Concrete has not traditionally been seen as a low-carbon building material, but research from The Concrete Centre suggests using concrete blocks can create a highly sustainable building.
With the environmental performance of construction materials becoming an increasingly significant design consideration, the use of concrete in buildings can lead to questions regarding embodied CO2.
Concrete is often incorrectly perceived as having much higher embodied carbon than many alternative construction materials. In the case of timber-frame housing, the difference compared with masonry construction is actually quite small at around 4-6 per cent for the average house.
This is because, with the exception of the blockwork and timber frame, all the other construction materials used are common to both house types and account for most of the embodied impacts – for example, UPVC windows, brick façade, roof structure, carpets, etc.
Over the life of a masonry home this difference becomes even less significant, since the CO2 emissions resulting from heating, lighting and hot water will be much greater.
Improved thermal mass with blockwork
This puts the embodied CO2 issue into context, but there is another important factor to take into account: the effect of the thermal mass provided by the blockwork.
Over the life of the building, this has the potential to tip the overall CO2 balance in favour of masonry construction.
“Concrete is often incorrectly perceived as being much higher embodied carbon than many alternative construction materials”
Blocks and other heavyweight materials can enhance fabric energy efficiency in well-insulated homes by capturing and later releasing free heat from the sun and internal sources such as cookers, electrical appliances and people.
This in turn can reduce the load on the boiler and, over time, offset the slightly higher embodied CO2 in a masonry home.
Put another way, these savings can be viewed as payback for the investment in the embodied CO2 of a block inner leaf. This thermal mass benefit is, to some extent, now recognised in SAP and the new Fabric Energy Efficiency Standard, which may be introduced this year.
Concrete vs timber
This sounds good in theory, but to establish the credibility of the argument The Concrete Centre commissioned Arup to compare the embodied and operational CO2 emissions of a typical masonry home with an equivalent timber-frame home.
The research showed that the additional embodied CO2 in an average semi-detached masonry house can be offset in as little as 11 years if the design seeks to include the following:
- A high level of insulation and airtightness;
- Some south-facing windows to maximise solar gain in winter;
- Continuous, low-level background heating (beneficial but not essential).
The research also showed that extra thermal mass by, for example, including a concrete upper floor will increase the offset period due to the higher embodied CO2 but further reduces annual emissions, ultimately leading to the lowest whole-life CO2 footprint of all the options considered in the study.
To put these findings into context, the overall CO2 saved (embodied and in-use) over the 21st century when compared with the timber-frame house was around 7 to 10 per cent for a typical masonry home with a timber upper floor, and 10 to 15 per cent when a concrete upper floor is specified.
“The potential whole-life CO2 savings provided by blocks can outweigh any additional embodied impacts”
While this study dates back to 2006, it was revisited by The Concrete Centre in 2011 using the latest embodied CO2 data for construction materials. This produced very similar results to original study, confirming that the research remains valid.
There has also been a similar study undertaken recently by the NHBC Foundation, which resulted in a useful report entitled Operational and embodied carbon in new build housing – published in October 2011.
This study also found that the difference in embodied CO2 between timber-frame and masonry housing is very marginal, with a maximum difference of only 4 per cent.
The impact of climate change
In terms of in-use performance, the NHBC Foundation study did not identify any clear trend. This may be a consequence of using SAP to model whole-life dwelling performance, and also because the likely impact of climate change on heating and cooling was not included in the study.
The report highlights this as an area for further research, particularly with regard to the influence of thermal mass.
The Arup study from 2006 did take account of climate change, which unsurprisingly was found to result in steadily increasing cooling loads over the 21st century, leading to potential overheating problems for lightweight homes around 20-40 years sooner than in equivalent medium-weight masonry homes.
For heavyweight, high thermal mass homes, the difference was found to be closer to 60 years.
The potential whole-life CO2 savings provided by blocks can outweigh any additional embodied impacts when compared to timber frame construction. The extent of these savings is largely determined by dwelling design and level of fabric performance.
But with the emphasis now placed on a fabric-first approach to design, the significance of thermal mass in concrete blocks is set to increase in the coming years.
Tom De Saulles is a senior manager, building sustainability, at MPA The Concrete Centre