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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Thermal Insulation
In order to maintain a constant temperature within a building we need to
restrict the rate at which heat energy is exchanged with the surroundings.
Keeping heat inside a building for as long as possible conserves energy and
reduces heating costs.
Good thermal insulation will also reduce the flow of heat into a building
when the temperature outside is higher than the temperature inside. In
other words a well-insulated structure will, if ventilation and direct solar
gain are controlled, stay cooler in the summer than a poorly-insulated
structure. Some people are reluctant to believe that insulation will help
keep a building cooler in summer, but it may be helpful to imagine the
discomforts of living in a tent or garden shed. This ‘building’ will be very
hot in summer for the same reason that it will be very cold in winter: lack
of insulation. In a large building good thermal insulation will give savings
in the energy needed to run the cooling plant. Some current office buildings
use more energy for summer cooling than for winter heating.
Another benefit of good thermal insulation is that the risk of surface
condensation is reduced because the internal surfaces of a room are kept
at a temperature which is above the dew point of the air. Surface
condensation is unsightly, unhealthy and damages decorations. Well-placed
thermal insulation also reduces the time taken for a room to heat up to a
comfortable temperature, such as in room that is unoccupied during the
day.
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Side Note
What is the dewpoint temperature?
The dewpoint temperature is the temperature at which the air can now
longer hold all of its water vapour, and some of the water vapour
must condense into liquid water. At 100% relative humidity, the
dewpoint temperature and real temperature are the same,
and clouds or fog can begin to form. While relative humidity is a relative
measure of how humid it is, the dewpoint temperature is an absolute
measure of how much water vapour is in the air (how humid it is). In
very warm, humid conditions, the dewpoint temperature can reach 75
to 77 degrees F, but rarely exceeds 80 degrees.
Interesting facts:
SOUPY AIR: When the dewpoint approaches 75 degrees F, most
people can "feel" the thickness of the air as they breathe, since the
water vapour content is so high (about 20 grams of water vapour per
kilogram of dry air, or 2% of the air's mass).
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Insulating Materials
A thermal insulator is a material which opposes the transfer of heat
between areas at different temperatures. In present-day buildings the main
method of heat transfer is by conduction, but the mechanisms of convection
and radiation are also relevant. In some circumstances there is also a
contribution from the process of condensation which releases heat energy
when water vapour changes to water liquid.
A vacuum provides perfect insulation against conduction but is not practical
for everyday purposes. The best practical arrangement is to look for
materials that have atoms spaced well apart; such materials will also have
low densities. Gases have widely spaced atoms and gases therefore provide
good insulation against conduction. Air, which is a mixture of gases, is
commonly used as the ‘active ingredient’ for insulation in materials such as
glass fibre and aerated concrete. Air is not, of course, the active
construction material and the need for insulation must be balanced against
other requirements such as strength and rigidity.
For air to act as an insulator it must be held still, because if air is allowed
to move it will transfer heat by convection. The primary purpose of
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
fibreglass or expanded plastic is therefore to trap and to hold air still. Also,
any surface such as a wall or human skin holds a boundary layer of
stationary air which provides a certain amount of thermal insulation against
conduction.
Heat transfer by radiation is restricted by using surfaces that do not readily
absorb or emit radiant heat. Such surfaces, which look shiny, reflect the
electromagnetic waves of heat radiation and this insulation against radiant
heat depends only upon the surface appearance. When we use aluminium
foil as an insulator, for example, it is the shiny surface of the material that
is important. Aluminium is actually a good conductor of heat, but because
the foil is thin the conduction effect is small.
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Types of thermal insulator
Thermal insulators used in construction are made from a wide variety of
raw materials and marketed under numerous trade names. These insulation
products can be grouped by form under the general headings given below.

Rigid performed materials. Example: aerated concrete blocks.

Flexible Materials. Example: fibreglass quilts.

Loose fill materials. Example: expanded polystyrene granules.
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION

Materials formed on site. Example: foamed polyurethane.

