Transcript Tackling Overheating in Homes

Introduction
to Overheating
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TACKLING OVERHEATING
IN HOMES
Contents
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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.
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Who is this pack for?
Those working in…
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Sustainability, energy efficiency
and climate change
Environmental health
New development
Asset management
and maintenance
Operations and customer services
Risk management, research
and business planning
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1
Introduction
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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.
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2
Defining
Overheating
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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
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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
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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.
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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
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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
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3
Causes
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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.
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External
temperature
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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
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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.
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Location
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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.
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Internal sources
of heat
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Inadequate ventilation
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Image from the Good Homes
Alliance, “Preventing
Overheating” (2014)
Mechanical ventilation
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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.
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Thermal mass
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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
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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
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4
Risk
factors
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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.
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“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
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Look out for: Location
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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.
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Look out for: Micro-climate
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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.
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Look out for: Built form
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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.
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Look out for: Orientation
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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.
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Look out for: Ventilation
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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.
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Look out for: Internal gains
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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.
.
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Look out for: Occupants
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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.
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Combinations of risk
factors
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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.
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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
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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
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This slide
pack is part
of a series
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All are available
for download at
www.zerocarbonhub.org
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Evidence
reviews
___
See our Overheating in
Homes Evidence Review
Series.
All are available
for download at
www.zerocarbonhub.org
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