HEATING UP UK NEEDS HEAVYWEIGHT BUILDING RETHINK

This week’s June heatwave that follows record May heatwave temperatures of 30C suggests that climate change could see heatwaves in the UK starting earlier in the year and gaining in regularity and intensity. Time to build homes, schools and offices that work with summer temperatures rather than against it.

The heatwaves are part of a succession of such events. The Met Office reports that four of the last five years – 2022, 2023, 2024 and 2025 – have been the warmest on record since 1884. An example of the extreme heat was in 2022 when summer temperatures reached 40C. As a result, homes, schools and offices built using lightweight timber and steel construction are increasingly at risk of overheating and require energy intensive air conditioning to make them liveable and workable spaces. Such a response results in a carbon emission loop: increased energy use equals increased emissions equals increase climate change impacts equals more energy use.

There is a way the break this loop: realise the potential thermal efficiency of heavyweight concrete construction.

Unlike timber and steel construction, heavyweight reinforced concrete construction with its innate thermal mass can do much to mitigate the impact of heatwaves. Thermal mass can absorb heat during the day and then release it at night. Often referred to as Fabric Energy Storage (FES), thermal mass is especially effective when used as part of an integrated passive design solution that includes building orientation, shading and natural ventilation.

Admittedly, the use of concrete construction can raise questions concerning the level of construction embodied CO2. However, having to mechanically ventilate and cool a lightweight constructed building means that the resulting operational CO2 emissions, over the lifetime of that building, will far outweigh any initial construction embodied CO2 savings. FES concrete buildings, due to reduced air conditioning requirements offer a significantly reduced whole life long-term carbon solution.

This was proven by a life cycle carbon analysis of a typical six storey residential building in London carried out by MPA The Concrete Centre*. The study compared a concrete structure with an equivalent cross-laminated timber (CLT) structure.

To develop appropriate cooling strategies, both buildings underwent a full overheating analysis using dynamic thermal modelling and the CIBSE TM59 methodology, along with weather data that took account of the likely future climate. It was found that, for the period 2020-40, the concrete apartments could remain cool predominantly using its exposed thermal mass and night cooling with the addition of some very low-energy ceiling fans. The fans enabled the percentage of time spent above 26°C in the bedrooms to be shifted from around 3% to below 1% of the occupied period, which represents the CIBSE TM59 overheating threshold. In contrast, the lightweight CLT version of the apartment building required active air conditioning from the outset.

The concrete apartments were found to have an average peak space heating demand around 25% lower than the CLT option, which is a result of the concrete’s thermal mass helping stabilise the internal temperature and smooth out the load profile.

Reducing the need for high energy air conditioning also relieves pressure on the national grid. This is set to become an important benefit of high thermal mass buildings, which can be actively controlled to store and release heat so their demand profile responds sympathetically to the peaks and troughs of the renewable energy feeding the grid. In this way, the building’s energy demand can be shifted away from periods of high grid carbon intensity.
Similar thermal efficiency arguments can be used for school and office buildings.

Steve Elliott, Chair of the British Association of Reinforcement, said: “Unlike timber and steel lightweight construction, reinforced concrete heavyweight construction can do much to mitigate the impact of heatwaves without recourse to energy guzzling air conditioning. With summers predicted to be getting warmer with more extreme heatwaves, the UK needs to rethink the way that it builds.”

*Life cycle carbon analysis of a six-storey residential building, MPA The Concrete Centre, 2022.