Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Introduction to Overheating ___ TACKLING OVERHEATING IN HOMES Contents ___ 1 Introduction 2 Defining overheating 3 What causes overheating 4 Risks factors to look out for 5 Further information This pack has been produced by the Zero Carbon Hub, in partnership with the Environment Agency’s Climate Ready Support Service, Public Health England, CITB, Cambridge City Council, Kingston upon Hull City Council, Southend Borough Council, and the Structural Timber Association. 2 Who is this pack for? Those working in… ___ Sustainability, energy efficiency and climate change Environmental health New development Asset management and maintenance Operations and customer services Risk management, research and business planning 3 1 Introduction ___ It is clear that the goal of delivering It is therefore important for those with year-round comfort, and avoiding housing responsibilities to feel confident overheating in homes, is a subject they have appropriate processes in which is rising in profile. place to address the risk of overheating This slide pack has been created by the Zero Carbon Hub to provide Housing Providers with a brief, non-technical introduction to the subject. It explores occurring within their own stock. For some, limited action may be needed in the short-term. For others, tackling this issue may be more pressing. how overheating is defined, what Why take action? If unwanted heat builds causes it and some of the risk factors to up inside homes, not only can the look out for. occupants become uncomfortable and Although the majority of the UK’s building stock does not currently overheat, there are properties with sleep deprived, but health issues can develop, particularly in vulnerable groups such as the elderly and young infants. characteristics making them more prone to high internal temperatures. This pack is part of a suite of Indeed, research suggests that up to materials produced by the Zero 20% of homes in England may already Carbon Hub intended to support overheat. As the climate across the UK Housing Providers in managing changes in the coming decades, overheating risk. See overheating is expected to become www.zerocarbonhub.org much more common. The information provided does not constitute legal advice. 4 2 Defining Overheating ___ Overheating, in general terms, describes situations where the temperature inside a person’s home becomes uncomfortably or excessively warm, including for prolonged periods. Researchers have tended to find that overheating occurs when issues with the design of the house make it difficult for the occupants to keep unwanted heat from the sun out, or difficult to get rid of the warm air and heat which has accumulated inside the building. Or both. For example, a property may have window openings that are too small to achieve the ventilation rates needed to expel or “purge” excess heat. Although overheating happens most often during warm weather in the summer, it can happen in cooler months too. It is not limited to heat wave periods. That is why it is important for organisations to get to grips with the issue and plan their long-term strategy for managing it. Thermal comfort ___ As hinted at, overheating is sometimes For example, guidance produced by the judged to occur when a person reports Chartered Institution of Building Services feeling “too hot” in their home. It Engineers (CIBSE) states that it is desirable therefore relates primarily to internal that bedroom temperatures at night should temperatures and levels of “comfort”. It is not exceed 26 °C, to help avoid sleep not a measure of outside temperatures, disturbance and overheating. although the weather outside does influence the conditions inside. Different internal temperature thresholds are allowed in living rooms. These thresholds Alternatively, “thermal comfort” can be are permitted to vary depending on recent assessed objectively using criteria. outdoor temperatures. This is called the If certain rooms exceed pre-defined Adaptive Thermal Comfort Model. See more temperatures for specified periods of time, by following the link below. then overheating is deemed to occur. Whilst there is no official legal definition of overheating in the UK, industry guidance is available. For more information see our “Defining Overheating” evidence review. Such thermal comfort thresholds are used by many designers in the construction sector to inform design choices. Modelling software is also frequently used to determine if the property or scheme in questions passes such thermal comfort criteria. CIBSE, Environmental Design, Guide A (2015) http://www.cibse.org/knowledge/cibse-guide/cibse-guide-aenvironmental-design-new-2015 6 Another way used to assess whether a If the internal temperature (averaged property being constructed is at risk of over day and night) is calculated to be overheating is the “overheating check” above 23.5 °C, it is determined to have in the Government’s Standard a “high risk” and may fail Building Assessment Procedure (SAP). Control checks. The overheating Energy assessors, when carrying out their SAP assessment, calculate the calculation is found at Appendix P of the SAP methodology. For more discussion on the advantages and limitations of using certain risk assessment methods see our “Assessing Overheating Risk” evidence review. risk of the dwelling having high internal Note that building designers use See more on the Appendix P temperatures in June, July and August. Appendix P with some caution as it is methodology on the BRE’s The property is given a risk rating of not intended to inform detailed design website: low, medium, or high. decisions. It is a basic compliance http://www.bre.co.uk/filelibrary methodology for energy performance /SAP/2012/SAP-2012_9-92.pdf assessments. 