Mother Earth Living

Green Building Materials: A Guide to Natural Building Systems

Natural building provides an opportunity to create homes that are healthier than their conventional counterparts.
By Dan Chiras
July/August 2006
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Rammed earth building uses slightly dampened subsoil or a sand-cement mix in between forms attached to a foundation.
Photo by Robert Reck
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With their thick walls, sensuous earthen plasters and flowing lines, homes built using natural materials offer beauty and comfort. And natural building also provides an opportunity to create homes that are healthier than their conventional counterparts. More than a dozen natural building methods are in use today, and many natural builders use combinations of these to create a wide array of beautiful homes.

The Basics: Natural Building Methods

Adobe: Adobe is a time-tested natural building material dating as far back as 6000 B.C. Builders create adobe homes using blocks made from clay-rich mud (subsoil) and straw, dried and hardened in the sun. The blocks are placed like bricks, using wet adobe mud as mortar.

Cast Earth: Cast earth is a modern invention and one of the most expedient natural building materials available today. Earth is mixed with water, a special type of gypsum and some proprietary chemicals to form a slurry that’s poured into forms. Unlike most other natural building methods, this one is reserved for professionals, as only certified contractors are licensed to build with the material.

Cob: Cob is an ancient building material used in Europe at least 500 years ago. In this method, a timber frame typically supports the roof. The builder then literally handsculpts the walls by mixing clay-rich mud and straw and applying them to the foundation in wet clumps (cobs) that are kneaded and massaged.

Cordwood: Cordwood's exact origins are unknown, but some of the earliest cordwood buildings were built in Siberia and Greece about 1,000 years ago. Split or unsplit firewood is stacked on a foundation (generally with insulation between the logs) and held in place on both sides by cement mortar. Visible round log ends form interesting patterns in finished walls.

Earthbags: A newcomer to the natural building scene, earthbag construction got its start in the United States. Polypropylene bags—typically 50- or 100-pound grain bags—are filled with dirt and packed into place, then arranged in an overlapping pattern. Rows are sometimes secured barbed wire. Plaster protects the bags from the elements.

Log: Log homes were popular with early America’s European settlers. Interlocking notched logs make up the walls, and the gaps between them are filled with a chinking material to seal the structure. Alternatively, logs can be notched lengthwise so they fit snugly together, creating a more airtight seal and making chinking unnecessary.

Papercrete: Papercrete, which evolved relatively recently in the American Southwest, is made from a slurry of shredded newspaper and water mixed with cement and sand. When it dries, it forms lightweight bricks that are stacked in a running-bond (overlapping) pattern like conventional bricks. Wet papercrete is applied as a mortar to hold them in place.

Rammed Earth: Rammed earth is one of the oldest forms of natural building, dating back thousands of years. Today, slightly moistened subsoil (or a sand-cement mix made on site) is packed between forms, either mechanically or by hand. When the wall is completed, the forms are removed, revealing massive walls that resemble blocks of sandstone.

Rammed-Earth Tire: Invented in the 1970s in New Mexico, rammed-earth tire homes (also known as “earthships”) are made from used automobile tires arranged in a U shape, then packed with earth to form thick walls, which are coated with thick layers of plaster. Because the homes typically are nestled into the ground (or earth-sheltered), builders must take special precautions to ensure that the walls are waterproof, especially in wet climates.

Straw Bale: Straw bale building was “invented” by American pioneers who eked out a living on the treeless, windswept plains of western Nebraska. Most straw bale homes are built using post-and-beam construction with straw bales between the framing. Some are built without framing; the bales simply are stacked on the foundation, and the roof is attached to a top plate that rests on top of the bales. Straw bale walls are finished with plaster.

Straw-Clay: Straw-clay has been used for at least 500 years, mostly in Europe. A mixture of straw and clay is packed into forms attached to a wooden frame that supports the roof. After the forms are removed, the wall dries and is then plastered.

Stone: Stone has been used to build homes—and castles—for centuries. Stone walls are built using locally available rocks that are carefully stacked on solid foundations, designed to support the walls’ additional mass. The stones are mortared in place using cement.

Natural Building Materials: The Breakdown

Adobe
Subsoil (containing clay and sand) is mixed with water and straw and formed into blocks. Dry blocks are mortared into place.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold climates if walls are well-insulated.
Pros:
• Time-tested form of architecture
• Uses a widely available building material
• Thick; structurally sound, fire-resistant and beautiful
• Ideal for passive cooling in hot, dry climates
Cons:
• Requires time-consuming block production (unless adobe bricks are purchased from a commercial source)
• Mortarting blocks in place takes time and skill
• Tends to be cold unless insulated

Cast Earth
Specially prepared slurry of earth, heated gypsum and other components is poured between forms; walls are typically load-bearing.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold climates if walls are well-insulated
Pros:
• The fastest of all natural building techniques
• Thick, structurally sound, fire-resistant walls
• Ideal for passive cooling in hot, dry climates
Cons:
• Only licensed builders can use propietary formula; not for do-it-yourselfers
• Not suitable for cold climates unless well-insulated

Cob
Subsoil (containing clay and sand) is mixed with water and straw, then mud is applied by hand or other means to build walls. Walls are generally load-bearing, although post-and-beam construction also can be used.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold climates if walls are well-insulated
Pros:
• Ideal for beginners
• Allows for artistic homes with curving, durable and fire-resistant walls
• Inexpensive building material
• Ideal for passive cooling in hot, dry climates
Cons:
• Requries slow, labor-intensive construction
• Buildings tend to be cold unless walls are insulated
• Difficult to insulate

Cordwood
Split or unsplit firewood (bark removed) logs are stacked on a foundation, held in place with concrete mortar; insulation is poured in the center. Walls may be either load-bearing or post-and-beam.

