Embodied energy is only one part of a building’s impact on the environment. Specifiers should look at the bigger picture, reports Andrew Frost, sustainability manager of The Concrete Centre
Embodied carbon dioxide (CO2)is considered a useful measure for comparing the global warming potential (GWP) of different construction materials. The figure is based on the energy used in the extraction and transportation of raw materials and their manufacture into the final product. Embodied CO2 is often expressed as CO2 per unit mass (kgCO2/tonne) or CO2 per unit volume (kgCO2/m3). However, there are various misconceptions about the embodied CO2 of different construction materials that become apparent once issues such as raw material transportation are taken into account (see table).
Also, a material’s initial embodied CO2 and GWP is not the whole story of its environmental sustainability. It’s necessary to look beyond that in order to measure a building’s true environmental impact. Over 50% of the UK’s carbon emissions result from the energy used to heat, cool and light buildings. Over the life of a building, the operational CO2 emissions are far higher than the embodied CO2 of the material used to build it. The whole-life performance and energy consumption of a building are, therefore, vitally important factors to consider when evaluating the sustainability of construction materials.
The current predictions from the UK Climate Impacts Programme show that by the 2080s, annual temperatures from the UK may increase by 2 to 3.5°C. In London, as a result of the urban heat island effect, the increase could be as high as 8°C, taking the peak summertime temperature above 30°C. This will have a considerable impact on the internal temperature of buildings, especially those of lightweight construction, which are likely to overheat by 2020. This will in turn increase the demand for energy-intensive air conditioning to make such buildings comfortable. Uptake of air conditioning in the UK is already rising 8% a year, and by 2020 this could result in 6 million extra tonnes of CO2 emissions every year.
A proven inherent benefit of concrete is its high thermal mass. In summer, exposed concrete absorbs heat and that, together with the provision of solar shading, can keep internal temperatures 6 to 8°C below the peak external temperatures. Night-time ventilation is then used to cool the building, priming it for the next day.
In winter, concrete’s thermal mass stores the energy from the heat system, passive solar energy and heat gains from the occupants and internal sources such as electrical equipment. This stored energy is then released at night, sustaining warmer overnight temperatures and reducing the need for heating.
Research commissioned by The Concrete Centre highlights the energy savings that can be achieved by using thermal mass in a changing climate. Comparing lightweight timber homes with mediumweight and heavyweight masonry and concrete homes, the research found that through optimising their thermal mass, the latter can have the lowest total energy consumption and therefore the lowest whole-life CO2, owing to the reduced need for air conditioning and reduced winter heating requirements (see graph).
The results for housing are of relevance to other buildings such as offices where a major design challenge is keeping cool. Here, adequate ventilation, solar shading and the use of thermal mass helps minimise overheating through passive design. The moderate to high cooling loads associated with office environments mean that significant energy savings can be made if thermal mass and night-time ventilation are used to avoid or minimise the need for air conditioning. This will in turn result in a reduction in the operational CO2 emissions offsetting the embodied CO2 of the building’s structure in a few years.
Reducing operational CO2 emissions is just one of a range of inherent sustainable benefits of concrete. Its fire resistance, good sound insulation and minimal vibration performance minimise the need for additional coverings and products. It needs no chemical preservatives to make it durable and its robustness means that it has a life of well over 60 years. Furthermore, concrete is easily recycled. The UK reinforcing steel within concrete is 100% recycled and when a building is demolished, the concrete can be recycled as aggregate and the reinforcement recycled again back into reinforcement bar.
Furthermore, unlike other construction materials, the UK can be self-sufficient in concrete. This is environmentally, socially and economically sustainable. It minimises the need for long-haul transport with its associated environmental impacts, supports the local economy and prevents the import of material from countries with less stringent social and environmental protection legislation. The latter is often a major problem with cut stone, timber and steel imports.
Sustainability is a complex area with interwoven environmental, economic, and social strands. It is, therefore, insufficient to only take account of one aspect of a construction material’s environmental impact. For a true picture, the whole-life performance of a material must be taken into account. After all, a building’s global warming potential does not stop once it has been built.
For futher information on the LCA boundary conditions on the data in the table, contact The Concrete Centre