinforced eco concrete. There are numerous other interesting wall construction methods, such as papercrete (a material made from a mixture of recycled paper, sandy dirt and a small amount of cement), log walls, load-bearing engineered wood panels and many others, but few examples of these exist in Australia. ‘Emerging Materials’ in Renew 132 includes a discussion of cross-laminated timber and its use in Australia.
While this article looks at individual wall-building methods, the entire house envelope should be considered holistically in order to attain the most sustainable outcome. This includes total life cycle of materials (natural materials such as earth usually win out here), as well as the health of the home (such as the number of air changes per hour, humidity regulation, off-gassing and the like)—which includes both the completed living space as well as the manufacture of the materials (some foams, for instance, require toxic chemicals to manufacture).
For a building to be sustainable, it’s not just about minimising ongoing energy use: the total environmental impact of the building envelope should also be considered.
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