7 As might be expected, the health sector Environmental Health Officers (EHOs) tends to think about overheating in are guided by a framework called the terms of the possible consequences for Housing Health and Safety Rating people’s health. System. The risk of “excess heat” is The focus is on determining the temperatures at which people start to fall ill or lead to fatalities. Such thresholds differ from person to person depending on many factors, such as included as a hazard, and EHOs have legal powers to order housing providers to make changes to properties to make them safe to live in if they are assessed as a hazard. their age and ability to acclimatise to 25 °C is quoted in the guidance as the changes in their environment. level at which mortality can start to rise. Both sudden spikes in temperature and prolonged period of excess heat can be difficult for people to cope with, especially if they have an underlying health condition. See guidance on the Housing Health and Safety Rating System at: https://www.gov.uk/government/publications/housin g-health-and-safety-rating-system-guidance-forlandlords-and-property-related-professionals 8 There is currently no statutory provision” has been made to “limit heat maximum internal temperature for new gains” in new dwellings. dwellings in Building Regulations in England or Wales, or in health and safety guidance. The section, called Criterion 3, states that “reasonable provision” can be demonstrated by achieving a low or It is therefore worth taking advice about medium risk rating in the SAP the range of different ways of Appendix P overheating check. There measuring and assessing overheating is no similar requirement for existing and deciding which is most relevant for dwellings. your organisation. The Zero Carbon Hub has also been coordinating views on whether a sector-wide definition can be agreed. We published a discussion paper on this in April 2016. However, provisions in Approved Organisations sometimes choose different ways of assessing risk, for example by using detailed dynamic simulation modelling or the Passive House Planning Package to check whether overheating criteria are met. Document Part L1A of Building Regulations require developers to demonstrate that “reasonable 9 3 Causes ___ Overheating occurs when too much heat builds up inside a building – from People are always seeking a external sources such as the sun, or comfortable balance between from internal sources such as being too hot and too cold. appliances, hot water pipes or people. This heat cannot then be easily rejected or removed. A building’s location, how it is orientated, how it is constructed, how well it is ventilated, how it is heated, and how it is used, all contribute to whether an issue is likely to occur. 10 External temperature ___ As would be expected, high external air temperatures and the sun’s heat can affect the conditions inside a home. If external temperatures are high, the fresh air brought into buildings – either through windows or provided by mechanical ventilation – will also be warm. Heat from the sun will also warm up the fabric of the building (although, when night-time temperatures are cool, this heat should be released). As outside summertime temperatures vary across the UK, the risk of overheating also varies. The temperature in London and the South of England is usually hotter than in, say, Edinburgh, contributing to a higher risk of overheating in buildings in the South and South East of England. However, cases of overheating have been observed across the country. Orientation ___ There are many detailed design choices South-west and west-facing rooms are Window size and orientation are that will affect the internal temperatures considered to be at greatest risk as they therefore key risk factors for overheating. in a building once in use. For example – receive direct sunlight in the late Identical homes with different designers must consider how to size and afternoon, after the building has already orientations can have very different position windows so they are optimised become warmed by the sun all day. internal environments. South-facing rooms can also experience There are many shading devices and If windows are not shaded, direct energy overheating, but the windows are glazing solutions available to help to from the sun will be transmitted through, generally easier to shade due to the high deal with this risk. See more in our warming up the inside of the building. angle summer sun. “Solutions to Overheating in Homes” for each orientation. evidence review. 