Understanding and Mitigating Overheating in UK Residential Buildings: A Comprehensive Analysis

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

Abstract

Overheating in residential buildings has emerged as a significant and escalating concern in the United Kingdom, particularly exacerbated within the context of contemporary energy-efficient homes. The widespread adoption of stringent building standards, which promote airtight, highly insulated structures, while unequivocally beneficial for reducing heating demands and carbon emissions, has inadvertently heightened the risk of excessive indoor temperatures during warmer months and increasingly frequent heatwave events. This report undertakes a comprehensive exploration of this multifaceted phenomenon, delineating the intricate factors contributing to overheating, meticulously evaluating a range of recommended prevention and mitigation strategies, scrutinising advanced tools for precise risk assessment, and thoroughly discussing the profound long-term health and comfort implications for occupants within the undeniable trajectory of a warming global climate. The imperative for integrated, anticipatory design approaches and responsive policy frameworks is underscored to ensure the resilience, habitability, and well-being of the UK’s housing stock.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

1. Introduction

The United Kingdom’s steadfast commitment to achieving ambitious carbon emission reduction targets, notably the legally binding net-zero emissions by 2050, has profoundly reshaped its building sector. This commitment has catalysed the widespread adoption of advanced energy-efficient building practices, often mandated by evolving Building Regulations. These practices encompass significantly enhanced levels of thermal insulation, meticulous attention to airtightness, and the increasing integration of low-carbon heating systems and renewable energy sources such as solar photovoltaics. The primary objective of these measures is to drastically decrease the operational energy consumption associated with space heating, which historically constituted a substantial portion of residential energy demand. (uel.ac.uk)

However, this laudable progress towards energy efficiency has inadvertently given rise to a critical and often overlooked unintended consequence: overheating during extended warm periods and particularly during summer months. Traditional UK housing stock, often characterised by lower insulation levels and inherent draughtiness, possessed a degree of natural resilience to warmer temperatures through uncontrolled air exchange. Modern, highly sealed envelopes, while excelling at retaining heat in winter, are significantly less adept at dissipating unwanted heat gains in summer. The profound implications of this paradigm shift were starkly highlighted by the unprecedented heatwaves of 2022, which saw UK temperatures exceed 40°C for the first time in recorded history, exposing millions of households to uncomfortably and dangerously high indoor temperatures. An alarming study by the University of East London, published in 2023, indicated that as many as 80% of UK homes could be experiencing some degree of overheating during summer months, underscoring the pervasive nature of the problem. (uel.ac.uk)

This phenomenon is not merely an inconvenience; it poses substantial risks to occupant health, comfort, and productivity. As climate change projections indicate an increasing frequency and intensity of heatwaves, the urgency of proactively addressing overheating in the design, construction, and operation of residential buildings becomes paramount. This report seeks to provide a detailed technical exposition of the contributing factors, a comprehensive review of effective mitigation strategies, an overview of diagnostic and predictive tools, and an examination of the long-term societal implications.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

2. Factors Contributing to Overheating

Overheating in homes is not attributable to a single cause but rather to a complex and dynamic interplay of interconnected factors, originating from external environmental conditions, building design and fabric characteristics, and internal heat generation. Understanding these drivers is fundamental to devising effective mitigation strategies.

2.1 Solar Gain

Solar gain, or solar heat gain, refers to the increase in indoor temperature resulting from solar radiation entering a building, primarily through glazed areas such as windows, skylights, and conservatories. This is arguably the most significant external contributor to overheating in modern, well-insulated homes. (arcc-network.org.uk)

• Window-to-Wall Ratio and Orientation: Contemporary architectural trends often favour large glazed areas to maximise natural daylighting and provide expansive views. While beneficial in winter for passive solar heating, these large windows, particularly when oriented towards the south, west, or east, become highly effective heat traps in summer. South-facing windows receive significant sun exposure throughout the day, while west-facing windows are particularly problematic in the late afternoon when external temperatures are often at their peak, leading to intense heat accumulation just before occupants return home. East-facing windows can cause rapid morning heat gain.
• Glazing Type: The type of glass employed has a profound impact on solar gain. Standard double or triple glazing, while excellent for thermal insulation (low U-value), may have a high Solar Heat Gain Coefficient (SHGC) or g-value (total solar energy transmittance), meaning they allow a large percentage of solar radiation to pass through and convert to heat indoors. Without specific solar control properties, these windows can contribute substantially to overheating.
• Lack of External Shading: The absence of effective external shading devices is a critical oversight. Internally mounted blinds or curtains, while offering privacy and some light control, are largely ineffective at preventing heat gain because solar radiation has already passed through the glass and been converted to heat within the room before encountering the shading device. This trapped heat then dissipates into the indoor air. External shading, conversely, intercepts solar radiation before it enters the building envelope, preventing heat build-up at its source. (arcc-network.org.uk)

