Overheating guidance and tools for building designers

[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 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 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] 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] Approved Document O Building Regulations

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] BB 101: Ventilation, thermal comfort and indoor air quality

This document describes the factors that affect the indoor environment of schools, setting out the regulatory framework for ventilation in schools and gives recommended performance levels for compliance with UK regulations. The document also provides non-statutory guidance on how to design schools to achieve adequate performance for ventilation, indoor air quality and thermal comfort.

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. Supporting the document are five simple spreadsheets BB101 Calculation Tools.

[edit] BS 40102 Part 2 Thermal Comfort, Indoor Air Quality, and Overheating

BS 40102 is comprised of two parts: BS 40102-1:2023 covers Health and well-being and indoor environmental quality in buildings, whilst BS 40102-1:2023 addresses Thermal Comfort, Indoor Air Quality, and Overheating. Key aspects related to overheating include:

• Health and Wellbeing:The standard emphasises the link between building design and occupant health, including the impact of overheating on physical and mental wellbeing.
• Indoor Environmental Quality (IEQ): the overall IEQ of a building, considering thermal comfort, air quality, and ventilation.
• Overheating Mitigation: Provides a best-practice approach to identifying and addressing overheating risks, including strategies for limiting solar gains and providing adequate ventilation.
• Retrofit Projects: Whole building retrofit projects, improving existing stock to meet modern standards for thermal comfort and air quality.
• Comparison with other standards: Aligns with other standards from DEFRA, WHO, BREEAM, and WELL.

for more information visit https://knowledge.bsigroup.com/products/health-and-well-being-and-indoor-environmental-quality-in-buildings-health-and-well-being-in-non-domestic-buildings-code-of-practice

[edit] CIBSE guide A CIBSE Guide A: Environmental Design

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

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] CIBSE TM52 The Limits of Thermal Comfort: Avoiding Overheating in European Buildings

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 TM55 Design for future climate: case studies

"This publication is presenting current practice and understanding, rather than commenting on the best way to carry out building adaptation strategies, or recommending changes to regulatory documents and procedures. The projects were undertaken on a commercial basis and so the extent of modelling, investigation and research were inevitably constrained by the overriding needs of the project, planning, programme and costs. In many cases it was the first time that the design teams had given serious attention to the concerns of future climate and so approaches may not necessarily be best practice. It must be noted that it is impossible to measure the weather in 40 years time today, and so the decisions and recommendations in these case studies were based solely on modelling using future weather scenarios without the ability to validate their models with real world measurement, which is not ideal."

"These case studies on real building projects illustrate the lessons learned by design teams on improving adaptation resistance and resilience of building projects and show the impact that embedded adaptation strategies can have on the design decisions adopted by clients. The case studies recommend use of the following adaptation measures: Thermal performance and dealing with overheating; water conservation and dealing with flooding risk; and material durability."

For more information visit: https://www.cibse.org/knowledge-research/knowledge-portal/technical-memorandum-55-design-for-future-climate-case-studies-2014-pdf/

[edit] CIBSE TM59 Design methodology for the assessment of overheating risk in homes

A 2017 publication assessing overheating risk in homes. A complex issue and not adequately assessed by building regulations. Indeed, it would be wrong to assume that a home that complies with building regulations that were designed to focus on energy conservation also gives sufficient assurance of avoidance of overheating. Hence the recommendation that comfort conditions are separately assessed if it is felt that there could be a risk.

Many factors influence overheating in homes, including the intensity of heat gains, occupancy patterns, orientation, dwelling layout, shading strategy and ventilation method. Dynamic thermal modelling can be used to simulate the internal temperature conditions and will therefore help establish whether threshold conditions of discomfort will be reached. Given the complexity of the factors influencing overheating it is important that a standardised methodology is used to assess risk and hence the need for this technical memorandum. It can be applied to dwellings, care homes and student residences. Early analysis of overheating risk is recommended so that mitigation strategies can be reviewed in design proposals.

In summary, the application of this technical memorandum, by standardising the assessment methodology, should play a key role in limiting overheating risk in new and refurbished homes.

