Quick Answer
Rammed earth and cob are both natural, sustainable building materials used for off-grid structures, but they have distinct differences in terms of construction, durability, and thermal mass. Rammed earth is generally more durable and has better thermal mass, while cob is more affordable and easier to build with.
Rammed Earth Construction
Rammed earth walls are constructed by compacting a mixture of soil, sand, and gravel into formwork. A typical mix for rammed earth in arid climates consists of 10% sand, 20% gravel, and 70% soil, with a 10% water content. Formwork is typically 2-3 inches thick and can be made from wood or steel. For a 10x10 foot wall, formwork will require 40-60 linear feet of material.
Thermal Mass and Desert Climates
Rammed earth walls offer excellent thermal mass, helping to regulate interior temperatures in desert and arid climates. A 10-inch thick rammed earth wall can store up to 4.5 kilowatt-hours of heat energy per square meter, reducing the need for air conditioning in summer and heating in winter. In contrast, cob walls have lower thermal mass and may require additional insulation.
Cob Construction Considerations
Cob is a more affordable and accessible building material, but it requires a higher water content (typically 20-25%) and can be more prone to erosion. Cob walls are often built with a mixture of soil, sand, and straw, and can be up to 2 feet thick. A 10x10 foot cob wall will require approximately 100-150 pounds of straw per square foot, depending on the mixture.
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