Overheating Assessments

Assessments types and methodologies:

CIBSE 52, 59 and BB101 are the three predominant technical guides for assessing the overheating risk in buildings and each guide has its own set of criteria for compliance in naturally ventilated buildings.

The criteria for each technical guide is as follows:

TM52 – The limits of thermal comfort avoiding overheating

Can be used to assess overheating risk in all buildings and uses three criterion to determine the levels of risk:

  1. Criterion 1 (Frequency) sets a limit for the number of hours that the operative temperature can exceed the threshold comfort temperature by 1oK or more during the occupied hours of a typical non-heating season (1 May to 30 September) – Threshold temperature exceeded ≯ 3% of occupied hours per year.
  2. Criterion 2 (Severity) deals with the severity of overheating within any one day, which can be as important as its frequency and the level of which is a function of both temperature rise and its duration. This criterion sets a daily limit for acceptability – Daily weighted exceedance (degree hours) ≯ 6.
  3. Criterion 3 (Maximum Tolerance) sets an absolute maximum daily temperature for a room, beyond which the level of overheating is unacceptable – Temperature ≯ upper limit.

TM59 – Design methodology for the assessment of overheating

Specifically looks at overheating risk in homes using standardised methodology to assess and address the way dwellings respond to external temperatures. The two criterion to determine the levels of risk in naturally ventilated dwellings are:

  1. Living rooms, kitchens and bedrooms – the number of hours during which external and internal temperature difference is greater than or equal to one degree (K) during the period May to September inclusive shall not be more than 3 percent of occupied hours (same as CIBSE TM52 Criterion 1)
  2. For bedrooms only – to improve comfort during the sleeping hours the operative temperature in the bedroom from 10pm to 7am shall not exceed 26oC for more than 1% of annual hours. (Note: 1% of the annual hours between 10pm and 7am for bedrooms is 32 hours, so 33 or more hours above 26oC will be recorded as a fail)

Where homes are predominately mechanically ventilated due to restrictions with window openings, the CIBSE fixed temperature test must be followed whereby all occupied rooms should not exceed an operative temperature of 26°C for more than 3% of the annual occupied hours.

Overheating in corridors risk with communal heating pipework

Whilst there is no mandatory target to meet, if an operative temperature exceeds 28oC for more than 3% of the total annual hours then this should be identified as a significant risk within the report.

BB101 – Guidelines on ventilation, thermal comfort and indoor air quality in schools

Recently updated (August 2018) and expanded upon, BB101 is a very good guide for assisting engineers, architects and schools with understanding how to design and operate a school that maximises the comfort of the users with a focus on ventilation, thermal comfort and indoor air quality. The guide highly emphasises the use of natural ventilation where possible which is assessed using TM52. Where buildings are required to be air conditioned, e.g. special needs or medical reasons, then the building should be assessed using recommended thermal comfort criteria in CIBSE Guide A.

If risk is identified in any of the above reports, the recommendations on how to overcome an overheating risk are to be detailed and mitigation strategies can be reviewed and then implemented within design proposals.