BAU-I-3: Cooling degree days | Umweltbundesamt

BAU-I-3: Cooling degree days

In the three summer climate regions according to DIN 4108-2:2013-02 , i.e. the authoritative standard for summer heat protection of buildings, the number of cooling degree days is increasing thus reflecting a significantly rising trend. Since 1999 the cooling degree days in the three regions have been consistently above the mean of the climate normal period 1961–1990. The requirements made for summer heat protection are increasing throughout Germany.

Increasing demands on summer heat protection

2018 and latterly 2022 have so far been the hottest years in Germany since meteorological records began. So far 2018 has been the only year in which more than 20 hot days were recorded when the mean temperature recorded nationwide amounted to 30 °C and more. The manufacturers of air-conditioning equipment and fans were blessed with a bumper summer in terms of revenue as in many offices and homes temperatures sored clearly beyond the comfort zone.

Heat protection by means of building design is to ensure amongst other things that this type of situation remains the exception to the rule and that the interior climate of buildings remains bearable even when the external summer temperatures are high. The Minimum Requirements for Protection from Heat (Mindestanforderungen an den Wärmeschutz) including heat protection in summer, are described in the relevant DIN (standard) 4108-2:2013-02 which bears the same name. The requirements laid out in this standard are also referred to by the Gebäudeenergiegesetz (GEG/Buildings Energy Act) which came into force in 2020 as a minimum requirement for heat protection of buildings in summer. They are to be observed both for new builds and for significant extensions.

In order to differentiate these minimum requirements spatially, this DIN standard divides Germany into three summer climate regions, i.e. A, B and C. The summer climate region A comprises the coastal areas of North Sea and Baltic Sea as well as the upland areas of the Alps, i.e. areas which tend to be relatively cool. Summer climate region C comprises areas which tend to be warmer. This includes Lake Constance and the Upper Rhine Graben (rift valley), the Rhine-Neckar and the Rhine-Main areas, the Moselle and the Middle Rhine Valley, the Ruhr Valley and the urban regions of Leipzig / Halle and Dresden. The other regions are comprised in Region B.

These three summer climate regions provide the spatial background to the time series illustrated for cooling degree days which were calculated based loosely on one of the procedures adopted by the European Environment Agency (EEA)¹³⁶. The evaluation of cooling degree days was used as a basis for estimating the temporal development of cooling requirements or the extent of heat protection required in these regions in summer. The cooling degree days constitute a derived value represented in Kelvin * day. This value – based on the amount of exceedance of a temperature threshold value which in this case amounts to 22 °C – is calculated by totalling the amount of exceedance per day for all days of the year in a weighted form. In this context, the weighting depends on whether the daily maximum, the daily mean or even the daily minimum exceeds the threshold value. In the case mentioned first, the weighting is lowest, in the case of the daily minimum it is highest. The data for the time series of summer climate region A were provided by the DWD stations Bremerhaven and Stötten in the Swabian Alps. For the summer climate region B the values from the DWD stations Potsdam, Essen and Hamburg-Fuhlsbüttel were used while Region C is represented by the Mannheim station.

Since 1951 all three time series have shown a significantly rising trend. A comparison shows that the cooling degree days in summer climate region C (i.e. the Mannheim station) increase faster than in the two other regions. Notwithstanding the above, a comparison with the climate normal period 1961–1990 shows that since 1999 the cooling degree days in all three regions were consistently above the mean of the period 1961–1990. This means that the requirements in terms of summer heat protection are increasing throughout Germany. Judging by current climate projections, this development will continue for the rest of the 21st century.

In the light of this perspective and bearing in mind international and national commitments to successively and significantly reduce the emission of greenhouse gases from buildings, it is imperative for experts involved in building design, to keep the demands on heat protection always in the forefront of their mind. However, DIN standard 4108-2:2013-02 defines only the requirements to be fulfilled by new builds, building extensions and add-on modules such as conservatories, referring only to the climate prevailing between 1988 and 2007. It is worth noting that, among other endeavours, and in view of the extraordinarily hot years of 2018, 2019 and finally 2022, and also in view of the GEG regulations to summer heat protection which refer directly to that DIN, there are currently moves afoot to drive forward an advanced development of DIN 4108-2:2013-02, in order to give more adequate consideration to the projected climate warming and associated impacts, with the objective to adapt the minimum requirements appropriately to summer heat protection. In the meantime, with a view to ever-increasing temperatures, it behoves developers to take adequate precautions and make appropriate provisions that go beyond the minimum requirements laid down in this standard. Starting points might be, for instance, a reduction in the amount of window space, adequate window tilt and orientation as well as the use of light surface colours on roofs and façades, the type of nocturnal aeration and the use of verdure on buildings as well as external shading, anti-sun glass and passive cooling systems (cf. Indicators BAU-R-2 and BAU-R-3). If supported by preventative urban planning and neighbourhood-based planning practices – including the provision of appropriate aeration and adequate provision of a green-blue infrastructure in urban spaces – the interior climate of buildings will be able to remain in the comfort zone even if temperatures keep rising (cf. Indicator BAU-R-1).

^{136 - Spinoni J., Vogt J., Barbosa P. 2015: European degree-day climatologies and trends for the period 1951–2011. In: International Journal of Climatology 35 (1): 25–36. doi: 10.1002/joc.3959.}