OVERHEATING CIBSE GUIDANCE
Unsatisfactory thermal performance
35 30 25 20 15 10 5 0
Top
Summer hours: 1 June – 31 August Tout
Tmax Figure 2. Outdoor air temperatures (Tout Tupp
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Top
Summer hours: 1 June – 31 August Tout
Tmax ) and indoor operative temperatures (Top operative temperature, and the likelihood of overheating, is clearly shown. temperature, Tmax (see fi gure 1), by 1K
or more during the occupied hours of a typical non-heating season – 1 May to 30 September. Criterion two deals with the severity of overheating within any one day, which can be as important as its frequency. This is a function of both temperature (∆T) and its duration. This
above Tmax criterion sets a daily limit for acceptability. London’s burning
New TM49 overheating guidance responds to rising temperatures and urban heat island effect A new CIBSE technical memorandum will help engineers to more accurately predict the risk of overheating in designs for London buildings. TM49 Probabilistic Design Summer Years
for London accompanies a range of hourly weather data from three London locations that designers can use to assess the likelihood of overheating when modelling buildings.
The updated guide complements the existing Design Summer Year (DSY), which is a moderately warm summer based on a near- extreme value of April-September average temperature at Heathrow airport in 1989. Three DSYs are available – all warmer summers than the existing DSY – for three locations, representing a range of weather conditions across London.
Data from Gatwick, Heathrow and the London Weather Centre is featured in the new guide, enabling variations in city-wide temperatures caused by the urban heat island effect to be taken into account. Designers can use data that is
representative of rural areas around London (Gatwick); suburban and urban areas outside central London (Heathrow); or central
24 CIBSE Journal August 2013
London (London Weather Centre). The new guidance takes account of climate change projections. This will lead to warmer summers than contained in the existing DSY, which the report’s authors found were no longer suffi ciently extreme to provide a basis for accurate overheating assessments in London.
The analysis indicates that according to the climate change projections the more extreme historical summers would become average summers.
It estimates that the current DSY will have a return period of only 1-3 years by the 2020s time period (2011-2040) – in other words, every one to three years there will be a summer as warm, or hotter, than the DSY. The return period is based on a new metric of summer warmth called ‘weighted cooling degree hours’, which is more closely related to the likelihood of thermal discomfort, according to the report’s authors.
The three DSYs represent a range of more extreme summers, with appropriate return times of between six and 11 years. These DSYs can be used for overheating risk assessments, and the guide recommends that more than one is used to investigate the sensitivity of designs to different weather conditions.
) for an exemplar building with little
thermal mass and no shading (left), and for the same building with exposed thermal mass, night cooling and solar shading (right). Also shown are the day-to-day values of Tmax
and Tupp (data from Tuohy et al, 20093). The effect on the Tupp
Satisfactory thermal performance
If each hour (or part-hour) in which the temperature exceeds Tmax
by at least 10
K is
multiplied by the number of degrees by which it is exceeded (∆T), then this ‘excess’ should not be more than six degree-hours (for example, it can exceed Tmax
by 100 six hours or 200
on). Criterion three sets an absolute maximum temperature of (Tmax
C for C by three hours, and so + 4)0 C for a room (shown as Tupp in fi gure 1),
beyond which the level of overheating is unacceptable. Figure 2 illustrates the use of the overheating guidance in a building simulation using a Design Summer Year weather fi le in TM49 (see box). It shows how the introduction of thermal mass, solar shading and night ventilation can make a building less likely to overheat. Similar analyses are shown in the technical memorandum, where monitored data from an actual building is used to illustrate the guidance. Simulation is also used to illustrate the increased risk of overheating as a result of climate change. TM52 also includes advice on the risk
of overheating in mechanically-cooled buildings. CJ
References
1. Nicol F, Hacker J, Spires B and Davies H, ‘Suggestion for new approach to overheating diagnostics’, Building Research and Information, vol 37 (4) pp348- 357 (2009)
2. BS EN 15251:2007 Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics
3. Tuohy P, Humphreys MA, Nicol F, Rijal H and Clarke J, ‘Occupant behaviour in naturally ventilated and hybrid buildings’, ASHRAE Transactions, 115 (1) pp16- 27 (2009)
FERGUS NICOL is Professor Emeritus at Oxford Brookes and a professor at London Metropolitan University
Other CIBSE publications relating to comfort and overheating Keeping Cool in a Heatwave: Top Tips for Facilities Managers and Keeping Cool in a Heatwave: Top Tips for Building Users (2010).
KS16 How to Manage Overheating in Buildings (2010)
Guide A: Environmental Design (8th edition, 2013)
TM49: Probabilistic Design Summer Years for London
All publications are available at
www.cibseknowledgeportal.co.uk
www.cibsejournal.com
Temperature (˚C)
Temperature (˚C)
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