Many builders and property owners are utilizing earth-friendly materials and construction techniques such as adobe, cob, rammed earth, poured earth, earth-bag, strawbale, cordwood, bamboo, and earthship. It is recommended that appraisers become familiar with these types of green building materials and techniques.
Adobe is moistened earth (usually a mixture of clay and sand), sometimes with straw added. It is often formed into rectangular bricks or blocks which are stacked to create walls. Sometimes cement is added to the mixture to provide additional weather resistance. Adobe holds heat (“thermal mass”), but it does not insulate very well.
Somewhat similar to adobe, cob is a mixture of sand and clay, but with more straw as part of the mix. Unlike adobe, cob is usually not formed into bricks or blocks, but is applied as non-uniform clumps, which are called “cobs.” Cob has essentially the same thermal properties of adobe, including its lack of insulating quality.
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3. Poured earth
This is similar to concrete, but the mixture uses native soil instead of the standard sand and gravel aggregate. The material is mixed and poured into forms, just like traditional concrete. This green building material is strong and weather-resistant. Insulation can be inserted into the forms during pouring, which provides for an energy-efficient structure.
4. Rammed earth
This green building material is a mixture of earth, clay, and water (sometimes straw and/or powdered cement is added) which is tamped into forms. Traditionally this has been done by hand, but modern rammed earth construction is characterized by the use of heavy machinery, which is faster and more effective at compressing the material. Rammed earth walls are typically at least one foot thick, and sometimes two feet thick or more.
Strawbale construction is exactly what it sounds like: bales of straw are used as the primary building blocks. There are essentially two types of strawbale construction: load-bearing and non-load-bearing. In load-bearing strawbale construction, the stacked bales provide the structural support and must be covered and kept out of the weather, because moisture is the enemy of a strawbale structure. In non-load-bearing strawbale construction, a frame is constructed (usually a wood timber frame) and then the bales of straw are used as in-fill. In either case, the result is an energy-efficient, sustainable home.
Bags of earth or sand are stacked to create an efficient home. There are dozens, if not hundreds, of variations on this technique. Traditional burlap bags or recycled plastic bags can be used. The fill material can be earth from the site, or sand, or another material such as crushed volcanic rock or rice-hulls. To protect them from the elements, the bags can be covered on the exterior with plaster, adobe, or lightweight concrete.
Unlike traditional wood, bamboo is considered a sustainable building material because of its amazing rate of growth. It grows faster than it is harvested. It is strong for its weight; it can be used for structural as well as finish applications.
Short pieces of wood (which resemble firewood) are cemented together with a cement or adobe mixture. These can be load-bearing or non-load bearing, depending on the builder’s intention and local codes. Proponents of this type of green building construction tout its beauty, as well as the fact that the wood used in the process is considered by many to be a waste material.
This type of home incorporates the passive solar advantages of berming with the use of recycled materials (generally, old automobile tires) to create an efficient home. Many earthship homes also incorporate other environmentally-friendly ideas, such as roof gardens, use of rainwater, or recycling graywater.
Interested in green home appraisal? In addition to studying up on green building materials, learn about the basics of solar in our recorded webinar: Appraising Solar Panels: A Primer.