Basic mud bricks are made by mixing earth with water, placing the mixture into moulds and drying the bricks in the open air. Straw or other fibres that are strong in tension are often added to the bricks to help reduce cracking. Mud bricks are joined with a mud mortar and can be used to build walls, vaults and domes.
The appearance of mud bricks reflects the materials they are made from. They are thus earthy, with their colour determined by the colour of clays and sands in the mix. Finished walls can range from a strong expression of the brick patterns to a smoothly continuous surface.
With thick enough walls, mud brick can create loadbearing structures up to several storeys high. Vaults and domes in mud brick prove that it can be used for many situations other than vertical walls. It may be employed as infill in a timber frame building or for loadbearing walls, although its compressive strength is relatively low. Typically, Australian mud brick structures are single or double storey. In the Yemen buildings eight storeys high and more have stood for centuries!
Mud brick walls can provide moderate to high thermal mass. For most Australian climatic conditions, as a rule of thumb, walls should be a minimum of 300mm thick to provide effective thermal mass (see Thermal mass).
Contrary to popular belief mud bricks are not good insulators. Since they are extremely dense they lack the ability to trap air within their structure, the attribute of bulk insulation that allows it to resist the transfer of heat.
To achieve the levels of insulation needed for sustainable house construction and to achieve Building Code of Australia compliance across most of Australia, it is almost always necessary to add insulation linings to external mud brick walls. In some milder climate zones, where thermal insulation is less critical to the overall building performance, mud brick walls may not need additional insulation (see Insulation).
One way of dealing with mud brick’s lack of insulation is to construct some or all of the outer walls with framed construction, and use mud brick for partition walls and as an internal ‘reverse brick veneer’ on some external walls. This approach allows the building to reach ‘lock-up’ very quickly and provides a protected space to make and dry the bricks.
Traditional earth buildings often used walls up to a metre thick: these would provide reasonable insulation and enormous mass to stabilise internal temperatures.
A well-built mud brick wall has very good sound insulation properties. In fact, it can be almost equivalent to a monolithic masonry structure in its capacity for sound attenuation (see Noise control). Some modern mud brick homes use mud brick for external walls and light partition walls internally; it is more effective for thermal and acoustic performance to use mud brick for the partition walls and lightweight, well-insulated external walls.
Fire and vermin resistance
Since earth does not burn, and earth walls do not readily provide habitat for vermin, mud brick walls generally have excellent fire and vermin resistance.
Durability and moisture resistance
Mud brick walls are capable of providing structural support for centuries but they need protection from extreme weather (e.g. with deep eaves) or continuous maintenance: the ancient structures of the Yemen have been repaired continuously for the centuries they have been standing. Although some soils are very resistant to weathering, as a general rule mud brick needs protection from driving rain and should not be exposed to continuous high moisture.
Breathability and toxicity
Mud bricks make ‘breathable’ walls but some mud brick recipes include bitumen, which potentially results in some outgassing of hydrocarbons. Ideally earth should be used in, or as near as possible to, its natural state.
Mud bricks could have the lowest impact of all construction materials. Mud brick should not contain any organic matter — the bricks should be made from clays and sands and not include living soil. They require very little generated energy to manufacture, but large amounts of water. Their embodied energy content is potentially the lowest of all building materials but the use of additives such as cement, excessive transport and other mechanical energy use can increase the ‘delivered’ embodied energy of all earth construction (see Embodied energy).
In a similar way, the greenhouse gas emissions associated with unfired mud bricks can (and should) be very low. To keep emissions to an absolute minimum, the consumption of fossil fuel and other combustion processes have to be avoided. If, say, 5% cement is added to a 300mm mud brick wall, it makes a fairly high energy/high emission building material, close to the embodied energy of a 125mm unenforced concrete wall.
Buildability, availability and cost
Mud bricks are a forgiving medium well suited to owner builder construction. A number of proprietary mud brick makers and builders provide good information in Australia. A strong owner builder oriented network includes a broad based national organisation, the Earth Building Association of Australia (EBAA), which is a not-for-profit group ‘formed to promote the use of unfired earth as a building medium throughout Australia’.
Materials for making mud bricks are readily available in most areas and in some cases may be sourced directly from the building site.
Low costs in construction can only be effectively achieved by self-build, reducing the labour costs associated with the manufacture and/or laying of bricks. Commercially produced mud brick construction can be as expensive, or even more expensive, than brick veneer.