2.2 Internal Heat Loads

Internal heat loads are generated within the building by its occupants, activities, and operational systems. In highly airtight and well-insulated homes, these internally generated heat gains can accumulate rapidly, significantly elevating indoor temperatures, especially when combined with insufficient ventilation. (nhbc.co.uk)

• Occupant Metabolic Heat: Human bodies continuously generate heat as a metabolic byproduct. An average adult at rest typically emits around 100 Watts of heat, increasing with activity levels. In a well-populated, sealed dwelling, the cumulative heat from occupants can become a non-negligible factor.
• Electrical Appliances: Modern homes are replete with electronic devices and appliances, many of which generate significant amounts of waste heat. Refrigerators and freezers run continuously, contributing a steady heat load. Televisions, computers, gaming consoles, ovens, hobs, dishwashers, washing machines, and tumble dryers all release heat during operation. Even seemingly minor devices, when aggregated, can contribute substantially. For instance, a typical oven operating for an hour can release several kilowatts of heat into the kitchen space.
• Lighting: While the widespread adoption of LED lighting has significantly reduced heat output compared to older incandescent or halogen bulbs, lighting fixtures still contribute some heat. In spaces with extensive artificial lighting, this can become a minor but consistent internal heat source.
• Hot Water Systems and Pipework: Uninsulated or poorly insulated hot water tanks, cylinders, and pipework can radiate heat into surrounding spaces, contributing to the overall internal heat load.

2.3 Ventilation Challenges

Effective ventilation is paramount for dissipating internal heat gains, expelling warm air, and maintaining good indoor air quality. However, in contemporary highly insulated and airtight homes, the traditional natural ventilation pathways are often deliberately restricted, leading to inadequate heat removal. (nhbc.co.uk)

• Airtightness: While essential for energy efficiency and preventing uncontrolled heat loss in winter, high levels of airtightness (low air permeability) drastically reduce unintended air changes that might otherwise help cool a building. This means that any heat gain, whether from solar radiation or internal sources, becomes trapped more effectively within the building envelope.
• Limited Natural Ventilation: Reliance on natural ventilation through opening windows and doors is a common strategy in UK homes. However, this strategy is frequently hampered by a range of external factors:
  • Noise Pollution: Dwellings located near busy roads, railway lines, airports, or commercial areas often experience significant noise pollution, deterring occupants from opening windows, particularly at night when external temperatures are lower and night purging could be most effective.
  • Air Pollution: Similar to noise, high levels of outdoor air pollution (e.g., from traffic, industrial sources, or particulate matter) can discourage window opening, posing a dilemma between thermal comfort and indoor air quality.
  • Security Concerns: In ground-floor flats or homes in urban areas, security concerns can prevent occupants from leaving windows open, especially overnight or when away from home.
  • Rain and Wind: Adverse weather conditions, such as driving rain or strong winds, can also deter window opening, even when internal temperatures are high.
• Lack of Cross-Ventilation: Many modern apartment blocks or terraced houses are designed as single-aspect dwellings, meaning windows are only present on one side. This severely limits the potential for effective cross-ventilation, where air flows freely across a space from one opening to another, effectively flushing out warm air.

2.4 Building Fabric and Thermal Mass

• High Insulation Levels: While insulation is crucial for preventing heat loss in winter, it can also slow the escape of heat in summer. If external temperatures remain high for extended periods, and there are significant internal or solar gains, the building’s highly insulated envelope can effectively trap this heat inside.
• Thermal Mass Mismanagement: Thermal mass refers to the capacity of materials to absorb, store, and release heat. While beneficial when used correctly for diurnal temperature moderation, if a building’s thermal mass is primarily internal and absorbs heat during the day without sufficient night-time cooling, it can become a heat reservoir, slowly radiating warmth back into the space throughout the night, preventing temperatures from dropping.

