How to retrofit houses to reduce overheating

Summertime overheating in new-build, existing and retrofitted domestic building stock is a growing public health issue in the UK and will get worse with the predicted rise in global temperatures and more frequent, intense and long-lasting heat waves.

In summer 2022, the UK experienced an unprecedented 40°C heatwave, causing 3,271 deaths in England and Wales. By the mid-2030s, 90 per cent of the UK’s housing stock could be susceptible to overheating. UK Building Regulations do not require buildings to pass the overheating criteria for predicted weather in future years. Flats are particularly at risk, partly because of their smaller size, shared external surfaces, lack of cross ventilation and urban location.

Severity of overheating can depend on several factors including building form and design, location and occupant behaviour. Additionally, higher insulation and airtightness standards aimed at reducing carbon emissions can increase overheating risk if not coupled with passive cooling measures. The problem can be mitigated by thoughtful design and retrofit.

There are several strategies for building resilience and reducing the risk of overheating in existing homes. These include reducing heat build-up within the microclimate of the building, minimising the impact of direct solar radiation on the building and limiting internal heat gain. Broadly speaking, overheating resilience measures can be categorised as either passive or active solutions.

Passive solutions

Natural ventilation Opening windows to allow airflow and heat escape can be effective, especially when cross-ventilation can be used. However, occupants may not want to take advantage of natural ventilation because of concerns over noise, security, bugs, smells and dust, so these concerns need to be considered. The ability to purge heat at night is a necessary feature of natural ventilation in the summer and can be especially important in thermally massive buildings. Passive cooling is more effective when coupled with other methods to reduce initial heat gain such as shading or cool envelopes. In existing buildings, operable skylights in the loft can provide a stack effect to exhaust heat buildup.

Shading This is by far the most effective passive method of reducing overheating. Shading can be achieved through balconies, simple overhangs, fins, louvres, PV arrays and awnings. The primary goal of a shading product is to reduce unwanted solar energy – heat – from entering a building or room. This means external shading is significantly more effective than internal shading. When designing shading, there are other energy-related consequences to consider. For example, shading should not block beneficial solar gain in winter (heating season) and it should not block natural ventilation or minimise daylight exclusion. Examples of synergies with shading devices include balconies, which can provide both shading and areas of refuge when buildings get overheated; photovoltaics as the angle for shading direct sunlight is also the most effective angle for capturing energy via photovoltaics. Movable shading in any of these forms gives the user more control over the benefits and potential limitations.

Green and cool surfaces Increasing the surface reflectance of the exterior face of the fabric, including the roof and walls, is highly effective at reducing the transfer of heat through the fabric from direct solar radiation. Planted green surfaces can also slow the transfer of solar radiation through the fabric and release heat through evapotranspiration. Green and reflective surfaces are also more extensively beneficial in that they also reduce the heat within the microclimate surrounding the building.

Active solutions

Fans These are an inexpensive way of improving comfort while consuming very little energy. Fans do not cool the air, but they can draw in cooler air from outside, increase heat loss from spaces coupled with natural ventilation, and provide a sensible cooling sensation for individuals. Fans increase the comfort of occupants, not by decreasing the indoor temperature but by removing heat from the body. Where active cooling is used, this effect can reduce cooling demand. Additionally, ceiling fans at low speeds (below 0.2 m/s at head height) can help de-stratify warm air, reducing heating costs.

Heat pumps Where passive measures are insufficient, heat pumps can reverse refrigerant flow, thereby removing heat from the interior environment.

Hot water pipe placement Internal heat gain can be prevented by carefully considering the impact of hot water pipes, where they go, their insulation levels, and their potential contribution to internal heat build-up. Solutions include increased pipe insulation and ventilation in areas where hot water piping is routed. Mechanical ventilation with heat recovery also needs to be equipped to disable heat recovery in the non-heating season (summer bypass) and occupants need to know how to make this happen.

Climate resilience measures in energy retrofits

We can integrate climate resilience measures as part of energy retrofits using the following approaches.

Light-touch assessment for energy retrofits An assessment can identify key factors that contribute to overheating risk and potential mitigation measures. The Good Homes Alliance overheating tool for retrofits is a risk-based tool developed by GHA, Oxford Brookes University and partners, and is a light-touch assessment tool intended for use at the early stages of residential retrofit projects or on existing homes.

Detailed assessment for high-risk/vulnerable homes Beyond light-touch, these more detailed assessments can include the Simplified Method based on design criteria as reflected in the new Part O of the Building Regulations, steady state models, eg, SAP-based assessment, and/or dynamic thermal simulation and thermal comfort criteria following the CIBSE TM59 approach. These models can demonstrate the impact of overheating and the benefit of adaptation measures. When coupled with visual modelling software, they can also tackle barriers of aesthetic concern and the difficulty in communicating and convincing building owners and occupiers of the need to mitigate future risk.

Indoor temperature monitoring using smart technology in high-risk retrofits This can be installed in different rooms of a building post-retrofit. Such a system could operate as a notification alert for able occupants to operate shading, ventilation, turn off heat etc, or as a heat alert system protecting the elderly, ill and disabled by alerting them and their carers to impending elevated temperatures.

Provision of user manuals to manage heat in summer This will likely look like initial walkthrough explanations and a homeowner’s manual with clear operational instructions with days of the year identified as reminders. Effective overheating avoidance may require seasonal behavioural shifts, eg opening or closing an external shading device or setting summer bypass on heat recovery ventilation along with changing filters.

To avoid excess summer deaths, especially amongst the most vulnerable, it is vital that climate resilience and retrofit efforts are integrated so that avoidance of summertime overheating becomes mainstream.

Rajat Gupta is professor of sustainable architecture and climate change, and director of the Oxford Institute for Sustainable Development, Oxford Brookes University

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