Overheating guidance for buildings

1 Introduction
2 ISO 13792:2005
3 CIBSE guide A 2006
4 Climate modelling
5 BB101
6 Adaptive comfort theory
7 CIBSE TM52
8 CIBSE guide A update 2015
9 Zero Carbon Hub, defining overheating
10 Good Homes Alliance risk assessments
11 Building regulations approved document O
12 Related articles on Designing Buildings

[edit] Introduction

Overheating is an element of building fabric control that will have an increasing importance as climate adaptation measures need to be balanced with climate mitigation measures. Its effects are indirectly impacted by fabric quality, design, layout and strategy as well as material choices and servicing, that can in turn impact energy use.

Building Regulations Part O 2022 establishes simplified and more complex dynamic modelling routes to provide evidence that certain design criteria that reduce the risks of overheating have been met. Below is a brief overview of differing developing guidelines in the past that have led up to Part O.

[edit] ISO 13792:2005

Overheating in buildings is most commonly defined in terms of thermal comfort, but it can also be related to and assessed in terms of health, well-being and productivity. The International Standard defines thermal comfort as ‘that condition of mind that expresses satisfaction with the thermal environment’ (ISO 2005). The approaches taken to assess overheating in buildings differs, depending on the type and use of the building.

For more information visit: https://www.iso.org/standard/37059.html

[edit] CIBSE guide A 2006

A general temperature limit for buildings (CIBSE Guide A-2006) is where an indoor comfort criteria is given as 25ºC, and 28ºC is given as the maximum allowable temperature, for no more than 1% of the occupied hours, when using a design summer year weather file for a climate modelling approach.

For more information visit: https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I79JAAS

[edit] Climate modelling

Design Summer Year (DSY) is a weather data reference file, that represents an averaged year of weather. It shows the weather as if it were single and continuous, rather than made up from average months. Initially a baseline it was adapted to create probabilistic DSYs that better describe overheating events, their relative frequency and severity. Although still used its simplicity is less able to account for individual extreme temperature events or incident solar radiation.

Test Reference Year (TRY) is a similar weather data reference file but one that models or represents a typical year in a certain location. It has a greater number of reference points, so more complex but more accurate (depending on the years selected). TRY files have been updated at various points in time and are available for 14 locations across the UK. More recently with the onset of climate change, many organisations start to use TRYs for different future climate scenarios.

Future Climate weather files; DSY's and TRY's incorporate different climate change scenarios (UK climate projections) with different percentiles of likely hood for three different time periods: 2020s (2011-2040), 2050s (2041-2070) 2080s (2071-2100). UK climate projections are based on different emission scenarios (Low - High) relating to climate mitigation efforts, which directly relate to the representative concentration pathways (RCPs) used show likely global mean temperatures (RCP2.6, 4.5, 6.0 and 8.5). Percentiles describe different probabilistic assumptions within the scenarios, for example, a 90th Percentile high emissions scenario means a 10% chance that temperatures will fall above the given threshold for a particular time period (worst case scenario), whilst 10th percentile low emissions scenario is a better case.

for more information visit: https://www.cibse.org/weatherdatasets

[edit] BB101

Schools introduced the use of three overheating criteria in BB101. The duration of overheating, when indoor temperature may rise above 28ºC but for no more than 120 hours per year. The severity of overheating; where the averaged difference between the internal temperature and the external temperature (Tint – Text) remains less than 5ºC per day. As well as an upper temperature limit of 32ºC, that should not be exceeded.

For more information visit: https://www.gov.uk/government/publications/building-bulletin-101-ventilation-for-school-buildings

[edit] Adaptive comfort theory

An adaptive comfort model take these three criteria, but also considers how an individual’s thermal expectations or preferences are impacted by their recent experiences of (outdoor) temperature, as well as a range of contextual factors. In practical terms this allows the human physiology to naturally adapt to extended periods in warmer (or cooler) environments.

