Temperature and Heat Loss
The following topics are covered in this presentation:
• Temperature and temperature scales
• Transfer of heat
• Thermal conductivity, resistivity and resistance
• Total thermal resistance
• U-value
Temperature and Heat Loss
The term temperature is used to express the degree of
hotness of a substance.
It is measured by using a device called the thermometer.
Mercury in glass and the electronic thermometers are
widely used.
Mercury
thermometer
Electronic
Thermometer
Temperature and Heat Loss
There are three temperature scales:
Celsius (ºC), Fahrenheit (ºF) and Kelvin (K)
ºC
ºF
K
Boiling point of water
100
212
373
Freezing point of water
0
32
273
‘Absolute zero’
-273
0
Only used in
USA
Temperature and Heat Loss
HEAT TRANSFER: Heat transfer from one substance to another
or from one point to another in a substance may take place due to
conduction, convection and radiation.
Heat
Conduction: As a material is heated, the atoms start to vibrate with
the increased thermal energy. The vibrating atoms make the
adjacent atoms to vibrate and the process continues. As a
consequence, thermal energy (or heat) travels from one end of the
material to the other.
Temperature and Heat Loss
Convection: All fluids are heated by convection
Heat
The vessel shown contains water and as it is heated, the particles
at the bottom of the vessel become hot and expand. On expansion
the particles become lighter and rise.
Cold water particles at the top are heavier than hot particles. They
descend as the hot water particles rise, and the process continues.
Temperature and Heat Loss
Sun
Radiation: Heat energy travels
from one place to another without
requiring a medium.
Earth
Heat energy from the Sun is
received by us due to radiation
as there is no medium (air)
beyond earth’s atmosphere.
Temperature and Heat Loss
Thermal conductivity: Thermal conductivity or λ-value (also known as
k-value) of a material is a measure of the rate at which heat is
conducted through it under specified conditions.
Flow of heat through a material is
directly proportional to:
θ2
• surface area (A)
• temperature difference between the
opposite faces (θ2 – θ1)
Heat
• time for which heat flows (t)
in, Q
Flow of heat is indirectly proportional
to the thickness of material (d)
θ1
Heat
out
A
d
Combining the above factors, the amount of heat flowing through a
material, Q, is proportional to: (see next slide)
Temperature and Heat Loss
t.A.(θ 2  θ1 )
Q
d
A.(θ 2  θ1 )
Q

d
t
Q
λ.A.(θ 2  θ1 )
=
d
t
 λ (or k) 
Q. d
t.A. (θ 2  θ1 )
where λ (or k) is the thermal conductivity of the material
(continued)
Temperature and Heat Loss
To work out the unit of thermal conductivity, the symbols in the
equation are replaced with appropriate units:
• Quantity of heat, Q: Joules (J)
•Thickness of material, d: m
•Time during which heat flows, t: seconds (s)
• Surface area of the material, A: m2
• Temperature difference, θ2 – θ1 = ºC or K
 λ (or k) 

J. m
s. m 2 . C
W
W
or
m C
mK
Temperature and Heat Loss
Thermal conductivity (λ) is the rate at which heat is conducted
through a material of unit measurements, to maintain the opposite
faces of the materials at a temperature difference of 1 ºC.
Thermal resistivity (r) is the reciprocal of thermal conductivity.
1
r
λ
m 2 . C
Units 
W
m2. K
or
W
Thermal resistance (R) takes into account the thickness of the
material.
d
R 
λ
where d is the thickness of the material in metres
Temperature and Heat Loss
The Total Thermal Resistance of a material is a combination of material
resistance and surface resistance
Material resistance: All materials offer some resistance to the
transmission of heat.
Material resistance = d/λ
Unit: m2K/W
Surface resistance: The surfaces of all building materials have
irregularities which trap air. Stationary layers of air are formed which
resist the transmission of heat.
Building material
Outside surface
(Rso)
Inside surface
(Rsi)
Temperature and Heat Loss
Airspace resistance (Rairspace): If there is a cavity in a component, the
airspace resistance needs to be considered as well, for example in a
double glazed window, cavity wall etc. The airspace resistance depends
on the width of the cavity.
Glass
Airspace
Double-glazed
window
Temperature and Heat Loss
The total thermal resistance of a component is given by:
Rtotal = Rsi + Rmaterials + Rairspace + Rso
Where Rtotal is the total thermal resistance
Rsi is the thermal resistance of the inside surface
Rmaterials is the thermal resistance of the materials
Rairspace is the thermal resistance of the cavity
Rso is the thermal resistance of the outside surface
Temperature and Heat Loss
Thermal transmittance or the U-value indicates the amount of heat
energy that will flow per second through one square metre of a
building element when the temperature difference between the inside
and outside surfaces is one Kelvin (or 1 ºC).
The U-value of an element/component can be determined from the
following relationship:
U
1
R total
Unit: W/m2K