Overheating
1.
Introduction ...............................................................................................................................................2
2.
Causes of Overheating..............................................................................................................................4
3.
Reduction of overheating .........................................................................................................................6
i) Reduction in solar gains .........................................................................................................................6
ii) Reducing internal gains.......................................................................................................................11
iii) Removing Heat ...................................................................................................................................12
4.
Summary ..................................................................................................................................................15
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1. Introduction
Overheating has many different definitions depending upon the type and use of the building in
question. These can take the form of the number or percentage of occupied hours greater than
some temperature (e.g. >28°C), or not to exceed a set limit (e.g. >32°C), or the difference in
temperature between outside and inside to be less than some limit (e.g. <5°C when external
temperatures >20°C). These limits while varied all have the same objectives, to improve
thermal comfort within the building, to avoid heat stress on the occupants and to maintain
productivity. This is true whether the building being considered is a school, office, hospital or
even domestic dwellings.
But why is this important? A typical human with a mass 70kg, has a metabolic rate of ~150W
doing light work, if they were unable to lose heat to their surroundings their core body
temperature would increase similar to if they had a fever. As the human body is mainly water
and we can approximate the specific heat of a person to be 4187 J/kgK. Therefore in an hour
their core body temperature would rise by:
(150W × 3600seconds) ÷ (4187 J/kgK × 70kg) = 1.8°C per hour
!
This of course assumes that the person cannot lose heat, but by what mechanisms can a person
lose heat. People can lose heat through radiation, their skin is hotter than their surroundings,
through convection and conduction though their skin and also via evaporation, sweating and
water vapour in breath. This can be represented by the following equation.
eqn. 1
M "W = E + R + C + K + S
Where: M is the metabolic rate of the body
W is mechanical work done by the body when not at rest.
! heat loss
E is evaporative
R is radiative heat loss
C is convective heat loss
K is conductive heat loss
S is heat stored in the body.
To avoid a core body temperature increase resulting in heat stress and heat related illnesses S
needs to be zero. The balance of eqn. 1 will change depending upon the activity of the person.
Estimates of different metabolic rates can be found below.
Table 1 Examples of metabolic rates by activity, for a typical human weight 65-70kg and surface area
1.8m2.
Basic activity
Lying
Sitting
Standing
Typing / writing
Walking on a level path at 2km/h
Walking on a level path at 5km/h
Going up stairs at 80 steps per minute
Transporting a 10kg load on a level path at 4km/h
Metabolic rate W/m2 (W)
45 (81)
58 (104)
65 (117)
85 (153)
110 (198)
200 (360)
440 (792)
185 (333)
As we can see from the table the metabolic rate of our bodies varies depending on our activity
level. Our bodies are inefficient, while some of the energy produced by our increased metabolic
rate will go into the activity (e.g. carrying your body weight up a flight of stairs, W in eqn. 1)
the rest of the energy will go into warming your cells. As the table shows there can be a large
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increase in heat output. This implies for buildings where there is a mixture of different activity
levels thought needs to be given to how to cool these spaces adequately.
Overheating can have an effect on not only the comfort of a building’s occupants, but also their
productivity and potentially their health. For a building such as an office the cost of
heating/cooling and electricity costs is a small fraction of labour costs. Therefore, decreased
employee productivity can have a significant cost attached to it. Seppanen (2006) created a
metric for the estimation of decrement of productivity with temperature. The findings of
Seppanen are shown in Figure 1, the optimum range of temperature is between 18°C and 26°C
with a peak at 22°C, outside this range of temperatures productivity decreases sharply.
Figure 1 Plot of relative productivity versus air temperature. Source: Seppanen et al, “Effect of
temperature on task performance in office environment”. Lawrence Berkley National Laboratory, 2006,
LBNL report 60946.
Heat stress is a potential problem if the building fails to moderate external temperatures during
heat waves. During 2003 35000 people died across Europe from heat stress and heat related
illnesses. The reason is simple, as it becomes warmer it becomes increasingly more difficult for
you body to lose the heat it is producing. The problem becomes even worse if relative humidity
levels are high as well as sweating becomes less effective and hence sweat rate increases
leading to dehydration. The majority of people who died during the heatwave of 2003 were
elderly or infirm.
Figure 2 shows a plot of air temperature within a building (retirement home) during the
heatwave of 2003 and the effect on core body temperature (rectal temperature) of the occupants.