Reflective materials. Example: quilts of fibreglass sheets of
aluminium foil.
The materials considered here are designed for the insulation of heat
transfer at the relatively low temperatures of weather conditions and
human comfort. There are other specialised materials for insulating against
heat transfer under the high temperature conditions encountered in boilers,
furnaces and flues.
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
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Properties of thermal insulator
When choosing materials for the thermal insulation of buildings the physical
properties of the material need to be considered. An aerated concrete block,
for example, must be capable of carrying a load. The properties listed below
are relevant to many situations, although different balances of these
properties may be acceptable for different purposes:







Thermal insulation suitable for the purpose
Strength or rigidity suitable for the purpose
Moisture resistance
Fire resistance
Resistance to pests and fungi
Compatibility with adjacent materials
Harmless to humans and the environment
The measurement of thermal insulation is described in the following
sections. As well as resisting the passage of moisture it is important that a
material is able to regain its insulating properties after being made wet,
perhaps during the construction of a building. The fire resistance of many
plastic materials, such as ceiling tiles, is seriously altered by the use of
certain types of paints and therefore manufacturers’ instructions must be
followed. Some materials are incompatible with one another and this should
be considered when installing the materials. For example, bituminous
products may attack plastic-based products. Iron and concrete are mixed
together to reinforce the structure. They have the same thermal expansion
ratio, so it is a perfect combination to reinforce concrete.
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Thermal conductivity, lambda value ().
In order to calculate heat transfer and to compare
different materials we need a measurement to
quantify just how well a material conducts heat.

Thermal conductivity () is a measure of the
rate at which heat is conducted through a
particular material under specified conditions.
The unit is Watts per metre kelvin (W/m K)
Note:
k is a common UK
symbol for coefficient
of thermal
conductivity. Lambda
() is used worldwide
as a symbol for
coefficient of thermal
conductivity.
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Measurement of thermal conductivity
The coefficient of thermal conductivity(lambda value or k-value), is
measured as the heat flow in watts across a thickness of 1 m of material
for a temperature difference of 1 degree K and a surface area of 1 m 2.
Difference techniques of practical testing are needed for different types of
material but the measurements required are shown schematically in the
following figure. The following general formula is then used to calculate the
value of thermal conductivity for the material tested.
𝐻=
where
𝑄 𝜆𝐴(𝜃1 − 𝜃2 )
=
𝑡
𝑑
 = coeff. of thermal conductivity for that material (W/m K)
H = rate of heat flow (current) between the faces (J/s = W)
t = time (s)
Q = Heat (J)
A = cross-sectional area of the sample (m2)

1
-
2
= temperature difference between the faces (oC or K)
d = distance between the faces (m)
1
Heat
Supply
d
Bar of material
2
Measured
Heat flow
Area A
Insulation to prevent heat
loss from sides of bar
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Practical Example
Heat current in a refrigerator wall
Estimate the heat passed in one hour in the Styrofoam insulation in the walls of a
kitchen refrigerator if the total wall area of a refrigerator is around 4 m2, the
temperature outside is 25oC and the temperature inside is 5oC, the thickness of
Styrofoam is 30 mm and the thermal conductivity of Styrofoam is 0.01Wm-1k-1.
Answer:
𝑄 𝜆𝐴(𝜃1 − 𝜃2 )
=
𝑡
𝑑
 = 0.01Wm-1k-1
t = 60 × 60 = 3600s
Q=?J
A = 4 m2

1
-
2
= 25oC – 5oC = 20oC
d = 30 mm = 30 ÷ 1000 = 0.03 m
𝑄
0.01 × 4 × 20
=
3600
0.03
which implies
𝑄=
0.01 × 4 × 20 × 3600
= 96000J = 96KJ
0.03
Check Yourself
Suppose that the refrigerator in the above example uses a 30mm thickness of
fiberglass insulation of thermal conductivity of 0.04Wm-1k-1 instead of Styrofoam,
what will be the rate of heat flow in the fibreglass?
Which material renders the best performance?
So, in your words, what is the meaning of thermal conductivity?
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
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Values of Thermal conductivity
The thermal conductivities or lambda-values of some typical building materials are
given in table 4.1. These values are a selection of measured values which can be
used for standard calculations. It is important to remember that the exact thermal
conductivity of practical building material can vary for the following reasons:



Manufacturing variations in density and thickness
Changes in moisture content
Effects of time on insulating properties
Table 1 Thermal conductivity of some common building materials
Material
Walls
Lightweight aggregate concrete block
Autoclaved aerated concrete block
Concrete (medium density inner leaf)
Concrete (high density)
Mortar (protected)
Mortar (exposed)
Gypsum
Gypsum plasterboard
Sandstone
Limestone (soft)
Limestone (hard)
Fibreboard
Plasterboard
Tiles (ceramic)
Timber (softwood), plywood, chipboard
Timber (hardwood)
Wall ties (stainless steel)
Surface finishes
External rendering
Plaster (dense)
Plaster (lightweight)
Roofs and roof finishes
Density
(kg/m3)
K value or 
(W/m K)
1400
600
1800
2000
2200
2400
1750
1750
600
900
1200
900
2600
1700
2400
400
900
2300
500
700
7900
0.57
0.18
1.13
1.33
1.59
1.93
0.88
0.94
0.18
0.30
0.43
0.25
2.3
1.1
1.7
0.1
0.25
1.3
0.13
0.18
17.0
1300
1300
600
0.57
0.57
0.18
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Reinforced concrete (1% steel)
Reinforced concrete (2% steel)
Aerated concrete slab
Asphalt
Felt/bitumen layers
Screed
Stone chippings (“hardstone”)
Limestone (“torba”)
Tiles (clay)
Tiles (concrete)
Wood wool slab
Floors
Cast concrete
Metal tray (steel)
Screed
Timber (softwood), plywood, chipboard
Timber (hardwood)
Insulation
Expanded polystyrene (EPS) board
Mineral wood quilt
Mineral wood batt
Phenolic foam board
Polyurethane board
2300
2400
500
2100
1100
1200
2000
1300
2000
2100
500
2.3
2.5
0.16
0.70
0.23
0.41
2.0
0.8
1.0
1.5
0.10
2000
7800
1200
500
700
1.35
50.0
0.41
0.13
0.18
15
12
25
30
30
0.040
0.042
0.038
0.025
0.025
Note: Use manufacturers’ data for precise values.
Source: Taken from Document F – Conservation of Fuel, Energy and Natural
Resources. (Minimum requirements on the Energy Performance of Buildings
Regulations, 2006)
Thermal resistivity (r) is sometimes more convenient to use than
conductivity for thermal conductivity of materials. It is the reciprocal of
thermal conductivity, that is:
𝑟=
1
𝐾
The thermal resistivity can often be obtained from a manufacturer’s data
sheet on the materials.
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
This thermal image shows heat loss from a building. The areas shaded in red
are where most heat is escaping. Why do you think the roof is blue?
For more info, go to Thermal Images PDF file.
Go to the following site.
http://www.thermoguy.com/globalwarming-heatgain.html
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
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Methods used to measure thermal factors
A thermographic camera, sometimes called a FLIR (Forward Looking
InfraRed) or infrared camera less specifically, is a device that forms an
image using infrared radiation, similar to a common camera that forms an
image using visible light. Instead of the 450–750 nanometer range of the
visible light camera, infrared cameras operate in wavelengths as long as
14,000 nm (14 µm).
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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION
Exercise 1
1.
An aluminium pot contains water that is kept steadily boiling (100oC).
The bottom surface of the pot, which is 12mm thick and 1.5 × 104
mm2 in area, is maintained at a temperature of 102oC by an electric
heating unit. The remaining part of the surface is well insulated from
the surroundings. Evaluate the heat current entering the water
through the bottom surface.
2.
A piece of wood, in the shape of a 350mm by 350mm slab of thickness
15mm, conducts heat through this thickness under steady-state
conditions. The heat current in the slab is measured to be 14.3 W
when a temperature difference of 25oC is maintained across the slab.
3.
4.
a)
Determine the thermal conductivity of this wood.
b)
Would this material be classified as a good heat conductor or
as a good heat insulator?
A 250mm long wooden (k = 0.19 Wm-1k-1) tube has circular cross
section with inner radius a = 10mm and outer radius b = 20mm.
Fitting snugly within the tube is a circular aluminium rod of the same
length. A temperature difference of 150oC is maintained across the
ends of this compound bar, and heat loss at the lateral surface is
negligible. For steady-state conditions,
a)
what is the total heat current in the compound bar and
b)
how much energy is transferred through the bar in a hour?
A double-glazed window consists of two sheets of 3mm thick glass
separated by a 12 mm layer of air. Determine the heat current per
unit area for such a window if the inside and outside temperatures
differ by 20oC. The thermal conductivities are kglass = 1 Wm-1k-1, kair
= 0.02 Wm-1k-1. Assume a steady-state heat flow due to conduction
and neglect convection.
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