2.5 Urban Heat Island Effect

Cities and urban areas often experience significantly higher temperatures than surrounding rural areas, a phenomenon known as the Urban Heat Island (UHI) effect. This effect intensifies during heatwaves and can compound the overheating risk for urban dwellings. (arcc-network.org.uk)

• Impermeable Surfaces: Urban environments are dominated by dark, impermeable surfaces like asphalt roads and concrete buildings, which absorb and store solar radiation more effectively than natural landscapes. This heat is then re-radiated into the atmosphere, elevating ambient temperatures.
• Reduced Evapotranspiration: The lack of vegetation and green spaces in urban areas means less evapotranspiration (the process by which plants release water vapor, cooling the air), further contributing to higher temperatures.
• Anthropogenic Heat: Heat generated by human activities in cities, such as vehicle exhaust, industrial processes, and air conditioning systems, also contributes to the UHI effect.

2.6 Climate Change

The overarching context for the increasing prevalence of overheating is climate change. Forecasts for the UK indicate a future with warmer, wetter winters and hotter, drier summers, accompanied by an increased frequency and intensity of extreme heat events. This means that the problem of overheating, already pressing, will only intensify over time, making future-proofing building design an imperative rather than an optional consideration. (uel.ac.uk)

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

3. Prevention Strategies

Mitigating overheating risks necessitates a holistic, integrated design approach that prioritises passive strategies, minimising reliance on active cooling systems that consume significant energy and contribute to carbon emissions. A multi-layered defence system, implemented from the earliest design stages, is most effective.

3.1 External Shading

External shading is arguably the most effective passive strategy for reducing solar gain, as it intercepts solar radiation before it enters the building envelope. Its effectiveness is superior to internal shading, which only controls light and privacy once heat has already entered the space. (arcc-network.org.uk)

• Fixed Shading Devices: These are permanent architectural elements designed to block direct sunlight during warmer months while allowing beneficial solar gain in winter when the sun’s angle is lower. Examples include:
  • Overhangs/Canopies: Horizontal projections above windows, effective for south-facing facades. Their depth needs to be calculated based on sun angles to block summer sun but allow winter sun.
  • Brise Soleil: Vertical or horizontal fins fixed to the building exterior. Vertical fins are effective for east and west facades to block low-angle sun, while horizontal fins work well for south-facing windows.
  • Recessed Windows: Setting windows back into the facade provides self-shading.
• Movable Shading Devices: These offer flexibility to adapt to changing weather conditions and occupant preferences. Examples include:
  • External Blinds/Shutters: These can be manually or automatically operated and offer precise control over solar gain and privacy. They are highly effective at blocking heat.
  • Louvres: Adjustable slats that can be angled to control solar penetration and airflow.
• Vegetation: Deciduous trees strategically planted on the south, east, or west sides of a building can provide effective seasonal shading. Their leaves block summer sun, and once shed in winter, allow passive solar gain. Climbers on pergolas or trellises can also offer seasonal shading.

3.2 Thermal Mass and Phase Change Materials (PCMs)

These strategies leverage the heat storage capacity of materials to moderate internal temperature fluctuations.

• Thermal Mass: Materials with high thermal mass, such as dense concrete, brick, stone, or even water, have a high specific heat capacity, meaning they can absorb and store large amounts of heat without a significant increase in their own temperature. During the day, they absorb excess heat from internal gains and solar radiation. At night, when external temperatures drop, this stored heat can be passively released back into the cooler indoor air or, ideally, purged from the building through ventilation (night cooling). This ‘thermal flywheel’ effect helps to dampen peak indoor temperatures and reduce the amplitude of diurnal temperature swings. For thermal mass to be effective in summer, it must be exposed to the occupied space (not covered by insulation or lightweight finishes) and coupled with effective night-time ventilation to ‘recharge’ its cooling capacity. (nottingham.ac.uk)
• Phase Change Materials (PCMs): PCMs are advanced materials that absorb and release latent heat as they undergo a phase transition (typically solid-to-liquid or liquid-to-solid) at a specific temperature range. For buildings, PCMs are designed to melt (absorb heat) at comfortable room temperatures (e.g., 21-26°C) and solidify (release heat) when temperatures drop. This allows them to absorb significant amounts of heat during the hottest part of the day without a corresponding temperature rise, effectively flattening the internal temperature curve. PCMs can be incorporated into plasterboard, insulation panels, floor screeds, or even integrated into textiles. They are particularly useful where conventional thermal mass is limited (e.g., lightweight timber frame constructions) or where space is a constraint. The challenge lies in ensuring that the PCM can sufficiently solidify overnight to be ready for the next day’s heat absorption.

