Energy is needed not only to run a building - it also takes energy to create the building products and build it. Put at its simplest, embodied energy is the energy needed to transform a product from raw materials in the ground the final article. The embodied energy of the building is therefore the total energy required to construct it - that is to win the raw materials, process and manufacture them as necessary, transport them to site and put them together. It is the energy that has "gone in with the bricks" and which cannot be recovered during the lifetime of the building, no matter how efficiently it operates.
However, product manufacturers may give embodied energy figures for their materials which take into account only some of the stages above. Clearly defined energy analysis boundaries are therefore critical in drawing useful conclusions on the embodied energy of a particular product.
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Generally speaking, the more manufacturing processes a product goes through, the higher its embodied energy will be. For example, timber board materials have a much higher embodied energy than the equivalent size of rough sawn timber.
The energy embodied in new construction and renovation each year accounts for about 10% of UK energy consumption. Of this, approximately half is used in the winning and manufacturing of the materials and half is used in transport (i.e. getting them to the processing plant and/or to site).
G.1. Different theories about embodied energy
1. The use of the term embodied energy is open to different interpretations and published figures are not always explicit in their definitions of what has been included in the total. In simple terms the total energy consumption for which a building material or component is responsible, must be all the aspects identified. It is the total energy consumed in winning the raw materials, manufacturing the components and constructing the building on site. It included the energy consumed for transportation within and each of the stages leading to the complete building as well as the human energy, transportation of workers to the factory or construction site and attributable portions of the energy used to manufacture and maintain the machinery and the factories that house them. This sis termed the Gross Energy Requirement (GER) of a material or component. A full audit of the GER of any item can be very complex and there are diminishing returns with respect to the accuracy and usefulness of the calculation the further removed the analysis becomes from the item under consideration. It is common therefore to evaluate the Process Energy Requirement (PER).
Calculated in this way embodied energy figures are clearly site specific. They are related to the specific materials, suppliers and efficiency of distribution and delivery route. There is however generally no need for such a precise figure to be determined in order to provide useful data for building designers. It is common practice to consider 'Factory Gate' figures for embodied energy. These represent the energy embodied in the winning and manufacturing processes and exclude the final delivery to construction site. However there can be significant differences for similar materials produced through different supply chains.
Published embodied energy figures for common building materials vary enormously and there is often little indication of what has been included in the analysis or how they have been obtained. It is common to evaluate the PER but the transportation component is also often omitted or considered using gross simplifications.
2. The measurement of embodied energy from basic data is difficult, involving assessments of the energy expanded in, for example, quarrying and crushing operations for aggregates, or oil inputs and moulding of plastics. Various methodologies are available and there is no single clear, correct and easy way to do it. Fortunately for RSLs there is no need to bother with this level of detail. In this paper we offer general guidance gathered from recent literature and provide references for those who need more information. However, it is important when making decisions in relation to embodied energy to be sure that you are comparing like with like, and there are two particular points to note:
- Whether you are using kilowatt hours per tone (kWh/t), kilojoules per kilogram (kJ/kg), gigajoules per square metre (GJ/m2), or CO2 equivalents, try to be sure that your information is based on "primary", rather than "delivered" energy. Primary energy represents the total energy used, while delivered energy is the energy received at the point of use and it can be substantially lower. Delivered energy data will therefore give misleadingly low figures and mixing the two will give spurious results.
- Another reason measurements of embodied energy may vary is that transport may have been excluded from the calculation. For materials with high bulk (e.g. timber, brick, sand etc.) transport energy will be a substantial element in the total.
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If you see two different figures for the same material, it would probably be wise, for both of the reason above, to adopt the higher one.
3. Embodied energy figures are produced in the same ways as other environmental impact categories as part of an LCA and so the same caution should be applied to comparisons of figures from different sources.
- Though the 'embodied energy' of insulation materials is arguably important, the energy used to create and transport materials should be considered in the more important wider context: most of the energy a building consumes during its lifetime is 'operational/functional energy' include such as that is required to heat space, heat water or power appliances. An insulation material should be first considered for its thermal performance and, unless other contextual factors apply, only subsequently for its environmental impact.
- Embodied energy becomes important only when high levels of operational energy efficiency have been achieved (e.g. through Passivhaus and code levels 4, 5 & 6). In these instances embodied energy can increased to around 10% of overall energy expenditure.
- Embodied energy values are usually expressed in energy used per kilogram-but for any useful comparison to be made between materials, thermal performance and material density need to be included. Thus for example, based on rough assumptions, for 1 m2 of surface to attain a U-value of 0.2 W/m2K, the energy required would be (thickness of material x material density x embodied energy value):
Cellulose - 46 MJ
Sheep's wool - 100 MJ
Polyurethane - 424 MJ
Material Energy Carbon Density MJ/kg kg CO2/kg kg/m3 Cellular glass insulation 27 - - Cellulose insulation 0.94-3.3 43 Cork insulation 26.00* 160 Glass fibre insulation 28 1.35 12 Flax insulation 39.5 1.7 30* Rockwool (slab) 16.8 1.05 24 Polystyrene insulation 88.6 2.5 15-30* Polyurethane insulation 72.1 3 30
Material Embodied energy Expanded polystyrene 120 GJ/t Rockwool 25 GJ/t Cellular glass 27 GJ/t Cellulose 0.63-1.25 GJ/t Phenolic foam 27.78 KW/kg Sheep's wool 30 KWh/m3
Material Embodied Energy Embodied Carbon MJ/kg Kg CO2/kg Cellular glass 27 Cellulose 0.94-3.3 Cork 4 0.19 Glass fibre 28 1.35 Wool (recycled) 20.9 Rockwool (slab) 16.8 1.05 Expanded polystyrene 88.6 2.5 Polyurethane 72.1 3