12 Location ___ Buildings in dense urban areas are generally at higher risk of overheating due to the Urban Heat Island effect, and also if the building’s “microclimate” is particularly warm. During the day, hard surfaces such as tarmac, brick and concrete, absorb and store heat from the sun. This heat is then re-emitted at night, keeping built-up areas warm. Temperatures can be 5-9 °C higher in cities at night compared to the surrounding rural areas. Residents in the centre of cities may also be less able to keep windows open at night due to noise, Land Surface Temperature in London at 21:00 on 12 July 2006. Image from the Local Urban Climate model and its application to the Intelligent Development of cities (LUCID) project. See the ARCC Network’s website for more information and a list of organisations working on the project: www.arcc-network.org.uk pollution or for security reasons. This makes it more difficult to cool homes when people are trying to sleep. Careful consideration of how to provide secure and effective night-time ventilation is therefore especially important when designing homes in dense urban areas. 13 Internal sources of heat ___ 14 Inadequate ventilation ___ Image from the Good Homes Alliance, “Preventing Overheating” (2014) Mechanical ventilation ___ A growing proportion of new homes use mechanical ventilation systems, including Mechanical Ventilation with Heat Recovery (MVHR). The aim of such systems is normally to provide good indoor air quality – in compliance with Approved Document Part F of Building Regulations - rather than to cool the home. Current guidance from the NHBC Foundation suggests that air flow rates of 4 to 5 air changes per hour are needed for thermal comfort ventilation – which is much higher than most systems are designed to deliver, even in boost mode. Consideration should be given to how to purge hot air using natural ventilation, even when mechanical systems are used. 16 Thermal mass ___ Heavyweight building materials, such as brick, stone or concrete have the capacity to “soak up” and store heat – or cold. This ‘thermal mass’ can be used to maintain more uniform temperatures inside a building. The heat from the day is stored, then released at night when the temperatures are cooler. As a result, internal temperatures should have lower peaks, and only reach these peaks once outside temperatures are relatively cool. However, this mechanism is less effective in locations where the swing between day time and night time is less pronounced, or people are unwilling or unable to naturally ventilate their homes at night, e.g. in major cities. Other ventilation and/or cooling solutions could be needed to realise the benefits of the thermal mass. . Top image by BRE, bottom image by Alan Clarke 17 In summary, problems arise when we struggle to prevent unwanted heat entering the home, or struggle to get rid of it Options for tackling overheating include: Limit sources of heat gains – External Limit sources of heat gains – Internal Provide capacity to purge excess heat 18 4 Risk factors ___ Researchers from the Community Resilience to Extreme Weather (CREW) project found that overheating exposure varies up to a factor of ten times between types of dwelling. It can also be significantly greater for residents who are at home during the daytime, such as the elderly. See more at www.arcc-network.org.uk The risk of overheating occurring will Using common sense to look out for vary from building to building depending “red flags” in the early stages of on whether causes, such as those construction or major refurbishment outlined, are present. projects should help you to decide Most dwellings which have a high chance of overheating have recognisable risk factors. Some of these are set out in the next section of whether further in-depth modelling is needed, and what overheating mitigation strategies might be most effective. this pack, although not all will be Interviews by the Zero Carbon Hub relevant to every organisation. It is team with Housing Providers suggest hoped these will be developed further some are already using their by technical experts to fully support experience and knowledge to identify Housing Providers in carrying out risk riskier buildings. assessments. 19 “In many cases, the risk of overheating may be low and no action will be necessary for a Housing Provider. However, unless [they] assess their potential current and future levels of risk methodically, there is potential to be ‘caught out’ by changing external circumstances such as the climate.” TECHNICAL EXPERT 20 Look out for: Location ___ 1. The property is located in the South or South East of the UK 2. The property is located in a dense urban area with a “heat island” effect 3. The location of the property means it is likely to be impractical for occupants to open their windows Average external air temperatures are highest in the south of the UK. The further south you go, the more conscious of the potential for overheating you need to be, especially if other factors such as heat Island effects are also present. However, it should be noted that cases of overheating have been observed across the country, and are not limited only the South of England. External air temperatures in urban centres are significantly warmer than the surrounding rural areas, especially at night. This is known as the Urban Heat Island effect. Generally, the closer you are to the city centre, the higher the temperature will be. For example, in Birmingham, during the August 2003 heat wave, the night-time temperature, measured at a semi-urban location 3 km south-west of the city centre, was 4.7 °C warmer than outside the city. For example, buildings located next to busy main roads or train tracks may mean residents will not open their windows to ventilate the property due to concerns about noise or pollution. This issue is even more pronounced in ground floor properties where security also more of a factor. For properties in such areas, it may be more challenging to provide a secure means of night-time ventilation, at adequate levels. Designers need to give this issue careful consideration. 