Straw

The use of straw bales in construction can reduce the embodied carbon of a building, as well as reducing primary energy needs, and therefore operational carbon emissions. The low embodied carbon stems from the fact that the straw bales are a co-product of the growing of food crops. The crop from which the straw is derived will have absorbed carbon dioxide through photosynthesis. This makes straw bales not just carbon neutral, but carbon negative. The low U-value achieved by a straw bale wall (typically 0.17 W/m2K from a thickness of 450-500 mm) contributes to a low primary heat energy needed by providing a high level of insulation.
In a simple structure, straw bales can be used to support the roof on their own (known as load bearing). For more sophisticated multi-story buildings, straw works best as an insulating wrap around the outside of a structural frame.