LCA Study

An independently assessed life cycle analysis compared two apartment blocks – one with a concrete structure, one made of cross-laminated timber, but otherwise virtually identical. Below, we pick out five key findings

A concrete-framed building may not be significantly more carbon-intensive than a cross-laminated timber (CLT) one, according to a life cycle analysis (LCA) conducted on two notional apartment blocks.

The concrete structure could also help to make useful carbon savings in a National Grid increasingly supplied by intermittent renewable sources. These are two of the key findings of the LCA, which was commissioned by The Concrete Centre, with the aim of gaining a better understanding of carbon emissions over the lifespan of a relatively conventional concrete framed residential building – and by extension showing how future designs can be optimised.

The analysis focused on two study buildings, one made of concrete, the other CLT. Both were 2,500m², six-storey blocks in London, containing 22 flats. They were of the same size, shape and layout, with the same functional requirements, and the same heating, hot water and ventilation systems, all designed to meet the Future Homes Standard.

The concrete building comprised a reinforced concrete (RC) frame, with exposed 225mm-thick slabs supported on a foundation of RC ground beams, pile caps and piles. The frame was made using a C32/40 concrete with 50% GGBS. The substructure works used an FND3 and FND4 concrete with 70% GGBS. The internal walls were made with concrete blocks finished with a wet plaster to maximise their thermal mass, which was also the purpose of exposing the soffits. The CLT building comprised 160mm-thick, five-layer panels for the floors, spanning unidirectionally onto 100mm-thick, three-layer loadbearing wall panels.

These were finished with mineral wool insulation and plasterboard to provide the necessary fire resistance and noise transfer performance. This building was also supported on a foundation of RC ground beams, pile caps and piles. Analysis of the two buildings over a 60-year study period was carried out in early 2020, using IES ApacheSim for the dynamic thermal modelling and the OneClick tool for the LCA. Wherever possible, embodied carbon rates were determined using environmental product declarations (EPDs) for specific products. Where these weren’t available, generic data sources were used such as the OneClick tool and the ICE database.

The results offer a practical insight into the relationship between embodied and operational carbon, and the interplay between different building materials, systems and design needs. The study also provided useful lessons on undertaking an LCA. Below we pick out some of the key findings.

Above Floorplans were the same for both buildings. They also had the same heating, hot water and ventilation systems

1. The concrete building’s whole-life carbon emissions were only about 6% higher

The whole-life carbon emissions after 60 years were estimated to be around 710kgCO2e/m² and 670kgCO2e/m² for the concrete and CLT buildings respectively. Predicting whole-life emissions does, of course, come with a degree of uncertainty as it is looking many years into the future and depnds on LCA factors such as the future carbon intensity of grid-supplied electricity. But with this caveat, the difference between the average whole-life emissions was quite small, with the concrete building being only around 6% higher.

2. Both buildings meet the RIBA 2025 and 2030 Climate Challenge embodied carbon targets

The results also provide some insight into the relative contributions of operational and embodied emissions. In both the concrete and CLT buildings, embodied carbon was predicted to account for about 75% of the total – made up of approximately one-third structure, one-third services and the final third made up of architectural elements such as finishes and cladding.

When the carbon emissions from the operational energy are excluded, the embodied impact of both buildings was around 500kgCO2e/m², with the concrete building marginally higher. This meets two key industry benchmarks: the RIBA 2025 and 2030 Climate Challenge targets. The study built on this result by developing a “low2” scenario for the concrete building. This improved the carbon performance of the base design through seven material and system enhancements that worked within the fixed design constraints adopted for the study. These included increasing the GGBS in the superstructure from 50% to 70%, switching from PIR to EPS insulation and using a heat pump refrigerant with a lower GWP. Collectively, the changes reduced the embodied carbon to around 430kgCO2e/m² using this data set.

3. The concrete building had significantly better passive cooling

Both buildings adopted a high standard of solar shading and ventilation to reduce the risk of overheating as far as practicable, but the concrete design also made use of the structure’s thermal mass. Overheating analysis using the CIBSE TM59 methodology found that, for the period 2020-40, the concrete building could remain cool by using this thermal mass, coupled with night cooling and some very low-energy ceiling fans. The CLT building, on the other hand, needed active cooling in summer, so includes an air-source heat pump, serving chilled water fan-coil units. By 2041-80, summertime external temperatures are anticipated to rise by around 1°C, and under these conditions, the concrete building also requires a small amount of active cooling.

4. Operational energy consumption was about the same

The concrete building was predicted to use less energy for cooling than the CLT option and slightly more for heating, but overall the two balanced each other out and there was no significant difference in the total energy consumption for any of the time periods or occupancy scenarios. Overall energy consumption was close to 43kWh/m2/yr throughout the 2020-80 period. This is reduced to 34kWh/m2/y when energy produced by the roof mounted PV array is included. It’s worth noting that the study assumed a reasonable active cooling set point of 24°C for the modelling. In practice, occupants may of course opt for a lower setting closer to 20°C, resulting in more energy being used for cooling. The extent of any increase is however likely to be more modest in the concrete building, with its better passive cooling performance.

5. The concrete building’s peak space heating load was on average 25% lower

For the period 2020-40, the peak electrical load for space heating was on average 25% less in the concrete building, as a consequence of its higher thermal mass. When hot water heating was included, the total peak heat electrical demand was estimated to be around 15% lower than for the CLT building.

This matters because, by reducing peak electrical demand, the National Grid is better able to balance out supply and demand. This will become an important attribute of high thermal mass buildings, as they can be actively controlled to store and release heat in response to the peaks and troughs of renewable energy supply. In this way, the building’s energy demand can be shifted away from periods of high grid carbon intensity – that is, when fossil fuels are needed to meet a shortfall in renewable power. The net result is carbon savings at a national level.

To download a full review of the LCA, visit the Publications Library and download Life cycle carbon analysis of a six-storey residential building.