3.3 Advanced Natural Ventilation Design

Optimising natural airflow through a building is a fundamental and highly effective passive cooling strategy. This requires careful architectural design and site planning.

• Cross-Ventilation: Designing dwellings to have openings on opposing facades facilitates cross-ventilation, where prevailing winds drive air through the building, effectively flushing out warm indoor air and replacing it with cooler outdoor air. This requires thoughtful placement of windows, doors, and internal partitions.
• Stack Effect (Chimney Effect): Utilising the principle that warm air rises, vertical ventilation shafts, stairwells, or strategically placed high-level vents can create a ‘stack effect’. Warm air escapes through high openings, drawing cooler air in through low-level openings. This is particularly effective in multi-storey buildings.
• Night Purging (Night-time Cooling): This strategy involves opening windows and vents at night when external temperatures are typically lower to flush out accumulated heat from the building’s thermal mass and interior air. This ‘cool flush’ cools down the building fabric, allowing it to absorb heat more effectively during the following day. Effective night purging requires secure, controllable openings (e.g., trickle vents, secure night latches, or automated systems) that mitigate concerns about security, noise, and pests. (arcc-network.org.uk)
• Ventilative Cooling: This broad term encompasses strategies that use air movement to cool spaces. It can involve enhanced natural ventilation, but also hybrid systems where fans assist air movement when natural forces are insufficient. (en.wikipedia.org)

3.4 Mechanical Ventilation with Heat Recovery (MVHR) Systems

MVHR systems are increasingly specified in highly airtight homes to provide continuous, controlled ventilation and maintain indoor air quality. While their primary function is to recover heat in winter, their design and operation are critical to preventing overheating in summer. (nhbc.co.uk)

• Summer Bypass Mode: A crucial feature for overheating prevention is a fully functional summer bypass. In this mode, the heat exchanger within the MVHR unit is bypassed, allowing cooler incoming fresh air to be supplied directly to the dwelling without being preheated by the outgoing warm air. Without a summer bypass, an MVHR system can exacerbate overheating by actively bringing heat back into the building. The bypass should ideally be automatic, controlled by indoor and outdoor temperature sensors.
• Commissioning and Occupant Control: Proper commissioning is essential to ensure MVHR systems operate optimally. Occupants also need clear instructions on how to use their system, particularly how to engage summer bypass or boost ventilation rates when needed. Some MVHR units also offer the capability to draw air from a cooler source, such as a ground-coupled heat exchanger, though this adds complexity and cost.

3.5 Solar-Control Glass

Solar-control glazing is specifically engineered to reduce the amount of solar radiation that passes through the glass, thereby decreasing solar heat gain while still allowing adequate natural light. (build.saint-gobain.co.uk)

• Low-Emissivity (Low-E) Coatings: While primarily designed to reduce heat loss in winter, some low-E coatings can also offer solar control properties. They reflect specific wavelengths of the solar spectrum.
• Spectrally Selective Coatings: These advanced coatings are designed to selectively transmit visible light while reflecting or absorbing the infrared and ultraviolet portions of the solar spectrum, which are responsible for heat gain. This allows for high levels of natural daylighting with significantly reduced heat transfer.
• Tinted or Reflective Glass: While effective at reducing solar gain, heavily tinted or reflective glass can also reduce visible light transmittance, potentially leading to a darker interior and increased reliance on artificial lighting.
• Performance Metrics: When selecting solar-control glass, key metrics to consider are:
  • G-value (Solar Heat Gain Coefficient – SHGC): A lower g-value indicates less solar heat transmission.
  • U-value: Measures thermal transmittance (heat loss/gain through conduction, convection, radiation). A lower U-value means better insulation.
  • Light Transmittance (LT): Indicates the percentage of visible light passing through the glass. Balancing low g-value with adequate LT is crucial.

