Embodied energy (EE) is the amount of energy that's gone into making and delivering a product. The energy used to make building materials comes mostly from fossil fuels so there is a correlation between the EE of a product and the greenhouse gasses emitted due to its production. The data used to make the graphs below come from Victoria University in Wellington in NZ and should be seen as estimates due to variations in the way EE is measured and variations in the way products are made. The EE value for aggregate or crushed stone is used as a proxy for the EE of rock rubble used in building dry-stone walls.

The lower graph shows materials which could be used to build retaining walls, sea walls or river levees. Comparing MJ/kg is valid for these types of massive structures where the weight of solid material is needed for the structure to perform its design function. Concrete, brick and dimension stone are seen as low embodied energy building materials but it's clear that crushed stone is significantly lower on this scale. When comparing materials used to construct the walls of a building then MJ per square metre would be a better comparison, in which case crushed stone would not compare so favourably due to the thicker wall sections needed when building with this type of material.

The EE values from Victoria University are in reasonable agreement with another very good resource, the Inventory of Carbon and Energy (ICE) from the University of Bath in the UK. The ICE document has a good introduction to the reasons why there is some error associated with the measurement of EE and the associated greenhouse gas emissions.

Note: Although it's not directly applicable to comparing the EE of building materials, it is interesting to put some of the EE values in perspective with other types of energy. For example, the amount of energy in 1kg of black coal is about what's needed to produce 1kg of steel; the amount of energy used daily by the average Australian household could produce 1kg of plastic; the amount of energy in the 4 litres of petrol that goes into the average Australian car every day could produce 1kg of aluminium.