The effect on core body temperature was calculated using a dynamic version of ISO 7993: Hot
environments - Analytical determination and interpretation of thermal stress using calculation of
required sweat rate. While metabolic rate and re-hydration rates were altered to reflect the age
of the occupants the ISO does not allow for infirm people so all occupants are considered to be
fit and healthy. Therefore the results could be considered a lower limit. The data shown in
Figure 2 indicates that the occupants cannot lose heat effectively and their core body
temperature continues to rise throughout the duration of the heatwave. By the end of the two
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week period the core body temperature has reached dangerous levels and the occupants are
suffering from severe heat stress, they may faint or even die if their core temperature does not
decrease quickly enough, the occupants are also likely to be severely dehydrated.
Figure 2 Plot of air temperature within a building during the heatwave of 2003 and the resultant effect on
core body temperature.
What this shows us is that overheating can be a serious problem, the human body can cope with
relatively high temperatures for several hours so long as it has adequate water and it is allowed
to cool periodically (i.e. over night). A problem arises with heatwaves where the maximum
daytime temperature is high and the temperature does not fall low enough during the night. This
problem can be exacerbated in buildings where the occupants do not leave such as hospitals or
retirement homes. As was shown in the climate change chapter temperatures are set to rise
significantly and during heatwave events both maximum day and minimum nighttime
temperature are set to be dangerously high where not only the elderly and infirm will be in
danger of heat stress. Therefore there is the need to address the issue of overheating, preferably
without resorting to using air-conditioning as this is not only expensive but will exacerbate the
problem of climate change.
2. Causes of Overheating
The reason why building overheat is similar to why humans suffer from heat stress, the building
is either producing or receiving more heat than it is capable of shedding so energy is stored and
it heats up. What are the main sources of heat pertinent to a building?
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Figure 3 Plot of internal and solar gains (top) and air temperatures (bottom) for an open plan office
simulated with the Plymouth Design Summer Year.
In a typical building the main sources of heat are:
• Warm air via infiltration or ventilation (if it is warmer outside)
• Solar gains, both through windows and heat transmission through building fabric
• Heating system (if controls are poor, this should ideally not be a source of overheating)
• Lighting
• Equipment, computers, printers, TVs, fridges etc
• Metabolic heat (people, animals)
Obviously for specific types of building there may be other large sources of heat; for a
restaurant the ovens and cookers, for an aquarium, large tanks of water at 30+°C and so on.
Figure 3 shows a plot of the magnitude of the heat gains and the air temperatures inside and
outside an office. The open plan office has windows on the North and South facades allowing
for cross ventilation. The internal gain shown in the figure is the sum of the metabolic heat,
lighting and equipment gains, the solar gains in only transmission via windows. Notice how
solar gain alters throughout the year as the solar altitude varies, the amount of solar gain will be
heavily dependent on the design of the windows and the thickness of the walls.
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Figure 4 Plot of internal and solar gains and air temperatures for the last week of July and the first week
of August for an office block simulated with the Plymouth DSY.
Examining the data in more detail during the hottest part of the year (Figure 4), a two-week
period at the end of July and beginning of August. Figure 4 shows not only the sum of the
internal gains but also a breakdown. Note how the internal temperature closely follows the
external temperature especially once the windows start to open at 23°C. This indicates a
problem if cooling is to be provided via natural ventilation, opening a window cannot make it
cooler than it is outside, although it can alter your perception of thermal comfort. It is
interesting to note that the lighting and equipment gains are larger than the metabolic gain,
despite there being only 1 computer per person (assumed desktop PC with LCD screen).
3. Reduction of overheating
So having seen the impacts of overheating and examined the causes of overheating, what can
we do to adapt our building designs to mitigate against overheating? Since overheating is
caused by excess heat entering the building or large internal gains then limiting these before
considering ways of removing the heat would seem prudent.
i) Reduction in solar gains
The Sun is the single largest source of energy we are exposed to, it creates light for us to see by,
it warms the air, it drives the wind and our weather patterns. The two ways for direct energy
from the Sun to enter our buildings is either through windows or through the building fabric.
The surfaces of walls or roofs can become very hot (~60°C) when in direct sunlight, even light
coloured surfaces. This is due to the fact that half of the energy emitted by the Sun that
penetrates our atmosphere is in the infrared region (see Figure 5). Materials that are light
coloured and reflective in the visible region are often highly absorbing in the infrared. White
paint for example can reflect ~90% of visible light but absorbs ~90% of infrared radiation.