3.6 External Wall Insulation (EWI)

While primarily known for its role in reducing heat loss in winter, external wall insulation can also play a role in mitigating overheating risks in summer. (arcc-network.org.uk)

• Thermal Envelope Integrity: EWI forms a continuous, highly insulating layer around the building’s external walls. In summer, this helps to prevent solar gains from reaching the underlying thermal mass of the wall structure (e.g., brickwork or concrete) and transferring inwards. By keeping the main structural elements cooler, the building’s ability to resist external heat penetration is enhanced.
• Maintaining Thermal Mass Functionality: Unlike internal insulation, EWI keeps the building’s inherent thermal mass (the original wall structure) on the ‘warm side’ of the insulation. This allows the internal thermal mass to absorb internal heat gains during the day and release them to the cooler interior air at night, provided there is effective night-time ventilation.

3.7 High Albedo Materials

Albedo is a measure of the diffuse reflectivity of a surface: the ratio of the solar radiation reflected by a surface to the total solar radiation incident on that surface. Materials with high albedo reflect a large proportion of solar radiation. (arcc-network.org.uk)

• Cool Roofs: Applying light-coloured, highly reflective coatings or materials to roofs significantly reduces the amount of solar radiation absorbed by the roof surface. This directly lowers the temperature of the roof and, consequently, the heat transferred into the building below. This is particularly effective for top-floor apartments and bungalows where roofs are a major source of solar gain.
• Light-Coloured Facades: Similarly, using light-coloured paints or cladding materials for external walls reduces heat absorption compared to darker colours. This contributes to cooler wall surfaces and reduced heat transfer into the building.
• Urban Heat Island Mitigation: The widespread adoption of high albedo materials across urban areas contributes to reducing the overall urban heat island effect, leading to cooler ambient temperatures, which in turn benefits all buildings within that urban environment.

3.8 Landscaping and Urban Greening

Integrating green infrastructure into the built environment provides significant passive cooling benefits, both at the building level and the urban scale. (arcc-network.org.uk)

• Shading: Mature trees strategically planted around a building can provide substantial shading to facades and roofs, blocking direct solar radiation and significantly reducing heat gain. Deciduous trees offer the additional benefit of allowing winter solar gain.
• Evapotranspiration: Plants release water vapour into the atmosphere through a process called evapotranspiration. This process consumes energy (latent heat of vaporization), effectively cooling the surrounding air, similar to how human perspiration works. Green roofs, green walls, and extensive planting in gardens and public spaces can contribute to substantial localised cooling.
• Permeable Surfaces: Replacing impermeable surfaces (like asphalt and concrete) with permeable, vegetated surfaces (e.g., grass, permeable paving) reduces heat absorption and allows for natural water evaporation, further contributing to cooler ambient temperatures.
• Water Features: Ponds, fountains, and other water features can also provide a minor cooling effect through evaporation.
• Urban Heat Island Reduction: Collective greening efforts contribute significantly to mitigating the urban heat island effect, resulting in cooler microclimates within urban areas, indirectly reducing overheating risks for surrounding buildings.

3.9 Occupant Behaviour

While good design minimises the need for active occupant intervention, empowering residents with knowledge and control can significantly enhance thermal comfort outcomes. (arcc-network.org.uk)

• Night-time Ventilation: Educating occupants about the benefits and safe practice of night purging (opening windows/vents securely at night to flush out heat) is crucial. Providing secure trickle vents or restrictors facilitates this.
• Management of Internal Heat Gains: Encouraging simple habits such as turning off unused lights and appliances, particularly those that generate significant heat (e.g., computers, TVs on standby, ovens, tumble dryers), especially during the hottest parts of the day. Using extractor fans in kitchens and bathrooms during and after use can also help remove heat and moisture.
• External Shading Use: Instructing occupants on the proper use of external blinds, shutters, or curtains (e.g., closing them during the day on sunny facades and opening them at night) to prevent solar gain.
• Cross-Ventilation: Explaining how to maximise natural airflow by opening windows on opposing sides of a dwelling where feasible.
• Understanding Building Systems: Providing clear, accessible information on how to operate mechanical ventilation systems (e.g., engaging summer bypass on MVHR units) and other climate control features.

