Discussion

While it is important to consider the embodied impacts of new building designs this must not be the sole factor for selection. A full analysis of operational energy should be completed. Energy in operation presently has the largest impact over the lifetime of a property. With the current benchmarks it was estimated that the energy in operation would overtake the initial embodied energy within 7 years and the carbon would take almost 12 years (see Hammond and Jones, 2007 and 2008a). It was also discovered that the inclusion of a single garage, a driveway, a conservatory, and the contribution of a housing development would increase this to 12 years for energy and 19 years for carbon. This is a significant duration, especially considering the design life of such a house which may be 60 years, and over this time it will require further embodied energy and carbon inputs during refurbishment and routine maintenance. It is expected that this duration will increase over time as UK building regulations become more energy efficient and lower carbon. It is anticipated that by the year 2016 all newly built English dwellings will be zero carbon in operation. If this occurs embodied carbon will become the predominant life cycle impact.

These results demonstrate that the time required for the energy in operation to overtake the embodied energy is quicker than for embodied carbon, the climate change marker. This was mainly attributed to the release of non-fuel-related carbon into the atmosphere (as a result of manufacturing processes); cement is the key material with this additional release. The carbon in operation has originated from predominantly natural gas and electricity, which do not experience the same additional carbon releases. The net effect of this is to increase the relative size of the embodied carbon in comparison with operational carbon. This extends the duration for the operational carbon to overtake the embodied carbon. While it may be expected that organic materials, such as timber may negate this effect on a whole building, the chosen methodology did not allow for such carbon sequestration. The authors choose to neglect the effects of timber carbon sequestration until such a time that timber is utilized in a globally sustainable manner (consumption equals replenishment) and that the science of carbon pools and the carbon cycle is better understood.

Normalized results per unit floor area may appear particularly attractive as benchmarks: they are simple to apply and easy to understand. However, it must be noted that the relationship of embodied carbon to floor area is not linear, and therefore buildings of different sizes (but same type) normalize to give very different results. For example, the 'typical' semi-detached building in Table 23.3 was estimated to require 425 kgCO2/m2 based on its 73 m2 floor area. However, using the same benchmarks, a semi-detached property of 125 m2 is estimated to have an embodied carbon of 47.5 tonnes CO2 in total (Fig. 23.6). This normalizes to 380 kgCO2/m2, which is a significantly different (lower) value. Normalized results may therefore be unsuitable for domestic benchmarks. It would be preferential, and is recommended, that a model or formulae be applied (such as the results from Figs. 23.5 and 23.6, or the use of an equation [as in Hammond and Jones, 2007]). Normalized results of embodied energy and carbon may be particularly unsuitable for environmental policy making and legislation. The above analysis suggests that larger properties experience lower embodied carbon per unit floor area. If legislation required all semi-detached buildings to have an embodied carbon (per unit floor area) below a set threshold the easiest way to achieve this would be to increase its total floor area. This is naturally counterproductive and would increase the total embodied carbon of the property as a result of the larger floor area. In light of this, it is recommended that any environmental policy or legislation should set absolute benchmarks of embodied energy and carbon (such as Table 23.2,

Fig. 23.5, or Fig. 23.6) rather than normalized benchmarks (such as Table 23.3). Despite this recommendation valuable lessons can still be learnt from normalized embodied carbon results.

The study of a product or building over its life cycle is often geographically diverse; that is, the material inputs to a product may be drawn from any continent or geo-political region of the world. This unfortunately makes the improvement process complex, in that many businesses and economies are involved in the manufacture of a product. Therefore to achieve low or zero environmental burdens will require concerted efforts over a wide domain. Each business, nation, and individual must contribute in an altruistic manner, through acts of selfless wellbeing, rather than the current trend of egoism. Invariably such action may be too idealistic and is therefore unlikely to be achieved. But it may be encouraged with accepted international regulation and frameworks, providing examples of good practice and sustainable development.

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