On this page:
- Mud brick (adobe) 2 to 8 stories
What is super-adobe?
- Rammed earth to 5 stories
- Cob ‘stickier’ earth
- Car tyres to 2 stories!
- Hempcrete has strength limitations
- Trick: water for thermal mass
1. Mud brick (adobe) 2 – 8 stories
First of all, mud brick is also called adobe, but not cob. The adobe wiki clearly says: “Adobe buildings are similar to cob and rammed earth buildings, but cob and rammed earth are directly made into walls and not bricks.”
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…
…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! (see Construction systems)
What is Superadobe?
Calearth.org — the Californian Earth and Architecture group, explains below:
Superadobe (sandbag and barbed wire) technology is a large, long adobe. It is a simple adobe, an instant and flexible line generator. It uses the materials of war for peaceful ends, integrating traditional earth architecture with contemporary global safety requirements. Long or short sandbags are filled with on-site earth and arranged in layers or long coils (compression) with strands of barbed wire placed between them to act as both mortar and reinforcement (tension). Stabilizers such as cement, lime, or asphalt emulsion may be added….
….This concept was originally presented by architect Nader Khalili to NASA for building habitats on the moon and Mars, as “Velcro-adobe”.
… The innovation of barbed wire adds the tensile element to the traditional earthen structures, creating earthquake resistance despite the earth’s low shear strength. The aerodynamic forms resist hurricanes. The innovation of sandbags adds flood resistance, and easy construction, while the earth itself provides insulation and fire-proofing.
2. Rammed earth to 5 stories
As the government website yourhome.gov.au says:
Rammed earth walls are constructed by ramming a mixture of selected aggregates, including gravel, sand, silt and a small amount of clay, into place between flat panels called formwork. Stabilised rammed earth is a variant of traditional rammed earth that adds a small amount of cement to increase strength and durability. Most of the energy used in the construction of rammed earth is in quarrying the raw material and transporting it to the site. Use of on-site materials can lessen energy consumed in construction. Rammed earth provides limited insulation but excellent thermal mass….
….A five storey hotel in Queensland is built of stabilised rammed earth.
3. Cob ‘stickier’ earth:
Cob, cobb or clom (in Wales) is a natural building material made from sand, clay, water, some kind of fibrous or organic material (straw) and earth. Cob is fireproof, resistant to seismic activity, and inexpensive. It can be used to create artistic, sculptural forms and has been revived in recent years by the natural building and sustainability movements…
…The walls of a cob house were generally about 24 inches thick, and windows were correspondingly deep-set, giving the homes a characteristic internal appearance. The thick walls provided excellent thermal mass which was easy to keep warm in winter and cool in summer. Walls with a high thermal mass value act as a thermal buffer inside the home.
Straw has been used as a building material for centuries for thatch roofing and also mixed with earth in cob and wattle and daub walls. Straw is derived from grasses and is regarded as a renewable building material. Strawbale walls are surprisingly resistant to fire, vermin and decay. Finished straw bale walls are invariably rendered with cement or earth so that the straw is not visible. The final appearance of rendered straw bale can be very smooth and almost indistinguishable from rendered masonry, or it can be more expressive and textural.
Or as the wiki says:
Advantages of straw-bale construction over conventional building systems include the renewable nature of straw, cost, easy availability, naturally fire-retardant and high insulation value. Disadvantages include susceptibility to rot, difficulty of obtaining insurance coverage, and high space requirements for the straw itself. Research has been done using moisture probes placed within the straw wall in which 7 of 8 locations had moisture contents of less than 20%. This is a moisture level that does not aid in the breakdown of the straw. However, proper construction of the straw-bale wall is important in keeping moisture levels down, just as in the construction of any type of building.
5. Car tyres to 2 stories!
The Earthship movement prides itself on recycling rubbish. Car tyres form an enormous, expensive to recycle material that Earthships see as a key resource for recyling. They lay them flat, pound earth into them, and stack the next layer on top. This then becomes the next load-bearing wall. Clad it in stone or adobe for appearance. Earthips are best placed in the rural lifestyle and ecovillages, not New Urbanism because of their space requirements and height limits. But they’re beautiful, I’ve loved them for years, and so here I’m going to spoil them with a gallery.
Traditional Earthship tyre & adobe construction
6. Hempcrete has some strength limitations
Hempcrete is a mixture of hemp hurds (shives) and lime (possibly including natural hydraulic lime, sand, pozzolans or cement) used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, and Isochanvre. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints.
However, the typical compressive strength is around 1 MPa, around 1/20 that of residential grade concrete. Hempcrete walls must be used together with a frame of another material that supports the vertical load in building construction, as hempcrete’s density is 15% that of traditional concrete. Like other plant products, the hemp crop absorbs carbon dioxide gas as it grows, retaining the carbon and releasing the oxygen. 165 kg of carbon can be theoretically absorbed and locked up by 1 m3 of hempcrete wall over many decades.
7. Trick: water for thermal mass
Water has a higher thermal mass (heat storage capacity) than concrete! As Your Home shows:
Material to Thermal mass (volumetric heat capacity KJ/m3.k)
Compressed earth blocks: 1740
Rammed earth: 1673
Fibre cement sheet (compressed): 1530
Brick: 1360Earth wall (adobe): 1300
Autoclaved aerated concrete: 550
Source: Baggs and Mortensen 2006
How to include water into a house design cheaply? As Your Home shows:
Low cost mass options for upper storeys
PCMs or water filled containers have much greater thermal storage capacity than masonry and can be used as a mass substitute. PCMs are much lighter than masonry. Water has double the storage capacity of concrete and because of convection within the container, penetration rates are substantially higher. Thus water can supply similar storage capacity to masonry with significantly less mass and bulk. Accordingly, both can be cost effective mass options for upper storeys because they require no (or less) additional structural support.
With peak metal a concern, we must reduce the demand for metals. The materials above provide valuable alternatives. My favourite is wood as it lets us build soaring ecocities 30 to 40 stories tall out of a renewable carbon sequestering material that might even create a market for greening the deserts. If we can get the shape of our cities right we will be half way there. If we can build them out of wood, and even get some of our rural and suburban homes built out of the wonderful variety of materials above, we’ll not only reduce our environmental impact, but build a more beautiful home. It will be a fun and surprising journey. Let’s get started.