How long is the CO2 payback for thermal mass?

This very much depends on the building type and the way in which its thermal mass is used. For commercial buildings such as offices, it should first be noted that the embodied CO2 in a typical office design is more or less the same regardless of whether it has a steel or concrete frame; a conclusion reached by separate studies undertaken by both the steel and concrete sectors. So, for many buildings it’s a moot question. In the case of a very heavyweight office building, the additional concrete can result in a slightly higher level of embodied CO2, but the operational savings (reduced cooling load) afforded by the thermal mass will typically offset this in a matter of months rather than years.

For housing the situation is slightly different: The embodied CO2 in a typical masonry home is up to 4% higher than an equivalent timber frame home. Independent studies by Arup and the NHBC Foundation both arrived at this figure. The Arup study went on to look at operational impacts and found that the passive benefits of thermal mass during the heating seasons can result in CO2 savings that offset the figure of 4% in around 11 years.

In respect of summertime performance, the potential CO2 payback from thermal mass is largely through avoiding or reducing the need for domestic air conditioning. Whilst this is not widely used at present in the UK, this may change as the effects of our warming climate are felt in the future. The Arup study considered the impact of climate change and found that lightweight homes are at greater risk of frequent overheating in the coming years, even when enhanced shading and ventilation are used to mitigate higher temperatures. At this point, the use of air conditioning may become necessary, resulting in increased annual CO2 emissions compared to medium and heavyweight homes, which were shown to delay the occurrence of frequent overheating by a further 20 to 60 years, depending on the level of thermal mass present.