21 Look out for: Micro-climate ___ 4. Air drawn in from outside is accidently pre-warmed 5. The property is surrounded by heatabsorbing hard surfaces 6. The building’s materials create a warm “boundary layer” Exhaust air outlets from airconditioning and chiller units release hot air into the immediate environment. Even within urban areas, external air temperatures can vary significantly over relatively small distances depending on the amount of dark buildings and hard surface materials absorbing heat, or the shape of streets and whether they trap heat. Conversely, green space and open water can provide a considerable cooling effect. The external surfaces of a building also absorb heat from the sun. Depending on the nature of the materials and the prevailing weather conditions, this can result in a warm boundary layer next to the building, consisting of air which is hotter than the average external temperature. For properties in mixed use developments, it is sensible to avoid locating ventilation inlets or window openings directly above air conditioning units from commercial properties below (or other heat sources). Otherwise, pre-heated air may be drawn into them and significantly reduce the cooling effect of the ventilation. If opening the windows draw in this hot boundary layer air, it will heat up rather than cool the home. Ventilation intakes sited within this warm boundary layer can therefore cause problems. 22 Look out for: Built form ___ 7. High-rise apartment blocks with many small flats 8. Conversions, particularly loft conversions and rooms in roofs 9. Very air-tight properties Small flats are particularly prone to overheating. They tend to be more densely occupied than larger houses, and studies have shown that a high percentage do not have effective cross ventilation. Older homes sub-divided and converted into flats, or loft conversions, can suffer from overheating. Modern high standards of fabric insulation and air-tightness help keep heat in our homes during winter. However, outside the heating season, occupants also need to be able to get rid of excess heat generated inside the home from solar gains, people, electrical appliances and heating and hot water systems. Opening windows fully for ventilation in high-rise buildings can also be problematic due to high winds and safety concerns. Window restrictors are usually fitted. Subdivision of existing properties can close off previously effective cross ventilation routes and, particularly in the case of loft conversions and upper floors, where the roof is not insulated to modern standards, the heat from direct sunlight falling on the roof can be directly transferred into the home and cause overheating. Sufficient, controllable ventilation, for example by opening windows, is key to removing this excess heat which would otherwise become trapped and cause overheating. 23 Look out for: Orientation ___ 10. Properties with large areas of un-shaded glazing on the east, west or southfacing elevations 11. Bedrooms located on the west or south-west facing side of the property 12. Dark coloured, un-shaded, un-insulated walls Radiation from the sun transmits quickly through glass if it is not shaded or treated. This can very quickly heat up the rooms inside. If bedrooms are located on the sides of a property which are hit by the sun all day, particularly the late afternoon sun, then they may become very warm by the time the people living there go to bed. The ventilation strategy would need to work harder. Walls continuously and directly hit by the sun will absorb heat and gradually warm up. The amount of sunlight falling on a window depends on its orientation, as well as how high the sun is in the sky (the latter varies throughout the year). Otherwise identical homes but with windows that face in different directions can have significantly different internal environments. Homes with windows facing west or south and which are exposed to the sun are at high risk, as are homes with east-facing windows, which are occupied during the daytime. Some designers actively choose to locate bedrooms of the north side of the property to minimise this effect. The amount of heat absorbed and transmitted through into the interior of the home depends on factors such as the thermal mass of the materials. Insulating such walls slows down the transfer of heat to the inside. But if walls are made of dark coloured materials, more heat will be absorbed. Light-coloured materials would instead reflect much of the incident sunlight. 