Using natural materials: A comparison

by Malcolm Worby

Using natural materials for construction of dwellings and community buildings, is the oldest method of building since humans moved away from caves. In fact, more people in the world live in houses built of natural materials, than any other type of building material. It is therefore the most common method of building in the world.
There are many different types of natural building materials, and to describe and go into detail about each one would only serve to confuse many who are unfamiliar with natural building practices, as some are only applicable to certain areas of the world. So, in order to keep things simple, the more popular, and commonly known and used types, are outlined below, describing the basic methods of building with the material, along with its advantages and disadvantages. This is by no means meant to be comprehensive, and is intended solely to explain the basics to those who are unfamiliar with the different types of building and material. There is of course far more detail, and also other options of how to build than is outlined below, and it is hoped that anyone who having read this information, and is interested in pursuing building using natural materials will seek a professional to assist them to make the right choices.

ADOBE (MUD BRICK)
Adobe bricks comprise of a mixture of clay, coarse sand, fine sand, silt, and water, (the ideal clay content is no more than 20% of the mixture) which is placed in a form made to the size of the bricks required, and then removed to allow the bricks to bake in the sun until hard. A binder such as straw is added only if the clay content is low. The dried bricks are built on a solid foundation, ideally stone, built to a minimum of 200mm above ground level, which also acts as a ‘Damp Proof Course’ as it raises the adobe bricks wall from absorbing moisture from the ground. A similar clay-sand-water mix as used to make the bricks is used as mortar between the adobe bricks. Fairly large roof overhangs (600mm minimum) help prevent the walls being eroded over time, and on the north wall it helps keep the building cool in the summer. Usually a 2-coat earthen or lime plaster is applied as a final finish. Adobe wall structures lend themselves to having load-bearing walls, however, a wood or concrete ‘ring beam’ is recommended to support the roof structure.
The soil for making adobe bricks is usually of local material, and ideally from the property itself. This therefore makes adobe one of the most affordable building technologies, and is often completed without the use of engineers or architects, hence the term ‘non-engineered construction’. The walls are usually a minimum of 250mm thick for a single storey and 600mm double-brick thick for added thermal mass, and for two-storeys. Ideal areas for building with adobe would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot and humid climates, narrower thickness walls could be used, providing sufficient roof overhang is provided for shade.

Advantages:
• High thermal mass is very energy-efficient in both summer and winter, and ideal for passive solar heating and cooling. Indoor temperatures vary only about 5 degrees between summer and winter (17-22 degrees), making it naturally cool in summer and warm in winter.
• Environmentally friendly: Low carbon footprint and embodied energy
• Ideal material for owner builders and unskilled labour
• Relatively inexpensive for a long lasting building
• Lends itself to creative and free-form walls
• Rondavel (round) type Adobe buildings are capable of withstanding seismic activity
• Fireproof
• Excellent sound insulation
• Can easily be built up to 3 stories
• Can be recycled
• Approved by many local building departments.

Disadvantages:
• Fairly labour intensive
• Obtaining a bond from lending institutions is extremely difficult
• Adobe cannot be laid during very wet or freezing weather
• Insects, notably termites and small rodents can burrow into the walls weakening them. The use of dung in the mud mix, and lime plaster can negate this problem

COB
Cob, like adobe, is also comprised of a mixture of clay, coarse sand, fine sand, silt, and water; it also uses a binder of fibrous or organic material such as straw, or dung. The cob once mixed, can either be used ‘as is’ and installed in ‘lifts’ of about 600mm, or can be rolled into balls about 200mm in diameter. The building is a process of laying the straw-clay mixture or balls in layers on top of the foundation walls, which are built ideally with stone, to a minimum of 200mm above ground level. The walls start wide at the base (600mm+) and taper in as one builds up. Each layer of cob must be allowed to dry before laying the next. As with adobe, large roof overhangs (600mm minimum) help prevent the walls being eroded over time, and on the north wall it helps keep the building cool in the summer. Cob wall structures, due to their width, lend themselves to having load bearing walls, however, a wood or concrete ‘ring beam’ is recommended to support the roof structure. Usually a 2-coat earthen or lime plaster is applied as a final finish. The soil for making cob and cob bricks is usually of local material, and ideally from the property itself. Therefore cob is also one of the most affordable types of building material, and can be built often without the use of engineers or architects, as ‘non-engineered construction’. Ideal areas for building with cob would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot and humid climates, narrower thickness walls could be used, providing sufficient roof overhang is provided for shade.

