Operational and embodied CO2: commercial

The use of concrete in buildings can lead to questions regarding its embodied CO2, which is often thought to be much higher than other construction materials. In reality, the difference is typically quite small, and becomes insignificant when compared to a building's operational CO2 emissions over its life.

In buildings where the inherent thermal mass of concrete forms part of the cooling strategy, any additional embodied CO2 burden can in fact be offset many times over. This can be shown quite simply by comparing the embodied and operational CO2 emissions per m2 for a typical air-conditioned office with those of a typical mixed-mode office i.e. one that is cooled using thermal mass in conjunction with natural and mechanical ventilation.

A source of data for the embodied CO2 of office buildings is life cycle assessment research on steel and concrete framed buildings[1]. One of the conclusions of the study was that overall there is no significant difference between steel composite, reinforced concrete and precast concrete options with regard to embodied CO2. For a small to medium rise office, built to a developer's standard specification, the range of embodied CO2 for different options was equivalent to only 16 months CO2 emissions from an energy-efficient, air-conditioned office [1,2].

A source of data for operational CO2 emissions is the BCO Guide which reports that for a mixed-mode office that utilises thermal mass, the annual CO2 emissions are around 40-80 kg/m2 per year [2]. This is applicable to a solid in-situ concrete slab or a precast hollowcore option. The annual CO2 emissions from an energy-efficient air conditioned office are around 60-90 kg/m2 [2]. The range of values arise from variations in building location and activity within, as well as mechanical design and building management. Design and management are vital if operational CO2 emissions are to be minimised, but it is clear that the conceptual design choice of utilising thermal mass gives the best chance of minimising operational CO2 emissions.

If, conservatively, the 'mixed-mode office' building [2] is considered to have the greatest embodied energy from Eaton & Amato [1] and the air-conditioned building [2] is considered to have the least embodied energy from Eaton & Amato, then the comparative graph in Figure 1 can be drawn. From Figure 1 it can be seen that the additional CO2 in the thermally massive building is offset in only six years. More moderate assumptions would see the crossover even earlier.

To most effectively design for thermal mass, a slab thickness of over 200mm can be fully utilised by exposing it on both the soffit and the top surfaces; the latter by use of hard finishes or underfloor ventilation. If it can only be exposed on one surface a thickness of 150mm to 200mm concrete can act effectively in storing and releasing energy during particularly hot weather. For more details on thickness of concrete required to maximise thermal mass benefits download Thermal Mass from the Publications Library.

Radiance from thermally-massive surfaces is an important part of maximising thermal mass benefits. Whilst passing air over 'hidden' surfaces has a beneficial effect, unless occupants can see the surface of the thermal mass they do not gain the additional benefit of the radiance. It is to be noted that the thermal comfort of an occupant is dependent on air temperature and radiant temperature (the temperature of surfaces that can be seen). Therefore, exposed concrete slab soffits and concrete walls should be considered at the earliest stages of design.

The integration of services into a design where structure and slabs are exposed offers challenges to the design team. Utilisation of Thermal Mass in Non-residential Buildings from The Concrete Centre is a useful tool to designers facing this challenge.