Insulation or Thermal Mass: thermal mass and insulation
Climate Suitability: all climates
Pros:
• Suitable for a wide range of climates
• Structurally sound walls provide insulation and thermal mass
• Ideal for areas with ample wood supplies
Cons:
• Requires use of cement, the production of which contributes significantly to global warming
• Requires labor-intensive, fairly slow construction

Earthbag
Slightly moistened subsoil is poured into polypropylene bags and tamped until hard; walls are load-bearing.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold climates if walls are well-insulated
Pros:
• Easy to learn
• Ideal for do-it-yourselfers
• Inexpensive materials
• Ideal for domes and other curved structures
• Easy to plaster
• Durable and fire-resistant
Cons:
• Involves slow, strenuous work
• Tends to be cold unless well-insulated

Log
Peeled logs are either notched on the end so they interlock or notched lengthwise and stacked. Walls are load-bearing.

Insulation or Thermal Mass: thermal mass and insulation
Climate Suitability: all climates
Pros:
• Suitable for a wide range of climates
• Sustainable if logs are locally harvested from well-managed forests or standing dead timber
• Commercially built log homes widely available
Cons:
• Labor-intensive
• Felling trees is potentially dangerous for do-it-yourselfers
• Many log homes are built with logs unsustainably harvested many miles from the home site

Papercrete
Shredded newspaper mixed with water and concrete is formed into bricks that are stacked using papercrete mortar. Walls may be either load-bearing or infill.

Insulation or Thermal Mass: thermal mass and insulation
Climate Suitability: all climates
Pros:
• Suitable for a wide range of climates
• Ideal for beginners
• Lightweight blocks are easy to build with
• Reuses waste paper
Cons:
• Requires slow, labor-intensive construction
• Not widely recognized so building permits may be difficult to obtain

Rammed Earth
Slightly dampened subsoil or sand-cement mix is packed between forms attached to a foundation; walls are generally load-bearing.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold if walls are well-insulated
Pros:
• Inexpensive and widely available building material
• Thick, structurally sound, fire-resistant and beautiful
• Ideal for passive cooling in hot, dry climates
Cons:
• Best left to professionals as it requires extensive formwork and strenuous hours of packing dirt into forms
• Requires wood to build forms
• Not suitable for cold climate unless insulated

Rammed-Earth Tiles
Subsoil is rammed into used automobile tires stacked on a foundation in an overlapping pattern; walls are load-bearing.

Insulation or Thermal Mass: thermal mass
Climate Suitability: wide range of climates because walls are usually earth-sheltered
Pros:
• Ideal for earth-sheltered, passive-solar designs
• Suitable for do-it-yourselfers
• Recycles used automobile tires
Cons:
• Labor intensive
• Buildings can be humid
• Common designs often lack privacy

Straw Bale
Straw bales are stacked on a foundation to form load-bearing walls or stacked between framing members (typically between posts) as infill or insulation.

Insulation or Thermal Mass: insulation
Climate Suitability: all climates
Pros:
• Suitable for a wide range of climates
• High insulation value
• Thick, soundproof, aesthetically pleasing walls
Cons:
• Walls must be protected from moisture, insects and small animals
• Can be time-consuming to plaster
• Dry straw poses fire hazard during construction if site is not cleaned frequently

Straw Clay
Loose straw mixed with clay is placed in forms and tamped to form solid walls. This is typically used as infill in post-and-beam construction; straw-clay walls can be load-bearing only in very small buildings.

Insulation or Thermal Mass: insulation with some thermal mass
Climate Suitability: all climates
Pros:
• Suitable for a wide range of climates
• Inexpensive building materials
• Easy to learn and build with
• Easy to plaster
Cons:
• Requires slow, labor-intensive construction
• Slow to dry
• Building walls require considerable framing experience

Stone
Stones are stacked on a foundation using mortar to hold them in place; walls are generally load-bearing.

Insulation or Thermal Mass: thermal mass
Climate Suitability: warmer climates; appropriate for cold climates if walls are well-insulated
Pros:
• Inexpensive and widely available building material
• Beautiful, fire-resistant and durable
• Ideal for passive cooling in hot, dry climates
Cons:
• Construction is labor intensive and slow
• Tends to be cold unless well-insulated
• Requires sturdier foundation than many other natural building materials

Load to Bear

Recycled plastic (typically polystyrene) and cement blocks, marketed under the brand names Rastra and Cempo, have become an increasingly popular building alternative in recent years, but they’re not technically a natural building system. The blocks are stacked to form walls and are glued or clamped in place. Concrete is poured into the cavities to form a solid, energy-efficient wall. To improve environmental performance, concrete can be made from fly ash, a waste product from coal-fired power plants.

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