Advantages:
• High thermal mass is very energy-efficient in both summer and winter, and ideal for passive solar heating and cooling. Indoor temperatures vary only about 5 degrees between summer and winter (17-22 degrees), making it naturally cool in summer and warm in winter.
• Environmentally friendly: Low carbon footprint and embodied energy
• Relatively easy to build for owner builders and unskilled labour
• Relatively inexpensive for a long lasting building
• Lends itself to free-form walls
• Excellent sound insulation
• Cob buildings are capable of withstanding seismic activity, but must have a ring beam.
• Fireproof
• Can easily be built up to 3 stories
• Cob can be easily recycled

Disadvantages:
• Labour intensive
• Relatively slow to build
• Obtaining a bond from a lending institution is very difficult.
• Cob walls cannot be laid during wet or freezing weather
• Insects, notably termites and small rodents can burrow into the walls weakening them. The use of dung in the mud mix, and lime plaster can negate this problem

RAMMED EARTH
Rammed Earth construction is done by using a mixture of sand, gravel, clay (the proportions depend on the available soil), and water. The mixture is placed into formwork made of plywood supported by steel frames (or similar), placed on top of the foundation wall. The amount of mixture placed in a form at a time, known as a ‘lift’, is typically about 150mm deep, which is then compacted either manually, or by a pneumatic backfill tamper. This process is repeated until the desired wall height is reached. Door and window openings are created by using formwork, with lintels placed on top of the forms prior to compacting. The final result is a sculpted earth wall of exceptional strength. A stabiliser, preferably lime, but cement can be used, can be added prior to mixing, and is typically between 5% -13% of the mixture. Note: If cement is added as a stabiliser, a rammed earth wall 300mm thick, will have more cement content than a 115mm wide concrete block wall, and therefore the carbon footprint and the embodied energy is increased dramatically. For more creative builders, Rammed Earth offers the opportunity to mix colours of soil and when the lifts are done in different colours, it provides ‘stratification’ in the walls. This process when sealed with beeswax or similar, provides a beautiful finish with minimal maintenance. The thick earth wall is structurally very sound, but it is recommended that a wood or concrete ring beam be installed at the top of the walls. Ideal areas for building with rammed earth would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates which fully utilise the thick thermal mass for heat storage in winter, and for cooling during the summer. In hot climates, it is essential that the walls are shaded at all times.

Advantages:
• Low carbon footprint and embodied energy.
• The thermal mass is ideal for passive solar heating and cooling.
• Rammed earth walls are extremely strong
• Excellent sound insulation
• Fire proof
• Insects and rodents are not a problem
• Does not need to be plastered
• Can withstand seismic activity providing it has a concrete ring beam.

Disadvantages:
• Rammed earth walls are very labour intensive.
• Building the walls is a slow and precise process.
• The formwork adds considerable cost.
• Obtaining a bond from a lending institution is very difficult.
• The walls cannot be built during wet or freezing weather
• Difficult to recycle

SANDBAG
Sandbag construction consists of lightweight plastic bags filled with sand or other earth mixes. Ideally the soil or mix is locally available on site. Typically sandbag construction utilises the pillar and beam type of structural framework construction, whereby the full bags are used as ‘in-fill’ by laying in courses, on a foundation wall, between the pillars. If pillar and beam construction is not used, the building will need to have curved walls to create added strength. Once the walls are completed, typically chicken-wire is then attached to the sandbags, which will allow the walls to be plastered, ideally with lime plaster. Sandbag walls are relatively quick to build, and due to the pillar and beam framework, they are also strong, and relatively inexpensive. Sandbagging requires very little water especially compared to adobe, cob, or rammed earth, which can be an important factor in some areas. Sandbag walls are also a viable option for constructing temporary buildings, as the materials are mostly reusable. The thick walls offer a good thermal mass which helps regulate the interior temperature of the building during both the summer and winter months. Ideal areas for building with sandbags would be temperate climates with hot and cold seasonal swings, cold climates, and hot dry climates.

