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Last edited 12 May 2019
Overheating in residential properties
New homes 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.
- Addressing issues around the quality of new homes, the performance gap and overheating.
This article focuses on the issue of overheating in homes, providing a description of the problem, information on existing guidance and guidelines.
 Overheating and thermal comfort
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 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.'
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 homeostasis 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.
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
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 25°C provided as a threshold temperature above which mortality rates will increase.
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 26°C 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 and 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 Standard Assessment Procedure (SAP) compliance tool and the usage of dynamic thermal modelling tools following CIBSE Guidance on overheating.
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.
 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.
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.
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.
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.
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.
 Related articles on Designing Buildings Wiki
- BSRIA articles on Designing Buildings Wiki.
- Evolving opportunities for providing thermal comfort.
- Heat stress.
- Home Quality Mark high temperature reporting tool.
- Light shelf.
- Overheating - assessment protocol.
- Preventing overheating.
- Thermal comfort.
- Thermal pleasure in the built environment.
- Tips for staying cool in buildings.
- Urban heat island
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