3.10 Integrated Design Approach

Crucially, these strategies are most effective when considered holistically from the earliest stages of building design. A fabric-first approach, combined with passive design principles, minimises reliance on energy-intensive active cooling. This means orienting buildings to minimise unwanted solar gain, optimising window sizes and types, integrating shading, designing for natural ventilation, and utilising thermal mass as an integral part of the building structure.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

4. Tools for Risk Assessment

Accurate and robust assessment of overheating risks is not merely advisable but increasingly mandated by building regulations and best practice guidelines. This allows designers to predict performance, identify potential issues, and implement corrective measures before construction commences, thereby avoiding costly retrofits and ensuring occupant comfort.

4.1 Passive House Planning Package (PHPP)

PHPP is a highly detailed, Excel-based energy modelling tool developed by the Passive House Institute (PHI). While renowned for its role in designing ultra-low energy buildings that meet the stringent Passive House standard, its capabilities extend to predicting thermal comfort, including the risk of overheating. (atamate.com)

• Comprehensive Energy Balance: PHPP performs a comprehensive energy balance calculation for a building, considering all heat gains (solar, internal, transmission) and losses (transmission, ventilation) on an hourly or monthly basis. This allows it to model internal temperatures accurately.
• Overheating Criterion: PHPP defines an overheating criterion, typically based on the percentage of hours in a year where indoor temperatures exceed a comfort threshold (e.g., 25°C). The Passive House standard often aims for no more than 10% of the year with temperatures above 25°C. This threshold can be adjusted to reflect specific project requirements or regulatory compliance targets.
• Sensitivity Analysis: Designers can use PHPP to perform sensitivity analyses, evaluating the impact of different design choices (e.g., window size, shading type, ventilation rates, thermal mass, glazing g-value) on predicted indoor temperatures. This iterative process helps optimise designs for both energy efficiency and thermal comfort.
• Limitations: While powerful, PHPP is a static, steady-state tool, or a quasi-steady-state tool, not a full dynamic simulation. It relies on monthly averaged climate data rather than hourly data, which can limit its precision in capturing extreme short-duration heat events. For highly complex buildings or nuanced overheating assessments, more sophisticated dynamic simulation tools might be preferred.

4.2 CIBSE TM59: Design Methodology for the Assessment of Overheating Risk in Homes

CIBSE TM59 (Chartered Institution of Building Services Engineers Technical Memorandum 59) provides a specific, widely adopted methodology for assessing overheating risk in residential buildings in the UK. It is increasingly referenced in Building Regulations (e.g., Approved Document O for England) as the preferred method for demonstrating compliance for complex or higher-risk dwellings. (atamate.com)

• Dynamic Thermal Modelling: Unlike simpler tools, TM59 mandates the use of dynamic thermal modelling (DTM) software (e.g., IES Virtual Environment, EnergyPlus, DesignBuilder). DTM simulates the thermal performance of a building on an hour-by-hour basis throughout a typical year, integrating detailed climate data, building fabric properties, internal heat gains, ventilation strategies, and occupant behaviour profiles. This granular analysis provides a far more accurate prediction of peak temperatures and periods of overheating.
• Compliance Criteria: TM59 defines two key criteria for assessing overheating risk:
  • Criterion 1 (Hours of Exceedance): For each living room, kitchen, and bedroom, the number of hours where the operative temperature (a combination of air temperature and mean radiant temperature) exceeds a ‘comfort temperature’ (defined as the outdoor temperature plus 3°C, but not exceeding 28°C) should not be more than 3% of the occupied hours during the period May to September. This criterion addresses prolonged periods of discomfort.
  • Criterion 2 (Peak Temperature Limit): The operative temperature in bedrooms should not exceed 26°C for more than 1% of the annual occupied hours (87.6 hours in a year). This criterion is specifically aimed at ensuring comfortable sleeping conditions, which are critical for health and well-being.
• Ventilation Strategy Assessment: TM59 requires careful consideration of the ventilation strategy, including both natural (window opening patterns, trickle vents) and mechanical (MVHR, extract fans). It incorporates assumptions about occupant behaviour regarding window opening (e.g., windows can be opened if external conditions permit, e.g., low noise, acceptable air quality).
• Future Climate Scenarios: For robust future-proofing, TM59 encourages the use of future weather files (e.g., CIBSE’s UK Climate Projections (UKCP) 2018 weather files) which integrate predicted climate change impacts, allowing designers to assess performance under anticipated warmer conditions.
• Limitations: The accuracy of TM59 assessments heavily relies on the quality of input data and the assumptions made (e.g., occupant behaviour, internal gains). Discrepancies between modelled and real-world performance can arise if these assumptions are not robust or if systems are not commissioned correctly.

