CIBSE Weather File vs. 2020

Our data shows that the summer of 2020 was hotter than the climate data used in overheating modelling. Good quality homes should be designed to last a lifetime, yet overheating mitigation strategies are currently developed using 2020s weather files. It could be argued that these weather files are now out of date, as we are coming towards the end of 2020.

Taking external temperature measurements gathered as part of our post occupancy monitoring work, we have compared Summer 2020 to the CIBSE weather files for a Central London location.

In the following charts, the temperature profile of Summer 2020 is shown in blue, with the three CIBSE ‘Design Summer Years’ (DSY) in yellow, orange and grey.

The circled parts of Figure 1 show:

• Actual peak temperatures were higher than modelling assumes. It is this area that matters when considering overheating.
• Below 25^oC, actual 2020 temperatures were lower than modelling assumes. This makes the average 2020 temperature lower, despite the above.

By comparison, the CIBSE Design Summer Years for 2050s (Figure 2) show that temperatures are expected to rise further, with the duration above 25^oC significantly higher. Therefore, any overheating experienced by occupants now would be expected to worsen.

Designing to 2050s weather file projections will be challenging and goes beyond current regulatory compliance. However, it is possible to mitigate overheating for these future climate change scenarios if passive design measures are considered from the outset. Not doing so runs the risk of homes being built now overheating in the future.