Overheating in Historic Buildings

Thermal comfort and summertime overheating

Whilst thermal comfort is subjective, well-established measures can be used to predict what is comfortable for most people and their indoor activities. Thermal comfort can be influenced by a range of factors including airflow, humidity and radiant heat; however, the internal air temperature provides a strong overall indication of whether the internal environment is comfortable or not.

Heat build-up and high temperatures can arise from inside or outside a building. Internal heat gains can be generated by lighting, equipment and people. External heat gains are driven by weather conditions. Maintaining summertime comfort is a growing challenge as warmer weather drives up external heat gains.

Identifying overheating

The first step in addressing overheating is to identify if, how and where it occurs.

Occupancy feedback

Engaging with building occupants is crucial to understanding the nature of overheating issues. Users can highlight certain rooms or parts of rooms that overheat regularly and when. Taking on board their feedback can help develop incremental and targeted mitigation strategies, rather than a broad-brush approach across the whole building.

Temperature monitoring

Temperature logging is important. Recorded data can indicate if a building is too warm, how warm it gets, when it cools down, and how quickly temperature swings occur. Compare the temperature data with industry standards and guidance to gain a deeper understanding of overheating issues.

Tackling overheating

In addressing overheating, aim to strike a balance between winter and summer comfort levels. Take retrofit measures that will not poorly adapt buildings for either seasonal extreme. Any retrofit measures must not lead to further building conservation and management issues.

The building environment

Understanding the building’s environment is important in planning an effective mitigation strategy to improve thermal comfort. Every building’s microclimate is unique and depends upon its location, aspect, exposure to sun, prevailing winds and rain, and other environmental factors such as other buildings and trees nearby.

Minimising internal gains

Central heating controls

It is essential to ensure that heating is controlled adequately during summer months. Inadequate control of central heating systems can often be the main contributor to the internal temperature being too high during the daytime.

Heating should be turned off during warm weather and not controlled via setpoints which turn the heating on or off to a specified temperature. Setpoint control of thermostats can boost internal temperatures and exacerbate the impact of heat gains.

Insulate hot water pipes

Whilst the heating demand should diminish during summer months, use of domestic hot water may not. Where possible, insulate any exposed domestic hot water pipework to prevent the heat from it contributing to the internal heat build-up. Likewise, lagging hot water cylinders and storage tanks will minimise the heat they emit.

Install more efficient electrical equipment

All electrical equipment generates heat due to electrical resistance. Replacement of traditional lighting, such as fluorescent or halogen, with more efficient LED lighting will help reduce internal gains. Use of other equipment such as servers, computers and televisions will generate heat. Install high-efficiency equipment where possible to reduce this.

Minimising external gains

External gains may be in the form of solar radiation (sunshine) and external air temperatures. Understanding the orientation of a building and its surroundings can provide a useful insight into the way sunshine can heat up its structure and enter through windows and openings. Passive measures such as external or internal window blinds are best for minimising the impact of direct solar gain and may be used seasonally to benefit from direct sunshine in the winter. Glazing plays a significant role in admitting external gains into a building and should be the focus of any mitigation measures. However, good ventilation is also needed to provide comfortable conditions.

External shading – awnings, canopies and shutters

Blocking the path of direct sunshine by using awnings, canopies or shutters prevents infrared radiation from heating up objects inside the building. Whilst they are very effective in dealing with external solar gain, blocking out sunshine may create demand for additional artificial lighting. Installation of awnings, canopies or shutters on historic buildings may be subject to listed building consent. Check first to find out what permissions you'll need.

Internal shading – blinds, curtains and shutters

Internal shading provides a physical barrier that reflects and partially absorbs external solar gains whilst providing good control for occupants. As such, internal shading provides flexibility in balancing solar gain with the need for natural daylight, though it may still necessitate artificial lighting in occupied spaces of the building. Internal shading is a less effective barrier against heat than external shading as it can heat up and radiate heat into the occupied space.

Solar control film

Treatment of windows with solar control film limits the admittance of infrared radiation while allowing some visible light to pass through. Solar control films vary based on reflectivity, aesthetic impacts and reduction to daylight. Visually, solar control films may change the aesthetic of a facade by creating a tinted window effect and careful consideration must be paid to the character of the building. Any aesthetic change to a listed building may require listed building consent. The application of solar film is not a temporary solution and therefore it may diminish the benefit gained from direct sunshine during colder weather.

Dealing with heat build-up in a building

If a building feels too warm, there are several strategies using ventilation and thermal storage that can help lower the temperature to a more comfortable level.

Natural ventilation

Natural ventilation tackles overheating by bringing external fresh air in through open windows. The throughput of moving air can improve comfort and promote a more stable temperature across the building. Whilst this has benefits in minimising entrapment of warm air, it is most effective when the air temperature outside the building is lower. Security considerations may rule out the use of natural ventilation in unoccupied areas. 

Mechanical ventilation

Electrical fans can channel air around a building. Using mechanical ventilation offers some advantages: you can monitor the flow rate of air and it doesn't depend on external conditions in the same way as natural ventilation. Also, mechanical ventilation can make better use of lower external temperatures at night-time. This is referred to as ‘night cooling’ and can provide some relief against overheating during warm spells. Many buildings remain warm, even when the external temperature decreases during the night, which compounds daytime overheating problems. Night cooling can provide a means to overcome this.

Despite its merits in effective control of fresh air, retrofit installation of mechanical ventilation can be challenging. Additional space will be required for the routing of new ductwork and installation of air handling equipment. Structurally, this may have technical implications on the load-bearing capacity of existing structures and may be physically invasive to existing historic building fabric or cause permanent damage.

Old ventilation equipment may be of historic interest. Consider recording and conserving any building services before making modifications.

Mechanical cooling (air conditioning)

Mechanical cooling differs from mechanical ventilation by using a chiller to deliver cool air to a space. This involves installation of a chiller as well as ducting and air handling units which may have load-bearing capacity implications (see mechanical ventilation section above). Mechanical cooling is an energy-intensive process and therefore may not be considered as a sustainable solution, although controlled temperature environments may be critical for collection and archive conservation.

Government guidance is available about inspection, maintenance and cleaning programmes to provide healthy and comfortable environments for building occupants, limiting the escape of refrigerant gases and ensuring the safety of equipment. As many of the refrigerants used in chillers are ozone-depleting and/or hazardous gases, take care when disposing of or recycling redundant equipment.

Thermal mass

Heavy and dense materials can be very effective at absorbing thermal energy without exhibiting a significant change in temperature or experiencing large temperature swings. Historic building materials such as stone, and the thick walls of traditionally constructed buildings, have high thermal mass which allows them to remain thermally stable. Exposed stone or brickwork can help the absorption of thermal energy and act as a buffer to outside temperature fluctuations.

Phase-change materials

Phase-change materials (PCMs) are engineered materials that have high thermal mass and can stay at the same temperature whilst absorbing large amounts of thermal energy. They convert this thermal energy by changing their internal properties and the result is that the internal air temperature remains unaffected.

If you are planning an extension to a traditionally constructed building it may be worth considering incorporating wall linings or ceiling tiles made of PCMs into the design.

Overheating building regulations and guidance

Building Regulations Part O (2021) addresses overheating in newly constructed domestic buildings but does not account for existing and non-domestic buildings. In recognition of the different comfort conditions within domestic and non-domestic buildings, the Chartered Institution of Building Services Engineers (CIBSE), provide the following technical memoranda to identify and minimise overheating:

TM52: The limits of thermal comfort: Avoiding overheating in European buildings (2013)

TM59: Design methodology for the assessment of overheating risk in homes (2017)