24 Look out for: Ventilation ___ 13. Windows cannot be safely secured in an open position 14. The property has small window opening areas 15. The property has window openings on only one side of the property Properties where the windows cannot be opened at all are problematic. This may be a design choice (for acoustic reasons), the result of poor design, or windows may have been accidentally fixed shut. Windows must be well-designed and large enough to allow sufficient ventilation to reduce the risk of overheating. In order to provide controllable ventilation it should be possible both to open windows wide and to secure them at a varied range of opening widths. In “dual aspect” homes where windows can be opened on two or more sides, air can flow through the dwelling. The air movement in itself creates a cooling sensation for the occupants, as well as getting rid of heat that has built up inside the home. Some window configurations do not provide easily controllable or secure ventilation. For example, while patio doors can easily be opened wide, often they cannot be firmly secured in a range of opening positions, to allow continuous controlled ventilation for cooling the home, without compromising security and safety. Flats above ground-floor level, which often have restrictors fitted to the windows for safety, may struggle to achieve adequate levels of ventilation to cool warm rooms. In dwellings which only have opening windows on one side – “single aspect” - it is much harder to create the same amount of air movement and achieve the same levels of ventilation. 25 Look out for: Internal gains ___ 16.The property has a communal heating system with long stretches of hot water distribution pipes 17. Poorly insulated hot water cylinders, pipes or heat-interface units etc. located near bedrooms 18. The property has an MVHR system without a summer “bypass” mode Communal heating systems are now frequently installed in large residential developments. Examples of overheating in bedrooms are being linked to the location of the services in the property. Mechanical Ventilation with Heat Recovery (MVHR) saves energy in winter by taking waste heat from stale exhaust air and using it to pre-heat incoming fresh air. Most systems now have a summer mode that by-passes this feature so that incoming air is as cool as possible. Occupants need to be aware of this facility and how to make best use of it. Hot water generated centrally is continuously piped throughout the building at very high temperatures, usually above or below the corridors, ready to efficiently provide heat and hot water to individual dwellings on demand. In some apartment blocks, heat losses from the system are causing corridors and dwellings to overheat. Especially where the hot water pipes run along unventilated corridors and where the pipework is poorly insulated. For example, in one study the occupants of a development in the South of England were surveyed. Over half of them said they found rooms too hot in summer. This was attributed to heat lost from the towel rail and hot water cylinder, even in summer, as well as to occupants not opening windows to cool the thermal mass. . However, even in by-pass mode, MVHR systems, provide lower ventilation rates than fully opening windows, and so make a limited contribution to keeping cool in summer. . 26 Look out for: Occupants ___ 19. Properties intended for vulnerable residents, such as care homes 20. Complicated or badly labelled ventilation controls 21. Properties with no shutters, blinds, curtains or other forms of shading The elderly, the very young and those with pre-existing medical conditions are more physiologically vulnerable to heat and may also lack the knowledge or capacity to adapt their behaviour and environment. Occupants who are at home all day are also at risk of being exposed to high temperatures. Occupants need to be able to understand how to operate and manage their homes to reduce the risk of overheating. Whilst it is preferable to design to windows to avoid too much solar radiation passing through the glass, internal shading devices such as blinds and curtains do help to reduce solar gain and can be easily fitted. If a property is intended to be occupied by those who are particularly vulnerable to high temperatures, it may need to be designed so that it remains cooler than typical homes (and/or has cool areas). More care will be needed to minimise the likelihood of overheating in these cases. Controls for heating and ventilation equipment need to be intuitive and understandable, particularly where less familiar technologies have been installed such as MVHR systems. 27 Combinations of risk factors __ It makes sense that if one or It provides a nice example of more risk factors are present, how considering the location, then the chance of overheating the building and the occupants occurring will rise. Greater care together, helps to build up a will be needed to find workable profile of risk. solutions. The next slide shows a For example, a fully occupied, different way to present risk top floor, small, single-aspect factors. This helpful graphic apartment without roof was adapted by the Zero insulation, located in a dense Carbon Hub from the NHBC urban area, would be Foundation’s guide NF 44 particularly problematic. “Understanding Overheating - The extract of the infographic Where to Start (2012)”. to the right was produced by The guide also provides clear Arup. See ”Reducing Urban and simple examples of Heat Risk” (2104). practical , technical steps you can take to limit heat gains and purge excess heat. 28 29 5 CIBSE, 2014. TM55: Design for Zero Carbon Hub, 2016. Future Climate - Case Studies. Solutions to Overheating in Homes Chartered Institution of Building Services Engineers, London Further Information ___ Gething, Bill, 2013. Design for future climate NHBC Foundation, 2012. NF 44 Understanding Overheating - Where to Start. NHBC Foundation, Zero Carbon Hub, Richards Partington Architects 30 This slide pack is part of a series ___ All are available for download at www.zerocarbonhub.org 31 Evidence reviews ___ See our Overheating in Homes Evidence Review Series. All are available for download at www.zerocarbonhub.org 32