4.3 Other Assessment Approaches and Regulatory Context

• Approved Document O (AD O) of the Building Regulations (England): Introduced in 2021, AD O specifically addresses overheating in new residential buildings. It offers two routes to compliance:
  • Simplified Method: For smaller, less complex dwellings, this method uses a set of prescriptive design rules based on location (urban vs. non-urban) and cross-ventilation potential. It specifies maximum glazing areas, minimum opening areas for ventilation, and requires external shading for dwellings in urban locations that are at higher risk. This method is a simplified proxy for avoiding significant overheating.
  • Dynamic Thermal Modelling Method: For dwellings that cannot meet the simplified method (e.g., large areas of glazing, single-aspect flats) or wish to demonstrate better performance, this route requires a detailed TM59 assessment.
• Post-Occupancy Evaluation (POE): While not a predictive tool, POE involves monitoring actual indoor temperatures and gathering occupant feedback after a building is occupied. This provides invaluable data on actual performance, helping to identify unforeseen issues and informing future design practices. POE is essential for closing the ‘performance gap’ between predicted and actual building performance.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

5. Health and Comfort Implications

Prolonged exposure to high indoor temperatures is not merely an inconvenience; it poses significant risks to human health, well-being, and overall quality of life. As climate change leads to more frequent, longer, and more intense heatwaves, these implications become increasingly critical, particularly for vulnerable populations. (uel.ac.uk)

5.1 Physiological Impacts

• Heat Stress and Heat-Related Illnesses: Exposure to high ambient temperatures can overwhelm the body’s thermoregulatory mechanisms, leading to heat stress. This can manifest as:
  • Heat Exhaustion: Symptoms include heavy sweating, cold, clammy skin, fast weak pulse, nausea, dizziness, and fatigue. If untreated, it can progress to heatstroke.
  • Heatstroke: A medical emergency characterised by a body temperature of 40°C or higher, hot red skin, altered mental state, and confusion. It can cause permanent disability or death if not treated promptly.
• Dehydration: Elevated temperatures increase fluid loss through sweating, leading to dehydration if not adequately rehydrated. Dehydration can cause fatigue, headaches, dizziness, and strain on the kidneys.
• Cardiovascular Strain: The heart has to work harder to pump blood to the skin for cooling, placing additional strain on the cardiovascular system. This is particularly dangerous for individuals with pre-existing heart conditions.
• Respiratory Issues: High temperatures can exacerbate respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD). If windows are kept closed due to external noise or pollution, indoor air quality can deteriorate, accumulating allergens, pollutants, and CO2, further aggravating respiratory problems.
• Sleep Disturbance: Elevated bedroom temperatures significantly disrupt sleep patterns, leading to insomnia, reduced restorative sleep, and subsequent fatigue, impaired cognitive function, and irritability. Studies have shown a strong correlation between higher night-time temperatures and poor sleep quality. (committees.parliament.uk)

5.2 Vulnerable Populations

The health risks of overheating are disproportionately borne by certain demographic groups:

• Elderly Individuals: Older adults have a reduced ability to regulate body temperature, may have underlying health conditions (e.g., cardiovascular disease, diabetes), and may be on medications that affect thermoregulation. They may also be less mobile or socially isolated, reducing their ability to seek cooler environments or assistance.
• Very Young Children and Infants: Infants and young children have less developed thermoregulatory systems and a higher surface-area-to-mass ratio, making them more susceptible to rapid temperature changes and dehydration.
• Individuals with Chronic Health Conditions: Those with heart disease, lung disease, kidney disease, diabetes, and certain neurological conditions are at significantly higher risk of heat-related illness.
• Low-Income Households: These households may live in poorer quality housing stock, have limited access to cooling measures, or face ‘fuel poverty’ where they cannot afford to run what limited cooling systems they might have, creating a cycle of deprivation and vulnerability.
• Individuals with Disabilities: Mobility impairments or cognitive disabilities can limit an individual’s ability to react to overheating or move to a cooler part of the home.

