5.6 Mud brick (Adobe)

The ideal building material would be ‘borrowed’ from the environment and replaced after use. There would be little or no processing of the raw material and all the energy inputs would be directly, or indirectly, from the sun. This ideal material would also be cheap. Mud bricks can come close to this ideal.

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.

At its simplest, mud brick making involves placing mud in moulds which, after initial drying, are removed to allow the bricks to dry slowly (not in direct sun). Moulds can be made from timber or metal – anything that can be shaped to provide the desired size for the bricks.

Virtually all the energy input for mud brick construction is human labour (indirectly, fueled by the sun) and after a lifetime of use, the bricks break back down into the earth they came from. The most effective use of mud bricks in building healthy, environmentally responsible housing, comes from understanding their merits and accepting their limitations. Mud brick construction is often referred to as ‘adobe’ which is an Arabic and Berber word brought by Spaniards to the Americas, where it was adopted into English.

The use of earth construction is wellestablished in energy efficient housing. There are many aspects to earth construction and despite the fact that most of the world’s buildings are made of earth and it is one of the oldest known building materials, there is much about its properties and potential that remains undeveloped and poorly researched.

PERFORMANCE SUMMARY

Appearance

The appearance of mud bricks reflects the material they are made from. They are thus earthy, with colour determined by colour of clays and sands in the mix. Finished walls can express the brick patterns very strongly at one extreme or be made into a smoothly continuous surface.

Structural capability

With thick enough walls, mud brick can create load bearing structures up to several stories high. Vaults and domes enable adobe to be used for many situations other than vertical walls. The mud brick may be used as infill in a timber frame building or for load-bearing walls, although its compressive strength is relatively low. Typically, Australian adobe structures are single or double storey. In the Yemen there are buildings 8 stories high and more that have stood for centuries! [See: 5.5 Construction Systems]

[See: 5.6 Construction Systems]

Thermal mass

Adobe walls can provide moderate to high thermal mass, but for most Australian climatic conditions, as a rule of thumb, walls should be a minimum of 300mm thick to provide effective thermal mass. [See: 4.9 Thermal Mass]

Insulation

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. Insulation can be added to adobe walls with linings but is not intrinsic to the material, and, depending on the building design may not be needed in some climate zones. [See: 4.7 Insulation]

Sound insulation

A well-built adobe 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: 2.7 Noise Control]

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

Adobe walls are capable of providing structural support for centuries but they need protection from extreme weather (eg. with deep eaves) or continuous maintenance (the ancient structures of the Yemen have been repaired continuously for the centuries they have been standing). As a general rule, adobe needs protection from driving rain (although some adobe soils are very resistant to weathering) 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 its natural state or as near it as can be achieved.

Environmental impacts

Mud bricks have the potential to provide the lowest impact of all construction materials. Adobe 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. The embodied energy content of mud bricks is potentially the lowest of all building materials but additives, excessive transport and other mechanical energy use can increase the ‘delivered’ embodied energy of all earth construction. [See: 5.2 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. [See: 5.1 Material Use Introduction]

Buildability, availability and cost

Mud bricks provide a forgiving construction medium well suited to owner-builder construction. There are a number of proprietary mud brick makers and builders able to provide good information and a strong owner-builder oriented network. There are good networks in Australia including a broad based national organisation, the Earth Building Association of Australia (EBAA), which is a not for profit organisation “formed to promote the use of Unfired Earth as a building medium throughout Australia.”

The materials for making mud bricks are readily available in most areas and may be sourced directly from the site of the building in some cases.

Low costs in construction can only be effectively achieved by self-build, reducing the labour costs associated with manufacture and/ or laying of bricks. Commercially produced mud brick construction can be as expensive, or even more expensive, than brick veneer.

Principal Author:
Paul Dowton