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Figure 5 Power spectrum of solar energy.
This absorbed energy heats the material and is either re-emitted from the surface later or is
conducted through the material. The speed at which heat moves through the material is
dependent on the conductivity of the material. Obviously metals are highly conductive,
however, it is surprising how conductive stone and concrete are. The slower the transmission of
heat through roofs and walls the more likely it is that the heat will be re-radiated back to the
outside world. This process works both ways hence why we put insulation in our walls and
lofts.
Table 2 Thermal conductivities of some common materials used in buildings, source:
www.engineeringtoolbox.com
Material
k (W/mK)
Aluminum
Brick work
Concrete, light
Concrete, dense
Fiber insulating board
Glass
Glass, wool Insulation
Granite
Hardwoods (oak, maple..)
Insulation materials
Limestone
Marble
Plaster, gypsum
Softwoods (fir, pine ..)
Steel
Stainless Steel
Tin
Zinc
250
0.69
0.42
1.7
0.048
1.05
0.04
1.7 - 4.0
0.16
0.02 - 0.16
1.26 - 1.33
2.08 - 2.94
0.48
0.12
43
16
67
116
For solar energy transmitted through windows the case is slightly different, glass is mainly
transparent to visible light and semi-transparent to near infrared but is opaque to lower energy,
longer wavelength thermal infrared. Low emissivity glass such as Pilkington K glass and
equivalents enhance this property by coating the glass to make it more reflective in the thermal
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region without greatly affecting the visible transmission. Figure 6 below shows the relative
transmission and reflection for low emissivity glass.
Figure 6 transmission and reflection through a low E glass pane, source:
http://www.awa.org.au/documents/HowComfortPlusSolarControlLow-EGlassWorks.pdf
This allows solar energy to enter the building as visible and near infrared (the majority of the
solar power, see Figure 5) through the windows. This energy is then reflected around the room
until it is eventually absorbed or escapes out the window again. Once absorbed the energy will
be conducted elsewhere or re-radiated as heat (thermal infrared). The heat can become trapped
in the building if it is highly insulated and has good air-tightness as the glass is highly reflective
to thermal infrared.
Hence, we are faced with two choices when trying to limit the effects of solar gains, shading or
reflection.
Shading can take many forms we will discuss a few here;
Trees – these have the added benefit of providing shade in the summer when needed but less in
winter to allow passive solar heating of spaces. Trees also actively cool their surroundings
through the evapotranspiration of water from their leaves.
Smaller / less windows – this measure can reduce the amount of heat that directly enters a
space. However, it can make a space feel dark and enclosed, resulting in larger lighting energy
usage. Since windows have a far higher U-value than the wall they are replacing smaller
windows or less windows will reduce heating loads considerably.
Roof overhangs and external shades – these are a simple way of providing shading of
windows and the external walls. Ideally the overhang needs to be sized to provide shading
during the hottest months and to a lesser extent during the cooler months. Roof overhangs
typically only provide shade for the top floor, however extra shades can be applied to the
building to shade other floors. Figure 7 shows a simulation of different size overhangs on a
simple building in the spring (top) and summer (bottom).
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Figure 7 simulation of a 5×5×3m room with different roof overhangs. From left to right overhang is
0.25m, 0.5m, 0.75m and 1m. The window is 1.2m high and 1.22m off the ground. Top image is for midday
15th March and bottom is midday 15th June.
As we can see from the figure the size of the overhang is crucial and needs to be sized
according to the building latitude, the overheating potential of the building (which months
require shading) and the position of the windows. Figure 8 shows a building with a large roof
overhang to provide shade for a glass fronted café area. The picture taken in early February
shows no visible shading during winter months allowing for passive solar heating.
Figure 8 Picture of large roof overhangs employed on a building to provide shade for glass façade in
summer.
There is the concern that increased shading will adversely affect daylighting and hence increase
the use of artificial lights. This not only will lead to increased energy bills but also may
contribute to overheating. A possible adaptation to over come this is to use light shelves. These
devices provide shade for the majority of the window in the same way as a roof overhang but a
portion of the light is reflected upwards onto the ceiling of the room. This reflected light can
reduce the need for artificial lighting. An example of the light shelves is shown in Figure 9.
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Figure 9 Simulations of an office block with light shelves showing the external (top) and internal (bottom)
views.