5.3 Psychological and Economic Impacts

• Reduced Productivity and Cognitive Function: High temperatures impair cognitive performance, leading to reduced concentration, slower reaction times, and an increase in errors. This affects productivity at home (e.g., for those working remotely) and overall quality of life.
• Increased Irritability and Stress: Persistent discomfort can lead to psychological stress, irritability, and reduced overall well-being.
• Economic Burden: The health impacts of overheating translate into increased healthcare costs due to hospital admissions for heat-related illnesses. Furthermore, if residents resort to active cooling systems (e.g., air conditioning) to cope with overheating, this leads to increased energy consumption and higher utility bills, which contributes to carbon emissions and can exacerbate fuel poverty.
• Depreciation of Property Value: Homes with known overheating issues may become less desirable, potentially impacting property values and rental income in the long term, especially as climate impacts become more pronounced.

5.4 Thermal Comfort Standards

Ensuring thermal comfort is not merely a matter of avoiding extreme temperatures but contributing to overall quality of life and well-being. Thermal comfort is often defined by standards such as EN 16798-1, which incorporates the Adaptive Thermal Comfort Model. This model recognises that people can adapt to their thermal environment to some extent (e.g., by opening windows or changing clothing), and acceptable comfort ranges vary with outdoor temperature. However, this adaptive capacity has limits, and once temperatures exceed these thresholds, discomfort and health risks escalate.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

6. Conclusion

The rising incidence of overheating within residential buildings, particularly the energy-efficient housing stock of the United Kingdom, underscores a critical design and policy challenge born from the laudable pursuit of decarbonisation. While enhanced insulation and airtightness are indispensable for reducing winter heating demands and carbon emissions, their implementation without concurrent consideration for summer performance has inadvertently created dwellings that function as effective heat traps during warmer months. The increasing frequency and intensity of heatwaves, exacerbated by the urban heat island effect, further amplify this complex issue, making the creation of climate-resilient homes an urgent imperative. (uel.ac.uk)

Effective mitigation of overheating risks demands a paradigm shift from a purely energy-efficiency-driven design philosophy to one of holistic building performance. This necessitates a multi-layered, integrated approach, prioritising passive design strategies implemented from the conceptual stages of a project. Key interventions include the strategic application of external shading devices to prevent solar gains at their source, the intelligent utilisation of thermal mass and advanced phase change materials to buffer internal temperature swings, and the meticulous design of advanced natural ventilation pathways that allow for effective night purging while addressing concerns of noise, security, and air quality. Mechanical ventilation systems, specifically MVHR, must be specified with fully functional summer bypass modes to avoid exacerbating the problem. Furthermore, material selection, such as solar-control glazing and high albedo external finishes, and the integration of green infrastructure through landscaping and urban greening, all contribute significantly to a cooler indoor and outdoor environment. Critically, empowering occupants with the knowledge and means to manage their internal environment through informed behavioural adaptations is an essential, albeit not singular, component of a robust overheating strategy.

To ensure that new and retrofitted homes are truly future-proof, precise and robust risk assessment tools are indispensable. Tools such as the Passive House Planning Package (PHPP) offer valuable insights into thermal performance, while the rigorous methodology of CIBSE TM59, often supported by dynamic thermal modelling software, provides a comprehensive and increasingly mandated standard for evaluating overheating risk under current and future climate scenarios. The recent introduction of Approved Document O to the UK Building Regulations signifies a crucial regulatory acknowledgement of overheating as a health and safety concern, guiding designers towards compliant, thermally comfortable homes.

Ultimately, the societal cost of inaction on overheating is substantial, encompassing not only direct health impacts such as heat stress, sleep disturbance, and exacerbation of chronic conditions, but also broader implications for productivity, mental well-being, and potentially increased energy consumption from burgeoning reliance on active cooling. By proactively integrating sophisticated passive design strategies, judicious material selection, and effective ventilation systems, guided by advanced assessment tools, it is entirely possible to create residential environments that are not only energy-efficient but also genuinely comfortable, healthy, and resilient in the face of a warming climate. This comprehensive approach is paramount to delivering on the promise of sustainable, habitable homes for future generations across the United Kingdom.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

References

• (uel.ac.uk)
• (arcc-network.org.uk)
• (nhbc.co.uk)
• (en.wikipedia.org)
• (build.saint-gobain.co.uk)
• (arcc-network.org.uk)
• (atamate.com)
• (nottingham.ac.uk)
• (committees.parliament.uk)