ARE YOU SITTING COMFORTABLY?
CIBSE’s TM52 guidance fi nally gives a clear and comprehensive defi nition of overheating – a vital tool in simulating comfortable, low-energy naturally ventilated indoor environments. Fergus Nicol reports
O
verheating has become a key problem for building design. The rising cost of energy, coupled with the need to
reduce reliance on fossil fuels, has cut the number of options available for designing comfortable buildings that are resilient to climate change. Much research effort has been directed towards methods of controlling winter temperatures indoors, but this has often led to lightweight, highly insulated buildings that overheat in the summer. The CIBSE Overheating Task Force
was formed to tackle this problem and soon realised that there was no adequate defi nition of overheating. This is a particular issue for the design of naturally ventilated (NV) buildings1
The basis of CIBSE’s new guidance, , where control of
indoor temperature in summer is often left to occupants. In the past, overheating has been defi ned as a number of hours over a particular temperature in the given season. Research suggests that the temperature that occupants of NV buildings will fi nd uncomfortable changes with the outdoor conditions in a predictable way. One way – and probably the best way – to
fi nd out whether a building overheats is to ask the people who occupy it. But this is no help when it is not yet built. Even a building that has been occupied for a short time may not have been fully ‘tested’ by a heatwave, let alone the effects of climate change. The task force needed a method for predicting the danger of overheating using dynamic building simulation or, if available, building monitoring.
22 CIBSE Journal August 2013
TM52: The limits of thermal comfort: avoiding overheating in European buildings, is the European Standard EN 152512. This standard was developed in response to the European Energy Performance of Buildings Directive and contains information about the relationship between the prevailing outdoor temperature and the upper limit of temperatures in NV buildings. The standard introduces three categories of building: Category I – buildings whose occupants are sensitive or fragile Category II – normal expectation, recommended for new build or renovations Category III – moderate expectation, mainly applicable in existing buildings Note that each defi nes buildings by reference to the needs of their occupants, rather than any notion of “building quality”. Figure 1 shows the upper limits for acceptable temperatures in NV buildings for each of the categories. The CIBSE task force decided that the guidance should be based on category II – the most applicable to new and existing buildings. The continuous white line shows the upper limit for indoor temperature as a function of the running mean of the outdoor temperature. The running mean is precisely defi ned in TM52 and is a weighted average of the daily mean outdoor temperature over the previous few days – it should be noted that it is not related to the peak daily temperature. The category II maximum temperature – the white line in fi gure 1 – is given by
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