It is also relatively inexpensive, and it retains heat, which means it can help save heating and cooling costs.

It can be used in most conditions, provided it is designed and built to provide protection from rain. It has been a very common construction material all over the world for thousands of years.

Types of earth construction

Rammed earth

Rammed earth walls are constructed by ramming a mixture of earth, with gravel, sand, silt and clay, into place between formwork. Ramming is done with manual or mechanical rammers.

Sand is added to most soils to reduce shrinkage which can otherwise be a problem. Stabilised rammed earth has 5-10% cement added to give extra strength and durability.

The mixture must be carefully mixed and almost dry. The formwork can be removed immediately after the wall is finished.

Earth buildings should not contain any organic matter. This will rot, allowing mould and fungi to grow, and also weakening the structure.

External walls are usually 30cm thick, and internal walls may be 20cm. Curved walls are possible but need more expensive formwork.

The appearance of rammed earth walls depends on the materials used. It often has a stratified look caused by the ramming process.

Poured earth

In this method a wet mixture is made, poured into moulds and left to dry out. To prevent cracking, alternate sections are completed and then the gaps are filled in.  This method often uses about 10% cement.

Mud brick (Adobe)

Mud bricks are laid similarly to conventional masonry.

Mud bricks are cast in moulds and allowed to dry out slowly in the air. Often straw or other fibres are added to help reduce cracking.

Any shrinkage occurs during this drying. New bricks can be trimmed and cut before they dry out too much.

Mud bricks are laid with a thick layer of mud mortar between them. This mortar is also rubbed over the walls to fill in any gaps and cracks.

Mud brick allows a greater range of soils to be used compared with rammed earth.

A temporary roof should be constructed to give shelter to the walls as they are being built.

Pressed earth

Dry earth mixtures can be pressed into bricks. This saves the drying time needed for mud bricks, and gives a more uniform size. Pressed earth bricks usually contain cement. Making them is a slow and labour-intensive process.

Light earth

Light earth uses aggregates such as straw, wood shavings or pumice as a mix for bricks or to be applied in a timber frame. It gives better insulation than dense earth.

Cob

Cob uses a mix of clay, gravel and straw applied directly to build thick walls that are trimmed as they are built.

Key features of earth construction

Cost

Costs for an earth-built home are comparable to conventional construction. Earth building uses inexpensive materials but is very labour intensive. You can save money by doing the work yourself if you have the appropriate skills or a source of experienced oversight.

It is more cost effective to have a source of suitable earth on-site.

Strength

In general, earth buildings are very strong in compression, but not so strong in tension. However, it is very important to test the earth to ensure it will do the job properly - natural materials are highly variable. It will need to comply with the relevant New Zealand Standard. See the Earth Building Association of New Zealand’s web page on standards for earth building.

Ease of construction

Mud brick, cob and light earth building can often be done by owner-builders. But you need a great deal of time to make the bricks and a dry storage area to keep them before they are used.

Before starting it is essential to know how long it will take to make the bricks. You may need as many as 8000 bricks if you use them for all walls, and each weighs about 18kg. Two people may be able to make 200-300 bricks in a weekend.

Because of the specialised equipment and skill involved, rammed earth construction should be left to specialist builders.

Many building inspectors are unfamiliar with earth construction. To speed the planning process, have the project designed by someone who has experience with designing earth buildings.

Durability and weathertightness

Earth buildings can last a very long time and generally require very little maintenance as long as they are not exposed to driving rain. To provide this protection, overhanging eaves or verandahs are needed. The New Zealand Standard for earth building specifies the size of the overhang for each part of the country.

Some earth buildings have an exterior coat of plaster to help with weathertightness.

Foundations must prevent rising damp. Modern earth buildings have conventional concrete footings, either strip or slab.  A damp-proof course should be laid between the concrete and the wall to stop rising damp.

Earthquake-resistance

Earth walls can be reinforced to withstand earthquake loads.

Fire resistance

Earth walls are fire resistant.

Pest resistance

Earth walls are vermin resistant.

Is it compatible with passive design?

Earth walls aren’t good thermal insulators. You’ll need some other form of insulation. They are good sound insulators.

Earth will absorb heat during the day and radiate it out as the temperature drops at night. This helps to reduce heating and cooling costs.

Because the walls are so thick and there is a need for weather protection, it may not be easy to size windows to make the most of the sun’s warmth in winter.

Earth walls breathe, which helps to control humidity. There are no harmful emissions from earth walls.

Is it right for my site?

Topography and site impact

Earth walls are very heavy and only work well on stable sites with good foundations. Excavating a site will have an impact on its flora and fauna, and on stormwater runoff. Excavation can also increase the likelihood of erosion and soil instability.

Climate

Earth buildings can be suitable for all areas of New Zealand, provided they are protected against damp and wind-blown rain.

More information

From Smarter Homes

• Timber construction
• Light steel frame construction
• Concrete construction
• AAC construction
• Straw bale construction
• Insulated concrete formwork
• Construction site practice
• Exterior design
• Materials
• Passive heating
• Passive cooling
• Insulation

From ConsumerBuild

• The Building Act
• Leaky buildings; this includes information about building for weathertightness
• Construction

From other sites

The Earth Building Association of New Zealand has information on mud bricks or abode, rammed earth, pressed and poured earth construction. Also see The Earth Building Association of Australia’s site.

The Waitakere City Council website has a factsheet Earth Building (PDF, 305KB).

New Zealand’s oldest earth building is Pompallier House in Russell, which has stood for more than 150 years. The Historic Places Trust has a web page about the house.

The Department of Building and Housing’s website has information about building law and compliance, including the Building Code, and on weathertightness.

All home construction needs a building consent from a building consent authority (usually your local council). ConsumerBuild’s Council finder web page has contact details for New Zealand local authorities.

You can buy New Zealand Standards relating to various construction systems from the Standards New Zealand website.

You can buy BRANZ publications about construction systems from the BRANZ website (click on the link to the BRANZ bookshop).