For more information visit: https://www.cibse.org/getattachment/Networks/Regions/South-Wales/South-Wales-Past-Presentations/TM52-The-limits-of-thermal-comfort-Cardiff.pdf.aspx

[edit] CIBSE TM52

The CIBSE TM52 (2013) guidance, directed towards non-domestic buildings, again uses the same 3 criteria, but adapted to consider adaptive comfort models. It introduces a ‘running mean temperature’ (Trm) which is a rolling average of the outdoor air temperature, weighted according to time in the past and an operative temperature (Top) which is a combination of the air temperature and the mean radiant temperature. In this model the duration of overheating or operative temperature (°C or (He) ΔT ≥ 1) does not exceed 3% of the occupied hours. The severity of overheating uses a weighted exceedance (We), less or equal to 6 and finally where the maximum operative temperature does not exceed a temperature difference (ΔT) of 4ºC. This guidance was then adapted for schools to also consider the impact of cold drafts, with an additional requirement to mix ventilated air with room air to prevent this. It contains a broad range of guidance including various explanations and the use of predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) in the heat balance model of comfort.

For more information visit: https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I7f5AAC

[edit] CIBSE guide A update 2015

In 2015 the CIBSE Guide A was updated to incorporate considerations for adaptive thermal comfort. Here the Indoor Comfort Temperature (operative temperature in °C) is 0.33Trm + 18.8 and the Maximum Temperature (°C) or Comfort Temperature is +3. Using design summer years the Operative temperature can only surpass the max temperature for no more than 3% of the total occupied hours.

For more information visit: https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I79JAAS

[edit] Zero Carbon Hub, defining overheating

This review highlights that evidence-based ‘overheating’ thresholds related to different sectors have been developed on the basis of different environmental variables, quite often by researchers from different disciplines. As a result, they are commonly expressed in different metrics and are therefore not directly comparable with each other.

Indoor health-related thresholds are less well defined in comparison to thermal comfort-based thresholds, despite the well-characterised epidemiological relationships between outdoor ambient temperature and heat-related morbidity and mortality. This is partly due to the methodological complexity of linking indoor environments with health outcomes.

It defines more clearly the term, overheating in its context and indicates that future research should aim to establish an integrated approach towards defining overheating thresholds that cuts across comfort, wellbeing and health impacts.

For more information visit: https://www.zerocarbonhub.org/sites/default/files/resources/reports/ZCH-OverheatingEvidenceReview-Definitions.pdf

[edit] Good Homes Alliance risk assessments

The Good Homes Alliance published an overheating guidance and risk score sheet tool for new homes in July 2019 followed by the same for retrofit and existing homes in March 2022, these are tools for early stage assessment not detailed assessment, sitting between existing high-level guidance, more detailed calculations and modelling tools. These identify key factors contributing to overheating risks, and possible mitigation measures.

For more information visit: https://goodhomes.org.uk/overheating-in-new-homes

[edit] Building regulations approved document O

Building Regulations Approved Document O; The overheating mitigation regulations 40B took effect on 15 June 2022 in England and Wales. It takes its lead from CIBSE TM52 and provides a set of criteria to determine if a dwelling is at risk of overheating, to be evidenced either though a simplified method or more complex dynamic modelling.

It requires residential buildings to be categorised by location and risk level, for solar gains to be minimised by glazed area limitations (differing based on with and without cross-ventilation) and for excess heat to be removed via minimum openable free areas (again with or without cross-ventilation). This prepares the building design to meet; Criterion 1: For living rooms, kitchens and bedrooms the number of hours during which T is greater than or equal to one degree (K) between May - September inclusive shall not be more than 3% of the occupied hours. Criterion 2: For bedrooms only – The operative temperature from 10pm to 7am shall not exceed 26°C for more than 1% of annual hours (32.85 hours). Finally, the regulation acknowledges that mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it but also excludes consideration for external tree shading for mitigation.

For more information visit: https://www.gov.uk/government/publications/overheating-approved-document-o

[edit] Related articles on Designing Buildings

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Comments

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Brilliant article. We made a document that really simply covers the history of BB101 and what it is, and what it means for school ventilation systems. Don't feel any pressue, but have a look if you want a really simple updated definition.