Window recesses – the wall thickness where windows are located can have a profound affect
on the amount of daylight that enters a room. This effect should be considered not only when
calculating the daylighting potential of a space but also as a means of limiting solar gains in
summer.
Brise Soleil – These can be mounted horizontally or vertically and perform a similar job to
external shades. Brise soleil are available in fixed and adjustable variants, the angle of the fins is
chosen to shade the space at certain times of day / year. Adjustable versions can be controlled
by a building management system (BMS) to control lighting and overheating levels.
Figure 10 Images of vertical and horizontal brise soleil on buildings.
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Solar films – while these are not specifically shading they do reduce the amount of solar energy
that passes through a window into a building. These can take the form reflective or coloured
versions, the difference is largely aesthetic, although the colour of transmitted light has to be
considered as it alters the internal environment. They do have the downside that there will be
reduced daylighting potentially leading to higher artificial lighting usage in the winter months.
Reflective roofs – a shiny surface or white paint on a roof will reflect a lot of the solar power,
also special paints that are reflective in the infrared region are starting to be developed to help
reflect even more of the solar spectrum. By reflecting heat back into the atmosphere it is
prevented from entering the building thus reducing overheating. However, this can have the
downside that heating bills may be increased in winter. Reflective roofs or white painted
surfaces also require maintenance in order to operate effectively.
ii) Reducing internal gains
As we have seen from the data shown in Figure 4 internal gain are a major contributor to
overheating. There are several ways to limit the internal gains of a building;
Reduced occupancy – as we discussed earlier people generate ~150W while doing light work.
Also people use energy in the form of lighting, PCs and equipment. Reducing the occupancy of
the building either by simply having less people or by staggering occupancy will reduce the
amount of heat being generated at any given time. For staggered occupancy, while the number
of kWh of heat produced in a day is likely to be similar to the previous case there is more time
for that heat to escape the building. Such measures however, are not easy to implement.
More efficient equipment – items such as computers and photocopiers can use a large amount
of energy. The table below shows a comparison of various types of computer with a screen.
Table 3 Example energy usages compiled from various sources and measurements.
Machine
Powerful desktop
Compact desktop
Laptop
Thin Client
Sleep Power
Idle Power
Full Power
>8W
>4W
~1-5W
~5-10W
~150-300W
~48-150W
~10-50W
~7-24W
~300-500W
~130-365W
~45-100W
~7-26W
It is clear that switching from a power hungry desktop machine to a more economical laptop or
thin client arrangement can reduce not only energy usage but also the overheating potential.
Even switching from a mid-spec compact desktop (e.g. Dell optiplex) which uses 150W while
idle to another more efficient machine (iMac), which uses only 48W while idle can reduce
overheating levels considerably.
Lighting efficiency – reduced lighting load can be accomplished in a number of ways. The use
of more efficient fittings such as high efficiency fluorescent fittings reduces the energy
consumption and heat output while maintaining a set level of light. Using lights with a higher
colour temperature generates a higher lux level for the same amount of energy. This is due to
our eyes being more sensitive to blue light than red light (lux is a measure of this). The
downside of this is that the environment can seem harsh and ‘cold’.
Daylight dimming – this reduces the lighting load and heat output when there is sufficient light
entering a room via the windows. A room with multiple banks of lights can have separate
controllers and only the lights that are needed need be on. Considering that for a typical office
lighting levels can be 12-14W/m2 the reduction in heat output can be appreciable.
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Behavioural changes – this can be worthwhile in the long term for saving energy and money as
well as reducing heat loads. Encouraging users to turn off lights and machines when not in use
allows the building to lose heat and maintain comfort levels. Another option is to change
working practices, such as changes to working hours or working from home, this reduces the
internal gains and hence the overheating potential. A relaxed clothing policy in summer months
would allow greater comfort if the building is prone to overheating, energy savings could also
be made by increasing the cooling set point. These measures however can be difficult to
implement.
iii) Removing Heat
If it is still necessary to remove excess heat from a building then there are several options
available, these include;
Thermal mass – this is a concept, which describes how the mass of the building provides
“inertia” against temperature fluctuations, in this case heat is remove from the air and stored in
the building fabric. Factors which affect thermal mass or thermal capacitance as it is sometimes
called are, the density, specific heat of the material (how much energy is required to increase
temperature) and conductivity. All materials have a thermal mass but some more than others. As
Table 2 shows materials such as stone and concrete have a relatively high conductivity, these
coupled with the materials high density makes them ideal for adding thermal inertia to a
building. For example heat is transferred from the air by either radiation or convection into the
concrete, due to there being a far greater mass of concrete than air the concrete changes very
little in temperature compared to the air. This has the effect of reducing peak temperatures. The
stored heat can be lost to the air later once air temperatures have fallen, this is useful for storing
heat in a building structure overnight to reduce heating bills. If it is unfeasible to increase the
thermal mass in a building it is possible to increase access to the thermal mass. Concrete
ceilings are often hidden behind suspended ceilings, concrete and brick behind plasterboard.
