Increased temperature - Thermal mass vs. lightweight timber homes

Increased temperature - The Concrete Centre


embodied energy-moladi

embodied energy-moladi



When taking account of the environmental sustainability of a construction material it is disingenuous to consider only its embodied carbon dioxide. Embodied CO2 is only part of the story.

Embodied CO2 is considered a useful metric for comparing the global warming potential (GWP) of different construction materials. Based upon the extraction and transportation of raw materials and their manufacture into the final product, embodied CO2 is expressed as CO2 per unit mass (kgCO2/tonne) or CO2 per unit area for a completed building (kgCO2/m3). Interestingly, once issues such as raw material transportation are taken into account, the misconceptions about the embodied CO2 of different construction materials are made apparent ( see table: embodied carbon dioxide and construction materials).

However, when considering the sustainability of a construction material you need to look beyond the material's initial embodied CO2 and GWP. Only then can this facet of its true environmental impact be measured. Over the life of a building, the operational CO2 emissions have far more environmental impact than the embodied CO2 of the material used to build it.

Currently over 50% of the UK's CO2 emissions come from the energy we use to heat, cool and light our buildings. It is essential that the whole life performance, and therefore the energy consumption of a building, is taken into account when evaluating the sustainability of construction materials. The growing evidence of climate change due to CO2 emissions underlines the need to address the energy consumption of buildings.

The current predictions from the UK Climate Impacts Programme (UKCIP) show that by the 2080s, annual temperatures in the UK will increase by 2 to 3.50C. In London, due to the Urban Heat Island (UHI) effect, the increase could be as high as 80C, taking the peak summertime temperature to over 300C. This will have a considerable impact on the temperature of the internal environment of buildings, especially buildings of lightweight construction which are likely to be overheating by 2020. This overheating may increase the need for energy-intensive air conditioning to make them bearable. Uptake of air conditioning in the UK is currently increasing at 8% per year. By 2020, such an annual increase could result in six million extra tonnes of CO2 emission every year in the UK alone.

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 680C 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 thereby sustaining warmer overnight temperatures and reducing the use of heating.

Independent research, carried out by Arup Research and Development, highlighted the energy savings that can be achieved by utilising thermal mass. Comparing lightweight timber homes with medium weight and heavyweight masonry and concrete homes, the research found that the latter has the lowest total energy consumption and therefore CO2 emissions over the lifetime of the house, due to the reduced need for air conditioning and reduced winter heating requirements.

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 utilisation of thermal mass can avoid overheating through passive cooling. The moderate to high cooling loads associated with office environments enables significant energy savings to be realised if thermal mass and night ventilation are utilised to avoid or minimise the need for air conditioning. This will in turn result in a reduction in the operational CO2 emissions thereby enabling the embodied CO2 of the building's structure to be offset.

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 reinforcing steel within concrete is 100 per cent recycled and when a building is demolished, the concrete can be recycled as aggregate and the reinforcement recycled again back into reinforcement bar.

Sustainability is a complex area that encompasses intrinsically interwoven environmental, economic, and social facets. 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, not just one embodied impact from its manufacture. After all, a building's global warming potential does not stop once it has been built.

Courtecy of Sustainable Concrete - http://www.sustainableconcrete.org.uk/main.asp?page=68

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