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The recently published CIBSE TM59 (2017): Methodology for the assessment of overheating in homes guidance has a focus on naturally ventilated (free running) dwellings and provides a standardised approach for the assessment and reporting of residential overheating as a growing concern within the development industry.

This new guidance clarifies the previous guidance issued in CIBSE TM52. CIBSE TM59 (2017) is the guidance to be used for dwellings.

The aim of the CIBSE TM59 Methodology for overheating is to:

  • Allow different designs to be compared with a common approach, based on reasonable assumptions;
  • Support design decisions that improve comfort without cooling;
  • Provide consistency across the industry as all consultants will be using the same methodology for overheating risk prediction.

The main key points and updates of the new guidance are listed below:

  • Thermal comfort criteria draw from CIBSE TM52 and CIBSE Guide A, however compliance is limited to Criterion 1 for all habitable spaces except for bedrooms which should also comply with the CIBSE guide A fixed temperature threshold of 26°C for less than 1% of the occupied night-time hours;
  • Weather data to be using the predicted Design Summer Year (DSY) 1 2020’s, high emission, 50 % percentile scenario weather file as opposed to the current DSYs;
  • Standard occupancy and internal heat gain data to be implemented to account for extended occupied hours day and reflect different patterns of building use that will test if it performs reasonably well throughout the day;
  • Where communal heating is present this needs to be accounted for and reported in the modelling. The risk of overheating within the communal corridor spaces has to be assessed and failure to comply with thermal comfort requirements to be flagged;
  • A good sample size of assessed dwellings is recommended to account for different conditions across the scheme;
  • The use of blinds as a mitigation measure has to be agreed with the client to ensure it is included in the base build specification.