Perimeter gaps around suspended ceilings have been shown to increase airflow above ceiling
tiles and increase access to thermal mass lowering peak temperatures. Replacing plasterboard
with more plaster, which is more conductive and in direct contact with the mass behind it can
have a significant impact on the available thermal mass in a building.
Ventilation – increasing ventilation to remove excess heat from a building is an effective
measure. Humans appreciate the sensation of increased air velocity when experiencing raised
temperatures, however too much air flow will cause discomfort. The increased air velocity also
helps the human body cool itself by increasing the effectiveness of sweating. Natural ventilation
via windows or simple mechanical ventilation are limited by the external air temperatures. If the
external air temperature is uncomfortable then increasing ventilation will not necessarily
improve comfort levels. In fact if the external air temperature is higher than the internal air
temperature (this is likely if the building has appreciable thermal mass) then increasing
ventilation rates can result in increasing internal temperatures and decreasing thermal comfort.
Earth tubes – these simple devices consist of a tube that runs for a distance beneath the earth
through which air is drawn for ventilation. The soil temperature a few feet below the surface is a
relatively constant 5-10°C depending on location. Hence the air being drawn through the tube
into the building is cooled in summer (and potentially warmed in winter), the exact temperature
of the air will depend on the length of the tube and the speed of the air. This system can be
coupled to a mechanical ventilation system to provide free cooling, a damper system mixing
earth tube air and external air can be used to regulate incoming air temperatures to maintain
comfort levels.
Heat exchangers – these are devices that recycle heat. Heat from outgoing air is absorbed and
passed to incoming air, this is usually considered only for use in winter to recycle heat and save
heating energy. However, simple devices such as thermal wheels (a mesh wheel intersecting
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both incoming and outgoing air ducts that turns slowly) can be used to recycle coolth instead of
heat. If the outside air is warmer than the air inside the building but there is still a need for
ventilation these can be used to help prevent drawing excess heat into the building.
Evaporative cooling – this is a process where water is used to cool incoming air. There are two
main methods for this process direct and indirect. In direct mode water mist is sprayed into the
incoming air supply. If the relative humidity of the incoming air is less than 100% then the
water will evaporate using heat from the air. This has the downside that the humidity of the
incoming air is increased, which can impact upon comfort levels, this can be overcome by
situating some desiccant material downstream to lower the humidity. There are potential health
implications using this method such as legionella and precautions would need to be taken. In
indirect mode the water mist is sprayed into the exhaust air supply from the building. The air is
cooled in the same way but a heat exchanger is employed to transfer the coolth from the exhaust
air to the incoming warm air. If a non-mixing heat exchanger is employed then there is no cross
contamination and the humidity of the incoming air is unaltered.
Night Cooling – this allows the use of cooler night time air temperatures to restore the coolth of
a building. In a building with a large amount of thermal mass then heat will be released from the
building fabric into the air at night. In order for the thermal mass to as effective as possible
during occupied hours then as much heat as possible needs to leave the building. Although not
too much so that the heating system is required to heat the building up again. Since the rate at
which thermal mass will lose heat is governed by the speed and temperature of the air passing
over it, purging the building with cool air from outside will cool the building fabric rapidly.
This can be accomplished by running a mechanical ventilation system in the early hours of the
morning when air temperatures are at their lowest. Alternatively if natural ventilation is used
open the windows. Openings at low and high levels such as ground floor windows or vents and
roof lights allows for considerable stack ventilation. Security is often a concern since in order to
achieve a large airflow large openings are usually required. Therefore for naturally ventilated
buildings this needs to be considered at the design stage to allow provision for secure night
cooling.
Green and blue space – we discussed earlier the use of trees to provide shade to limit heat
gains. The inclusion of green space (grass, trees and plants) or blue space (ponds, streams and
lakes) either within the building or in the surrounding area will cool the air through the
evaporation of water. The latent heat (energy required to change state from liquid to gas) of
water is 2270kJ/kg so a large amount of heat can be removed by the evaporation of water. In hot
countries such a Morocco buildings have traditionally taken advantage of this fact. Figure 1
shows some typical Moorish architecture featuring deep courtyards with pools of water at the
bottom.