Advantages:
• Relatively low carbon footprint and embodied energy.
• Sandbags can be reused or recycled
• Strong structures are erected quickly using pillar and beam construction with sandbag ‘infill’
• Minimal water needed for construction.
• Good thermal mass for regulating internal temperatures
• Excellent sound insulation.
• In rural areas, ‘Mealie’ bags can be collected and recycled as sandbags.
• Services can easily be added during the construction phase

Disadvantages:
• Labour intensive
• Care must be taken to prevent water penetrating through to the sandbags
• Sandbags are often made of plastic and are imported from China, which increases its environmental footprint, and embodied energy. Locally made bags are available, but are more expensive.
• Pillar and beam technology using steel, cement blocks, and wood from non-sustainable forests, are not environmentally-friendly
• Walls must be plastered.
• Obtaining a bond from a lending institution is very difficult.

STRAW BALE
The straw bales used for building must be of grain stalks (oats, barley, wheat, etc), as opposed to hay bales, which are made from grasses. The use of straw bales as a building material is very environmentally-friendly, as the straw if not baled, is typically burned by the famers after harvesting, creating tonnes of air pollution. Building with straw bales either utilises the pillar (or post) and beam type of construction, with the straw bales used as ‘infill’, or the straw bales built as a load-bearing wall. In the load-bearing wall method, a wood roof or top plate is installed covering the full width of the top of the wall, which in turn is attached to the foundation, typically with wire straps, on approximately 1200 centres. The top plate acts not only as a bearing plate, but also as a ring beam, distributing the roof load evenly along the walls. Window and door openings will need to be structurally framed in with wood in both methods. The walls when completed, and the roof is installed, are typically covered with chicken wire, and then plastered with either mud or lime plaster on the exterior, and mud, lime, or gypsum plaster on the interior. It is recommended that there is a sufficient roof overhang to help prevent water saturation during heavy rains.
Straw bale building, due to its high insulating value, is most beneficial in hot, dry desert climates, high desert climates with large daily temperature swings, cold climates, and temperate climates which have relatively hot summers, and cold winters.

Advantages:
• Energy efficient, as straw bales have a very high insulation value.
• Straw bales are produced from a waste product that is bio-degradeable.
• Can be built using unskilled or semi-skilled labour.
• Not as labour intensive as other natural building methods.
• Structurally strong
• Excellent sound insulation
• Relatively inexpensive material to purchase

Disadvantages:
• Straw bale walls are susceptible to mould and deterioration unless protected from moisture, and allowed to ‘breathe’.
• No thermal mass for passive solar heating and/or cooling
• Transportation increases the embodied energy/carbon footprint, and also the cost, unless building on a farm growing cereal crops.

As published on Enviropaedia under the title Green Building: Using natural materials

Disclaimer

Malcolm WorbyMalcolm Worby studied at Bristol Polytechnic in the UK and has had his own award-winning architectural design firm ‘Malcolm Worby Designs’ for over 30 years specialising in natural, sustainable, and environmentally friendly building. He now specialises in providing consulting services for natural and sustainable building projects, including mud brick (adobe), straw bale, sand and earth bag design and building, passive and active solar heating and cooling, photovoltaic (PV), grey water recycling, rainwater harvesting, and composting toilets. He has designed buildings in various parts of the world including the USA, UK, South Africa, Mexico, and the Netherlands, and has worked on low income affordable community-build projects in South Africa, Malawi, Mozambique, South Sudan, Uganda, Zambia through his Non-Profit Organisation ‘Homeless And Poor People’s Initiative’ (HAPPI).

2 thoughts on “Using natural materials: A comparison

  1. Pingback: All good things must come to an end |

  2. Pingback: Understanding Earth: The beginnings of a Natural Builder |

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