For more information visit: https://www.cibse.org/knowledge-research/knowledge-portal/technical-memorandum-59-design-methodology-for-the-assessment-of-overheating-risk-in-homes/

[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] Good Homes Alliance overheating and risk assessment tools

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 on new builds visit: https://goodhomes.org.uk/overheating-in-new-homes

For more information on retrofit visit: https://kb.goodhomes.org.uk/tool/overheating-retrofit/

[edit] Guidance on preventing overheating in the home

This guidance back by the government from 2015 outlines precautions people can take to prevent homes from overheating. Energy efficiency improvements help make homes warm and cosy in the winter and keep energy bills down. They also help us to reduce our carbon emissions and tackle climate change. Some energy efficiency improvements can also make homes hotter in the summer months. However, if these improvements are designed and installed correctly, the impact on summertime temperatures within homes can be greatly reduced. Future climate change is expected to increase outdoor temperatures and we all need to take steps to adapt to a changing climate. Undertaking energy efficiency improvements provides a great opportunity to not only make improvements to the standard of homes, but to also make them more resilient to climate change. This document can help to identify the homes which may be most at risk of overheating and gives details of measures which, if implemented at the same time as energy efficiency improvements, can significantly reduce the risk of overheating, and the potential use of energy for cooling.

For direct download visit: https://www.cibse.org/media/jvohyswt/decc-overheating-guidance-document-19jun2015.pdf

[edit] ISO 13792 Thermal performance of buildings - Calculation of internal temperatures of a room in summer without mechanical cooling - Simplified methods

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.

ISO 13792:2012 specifies input data for simplified calculation methods to determine maximum, average, and minimum daily operative temperatures in a room during warm periods. This helps in assessing the need for cooling systems and designing buildings to avoid overheating. The standard also includes criteria for calculation methods to ensure they meet the standard's requirements.

While ISO 13792:2012 is the current standard, it's worth noting that the standard is periodically reviewed. For example, it was reviewed in 2021, and while no major changes were made, the review ensures the standard remains relevant.

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

[edit] Passivhaus trust: Avoiding summer overheating

Guidelines for summer comfort in Passivhaus buildings and the PHT Summer overheating tool

Summer overheating is a growing concern for both new and existing buildings, especially as climate change intensifies. A UK study showed that all modelled new-build homes failed CIBSE TM59 overheating criteria. While the Passivhaus standard includes limits on internal temperatures exceeding 25°C, accurate modelling is essential, and current models often rely on outdated climate data. Key factors affecting overheating include occupant behaviour, occupancy levels, and assumptions around ventilation and shading. A four-stage assessment approach is recommended: design strategies, occupant constraints, risk indicators, and stress testing. Tools like the Good Homes Alliance risk assessment can help identify early-stage risks. Overheating results from an imbalance between heat gains and losses, with solar gain being a primary driver. Localised risks, such as poorly ventilated or west-facing rooms, may not be captured in average whole-building models, so Dynamic Simulation Modelling (DSM) using TM59/TM52 guidelines is advised for detailed assessment and to ensure design resilience under varying conditions.

For more information visit: https://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/Avoiding%20summer%20overheating.pdf

[edit] Shading for housing, a design guide

A product section provides detailed information to help users to select the right product for a building’s shading needs. Each product page features a brief description, a table detailing its functionality, an in-situ product photograph, a ‘performance web’ visualising a product’s strengths and weaknesses and, where relevant, an architect’s comment on a product’s added value.

The guide also provides a short history of shading design, explores UK-specific design challenges and wraps up with best practice advice. The guidance is applicable to both new build and retrofit projects, and aimed at a range of stakeholders including architects, local authorities, planners, housing associations, developers, and policy makers.

For more information and to download the guide, visit: https://kb.goodhomes.org.uk/guidance/shading-for-housing/.

[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] Related articles on Designing Buildings

• Approved Document O.
• Better prediction of overheating in new homes.
• Comfort in low energy buildings.
• Design summer year (DSY)
• Evolving opportunities for providing thermal comfort.
• Future climate models.
• Good homes alliance overheating tool
• Heat stress.
• Heatwave.
• Human comfort in buildings.
• ‎Maximum and minimum workplace temperatures.
• Overheating - assessment protocol.
• Overheating in buildings.
• Overheating in residential properties.
• Preventing overheating.
• Temperature.
• Thermal comfort.
• Thermal indices.
• Thermal pleasure in the built environment.
• Urban heat island.

Comments

[edit] To make a comment about this article, click 'Add a comment' above. Separate your comments from any existing comments by inserting a horizontal line.

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.