Figure 11 Images of Moorish style courtyards with pools.
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The building is built around this courtyard pool, which not only acts as decoration but cools the
surrounding stone. Often fountains and water channels are employed to further increase the
ability of the water to remove heat from the building fabric.
The use of green space around buildings and green roofs can also cool buildings through the
endothermic evapotranspiration of water. Similar to evaporation of water from a surface or a
pool this uses heat from the surroundings to overcome the latent heat of water. Green roofs have
the added benefit that they not only shade the roof surface but also actively cool the building
fabric through this process. Figure 12 shows the heat flow through a roof top terrace garden and
the different surfaces contained within. As we see the heat flow is reduced for the planted areas
and for the shrubs the heat flow is negative for the whole day indicating heat is being removed
from the building fabric.
Figure 12 Measurements of heat flux through a building roof depending on surface. Source: Building and
Environment 38 (2003) 261–270.
Phase change materials (PCM) – these are materials that change phase (solid to liquid) and in
doing so remove heat from their surroundings to overcome their latent heat. A popular PCM is
encapsulated paraffin wax, which can be incorporated into plasterboard or ceiling tiles. The
PCM is contained in small bead-like capsules which are distributed throughout the plasterboard
(or mixed into the wet plaster), the capsules allow the PCM to melt but prevent it from leaking
out. The PCM melts at a temperature at the top of the comfort range (e.g. 26°C) but can be
tailored to operate at different temperatures. While the PCM melts it limits the temperature of
its surroundings until it is all melted then temperatures are free to rise again. Once the
environment has cooled the PCM freezes and heat is given out to its surroundings. This is a
good method of offsetting peak temperatures and limiting temperatures to a set range (if enough
PCM is present). The finite capacity of the PCM to absorb heat means that in periods of warm
weather the PCM material may be inadequate to keep the building cool, also if the PCM is
unable to freeze again due to elevated internal temperatures it will cease to work. Unfortunately
current versions of thermal modelling software do not incorporate latent energy and hence
incorporating PCM in thermal models is troublesome. Due to the PCM having a finite amount
of heat it can absorb in any one cycle (solid→liquid→solid) incorporating a work around into
thermal models is very time consuming. PCM plasterboard is relatively expensive as well
costing typically 10× more than normal plasterboard.
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4. Summary
Technology
Trees
Smaller / less windows
Roof overhang / external shades
Window recess
Brise soleil
Solar films
Pros
Provide seasonal shade.
Less capital cost, lower heating
bills.
Simple to employ.
Free, comes with building
Tailored for building,
adjustability, control by BMS.
Cheap.
Reflective roofs
Cheap and simple.
Eqpt. Efficiency
Saves energy and money.
Lighting efficiency
Saves energy and money.
Daylight dimming
Saves energy and money.
Changes to working practices
Changes to working hours
Thermal mass
Little incurred cost, can save
energy and money.
Spaces less crowded.
Can regulate temperatures,
potential for passive solar
heating.
Ventilation
Many options available.
Earth tubes
Free cooling of air.
Heat exchangers
Saves energy and money.
Evaporative cooling
Ability to cool air cheaply.
Night cooling
Green and Blue space
Phase change materials
Overheating
Works well with thermal mass
to restore coolth.
Cools local environment
through evaporation of water.
Offsets and decreases peak
temperatures, easy to retrofit.
Cons
Maintenance, require space on
site.
Higher lighting bills, perception
of space being dark / enclosed.
Reduced ventilation.
Adjustment not usually possible.
Adjustment not possible.
Can be expensive, limits
daylighting.
No adjustment for winter
months.
Maintenance, no adjustment for
winter months.
Often a higher capital cost.
Often a higher capital cost, light
can be perceived cold / harsh.
Control systems incur a capital
cost.
Difficult to implement and
regulate.
Difficult to implement,
potentially higher energy bills.
Often perceived as a nonsustainable option.
Cannot lower temperatures
below external temperature,
draughts can decrease comfort.
Capital cost and space issues.
Capital cost and extra space
requirements.
Capital cost and maintenance.
Potential health implications.
Security.
Maintenance, possible glare off
water.
Cost, difficult to model, finite
capacity.
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