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Overheating in Residential PropertiesDecember 2017

Tassos Kougionis, Sustainable Building Consultant, Principal Consultant – Residential

Define, identify and prevent: an overview


New homes of today need to meet high quality standards, provide a healthy and safe environment for the occupants, be energy efficient and sustainable.

In terms of existing homes, incentives such as ECO, the former Green Deal, local initiatives and the introduction of future policies as in the case of minimum EPC ratings for rental properties, also support their continuous energy efficiency and overall improvement.

Construction standards are driven by the Building Regulations, European Directives, Local Authorities plans and requirements posed by the stakeholder in support of improving their overall brand reputation.

Challenges faced by the sector today include:

  • A request for increased new homes delivery to meet the government’s target of delivering more than 250,000 new homes a year and,
  • Addressing issues around the quality of new homes, the performance gap and overheating.

This article is focusing on the issue of overheating in homes, providing a description of the problem, information on existing guidance and guidelines.

Overheating and Thermal Comfort

Experiencing overheating at home can directly impact the health and wellbeing of the occupant, their productivity and can turn into a costly retrofit in the future.

Defining overheating at homes has proven quite challenging due to its complex and subjective nature. A lot of work on overheating was conducted by the Zero Carbon Hub during 2014-15 which led to the following working definition used by the Zero Carbon Hub in its publications.

“The phenomenon of a person experiencing excessive or prolonged high temperatures within their home, resulting from internal and/or external heat gain and which leads to adverse effects on their comfort, health or productivity” Zero Carbon Hub, overheating definition.

From a purely biological point of view, the human body has the ability to adjust and adapt to a range of thermal conditions exposed to. This is achieved through the various thermoregulatory mechanisms maintaining the physiological homeostatis during different activities or at rest.

In that sense one can think of the human body as a residential property with a central boiler system
installed within and running constantly. The circulation pump is the heart, while the blood vessels act as the heating pipe network and the different tissue layers acting as insulation.

In terms of cooling the three main mechanisms in the body include cutaneous vasodilatation, which dissipates heat by radiant and convective heat loss and sweating, which releases heat by evaporation.

Overheating describes thermal conditions that are perceived as ‘uncomfortably hot’ by the person experiencing it. This can be the result of exposure to high–peak-temperatures for a short period of time or, prolonged exposure to averagely high temperatures.

"Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation." (ASHRAE, 2013).

Green areas in the urban environment can contribute to the mitigation of the Urban Heat Island and have a positive impact on people's health and wellbeing

Experiencing overheating is associated both with health concerns as well as the feeling of discomfort which can affect the wellbeing and productivity of the individual.

One of the most important contributors and a leading authority, in the understanding of human thermal comfort within buildings, was P.O. Fanger (1934-2006).

Fanger studies introduced the concept of Predictive Mean Vote (PMV) comfort index (Fanger, 1973), which has become one of the most recognised comfort models used across the world today, principles of which is also used also by ASHRAE Standard 55 and BS EN 15251:2007.

Overheating and Public Guidance

Even though current Building Regulations do not cover overheating explicitly, references and guidance exists in a number of government documents.

The Housing Health and Safety Rating System Guidance published by Government on 2006 described overheating as threats from excessively high indoor air temperatures (DCLG, 2006). Excess indoor heat is characterised as a hazard in the document with 25C provided as a threshold temperature above which mortality rates will increase.

Urban heat island – London

The Heatwave Plan for England acts as a good practice guide protecting the public from overheating hazards. Within the Heatwave plan it is advised that cool rooms maintaining temperatures below 26C should be provided in the case of hot weather in hospitals, care/nursing homes and other residential environments occupied by vulnerable individuals (NHS England, 2015).

The London Plan requires the risk of overheating to be demonstrated during planning applications of major developments. More specifically, London Plan Policy 5.9 sets out an overheating and cooling hierarchy for developments and buildings (GLA, 2016).

Overheating Assessment Methods and Standards

In terms of Thermal Comfort & Overheating assessment methods, these include simple internal / external temperature models, predictive models like in the case of the Fanger’s PMV and adaptive comfort models.

The two methods most frequently used in the UK for the assessment of overheating at homes include the usage of the steady-state conditions SAP compliance tool (Standard Assessment Procedure) and the usage of dynamic thermal modelling tools following CIBSE Guidance on overheating.

SAP is a compliance tool mainly focusing on the energy rating of homes. Appendix P of SAP, which assesses the internal temperatures of homes in summer.

Dynamic thermal modelling follows a more sophisticated approach allowing the assessors to use additional variables and specify important parameters that best describe each building’s particular case.

Design standards often used for the prediction of overheating include guidance provided by ASHRAE55, BS EN 15251:2007 (for free running buildings) and CIBSE including CIBSE Guide A 2006 & 2017, CIBSE TM52:2013, and CIBSE TM59:2017.

The most recent CIBSE TM59:2017 standardises the assessment method for dynamic thermal modelling of overheating at homes.

Design, Construction and Handover Guidelines

Identifying the right design strategy strongly links to the experience and expertise of the design and building engineers involved in the project. Current minimum Building Regulation ventilation requirements may not suffice and will not cover overheating.

Concerns around overheating may lead to additional policies and regulations being introduced by government in the future, if industry solutions fail to address the problem.

In terms of modelling it is strongly advised that appropriate thermal models, with correct input data, are developed and reviewed at an early stage of the design strategy and are being kept up to date as the project progresses.

Changes during the construction stage, due to value engineering, procurement or simply on-site ‘quick solutions’ need to be carefully documented and communicated back to the design team.

The Housing Supplementary Planning Guidance, published in March 2016, discusses property types that might be more susceptible to overheating or be affected by noise, as is single aspect flats.

As mentioned in the report dual aspect dwellings are favoured due to the ability to cross-ventilate while for dwellings that cannot have openable windows due to poor air quality or noise restrictions, careful consideration needs to be given to the location of air intake units and any increased potential for overheating in the summer due to the reduced opportunities for natural ventilation.

In terms of building services and HVAC systems, these need to be installed, tested and commissioned properly by certified engineers. BSRIA’s technical library offers a wealth of information on a variety of systems testing and commissioning requirements.

A good and efficient ventilation strategy can assist in heat dissipation and maintenance of good indoor air quality

Good construction quality and site conditions are critical for an efficient and well-thought through design to translate into a good end product. Potential shortfalls throughout the process may lead to a Performance Gap, introducing a hard to evaluate performance impact including the likelihood of the property to overheat.

Our homes should be easy to run and maintain. With complex controls, new building services and solutions used today the occupant might fail to operate the property as per the original design intent.

This raises the question of how user behaviour is considered within the original design regime and how effective hand over methods can be used to communicate critical home operation information to the occupant. A good practice example is the introduction of Building Manuals and Building User Guides.

“A building manual condenses important documents relating to the building such as O&M manuals, health and safety documents, standard operating procedures, which is then used to compile a Building User Guide” BSRIA 2011.

Finally – collecting feedback on the performance of existing properties, through research, both in terms of indoor conditions data monitoring but also through occupant surveys, is vital in identifying overheating occurrences, potential causes and in developing appropriate solutions.


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