Temperature is actually not a physical quantity but it
can be thought of as a symptom-as the outward
appearance of the thermal state of a body. If
energy is conveyed to a body, the molecular
movement within that body is increased and it
appears to be warmer.
▪ Temperature is measured by the Celsius scale.
▪ A position on this scale, i.e. the temperature of an
object is donated as: o
C but an interval or
difference in temperature is: deg C.
Heat is a form of energy, appearing as molecular movement in substances or
as 'radiant heat', a certain wavelength band of electromagnetic radiation
in space (700 to 10000 nm). As such, it is measured in general energy
units: joules (J).
Specific heat of a substance is the amount of heat energy necessary to
cause unit temperature increase of a unit mass of the substance.
It is measured in: J/kg degC.
Latent heat of a substance is the amount of heat energy absorbed by unit
mass of the substance at change of state (from solid to liquid or liquid to
gaseous) without any change in temperature. It is measured in: J/kg.
Thermal capacity of a body is the product of its mass and the specific heat
of its material. It is measured as the amount of heat required to cause unit
temperature increase of the body, in units of J/degC.
Heat energy tends to distribute itself evenly until a perfectly
diffused uniform thermal field is achieved. It tends to flow from
high temperature to lower temperature zones, by any or all of
the following ways:
The 'motive force' of heat flow in any of these forms is the
temperature difference between the two zones or areas
considered. The greater the temperature difference, the faster
the rate of heat flow.
The rate of heat flow is measured in Watts (W). In most practical
applications, the multiple of watt 'kilowatt' (kW), will be used. (1
kW = 1000 W)
CONDUCTI VI TY &
RESI STI VI TY
Thermal conductivity (or 'k-value') is defined as the rate of
heat flow through unit area of unit thickness of the material,
when there is a unit temperature difference between the two
The unit of measurement is W/m degC.
Its value varies between 0·03 W/m degC for insulating
materials and up to 400 W/m degC for metals. The lower the
conductivity, the better insulator a material is.
Resistivity is the reciprocal of this quantity (1 /k) measured in
units of: m degC/W.
Better insulators will have higher resistivity values.
Whilst conductivity and resistivity are properties of a material,
the corresponding properties of a body of a given thickness
are described as conductance (C), or its reciprocal resistance
C = 1/R
Conductance is the heat flow rate through a unit area of the
body when the temperature difference between the two
surfaces is 1 degC.The unit of measurement is W/m² degC.
Resistance of a body is the product of its thickness (b) and the
resistivity of its material:
R = b x 1/ k = b/k
It is measured in m² degC/W.
MULTI LAYER BODY
If a body consists of several layers of different materials, its
total resistance will be the sum of the resistances of the
The conductance of such a multilayer body (C) can be found
by finding its total resistance (R) and taking its reciprocal:
Rb= R1 + R2 + R3
= b1/k1 + b2/k2 + b3/k3
= Σ b/k
Cb = 1/ Rb = 1/ Σ b/k
Note that the conductances are not additive, only the
In addition to the resistance of a body to the flow of
heat, a resistance will be offered by its surfaces,
where a thin layer of air film separates the body
from the surrounding air. This is the surface or
It is denoted as 1/f (m² degC/W),
f being the surface or film-conductance (W/m² degC).
OVERALL AI R- TO- AI R
The overall air-to-air resistance (Ra) is the sum of the body's
resistance and the surface resistances:
Ra = 1/fi + Rb + 1/fo
1/fi= internal surface resistance,
Rb = resistance of the body,
1/fo = external surface resistance,
all resistance values in m² degC/W.
The reciprocal of the overall air-to-air resistance (Ra)
is the air-to-air transmittance or U-value.
U = 1 / Ra
Its unit of measurement is the same as that of
conductance - W/m² degC.
This is the quantity most often used in building heat
loss and heat gain problems.
CAVI TI ES
If an air space or cavity is enclosed within a body,
through which the heat transfer is considered, this
will offer another barrier to the passage of heat.
It is measured as the cavity resistance (Rc) which can
be added to the other resistances described above.
In convection, heat is transferred by the bodily movement of
a carrying medium, usually a gas or a liquid.
The rate of heat transfer in convection depends on three
temperature difference (difference in temperature of the
medium at the warmer and cooler points)
the rate of movement of the carrying medium in terms of
kg/s or m3
the specific heat of the carrying medium in J/kg degC or
These quantities will be used in ventilation heat loss or
In radiation heat transfer, the rate of heat flow depends on the
temperatures of the emitting and receiving surfaces and on
certain qualities of these surfaces: the emittance and
Radiation received by a surface can be partly absorbed and partly
reflected: the proportion of these two components is expressed
by the coefficients absorbance (a) and reflectance (r).
The sum of these two coefficients is always one:
a + r = 1
Light coloured, smooth and shiny surfaces tend to have a higher
For the perfect reflective theoretical white surface: r = 1, a = O.
The perfect absorber, the theoretical 'black body', would have the
coefficients: r = 0, a = 1.
For building design purposes, it is useful to combine the
heating effect of radiation incident on a building with
the effect of warm air. This can be done by using the
sol-air temperature concept.
Ts =To + [(l x a)/fo]
whereTs = sol-air temperature in ˚C
To = outside air temperature in ˚C
l = radiation intensity inW/m²
a = absorbance of the surface
fo = surface conductance (outside),W/m2
solar gain factor (θ)
The solar gain factor is defined as the heat flow rate
through the construction due to solar radiation,
expressed as a fraction of the incident solar radiation.
Its value should not exceed 0.04 in warm-humid climates
or 0.03 in the hot-dry season of composite climates,
when ventilation is reduced.
solar gain factor θ = (a x U) / fo
heat exchange in buildings
Just like the human body, the building can also be considered as a defined unit and its
heat exchange processes with the out-door environment can be examined.
The thermal balance, i.e. the existing thermal condition is maintained if:
Qi + Qs ± Qc ± Qv ± Qm - Qe = 0
If the sum of this equation is less than zero (negative), the building will be cooling and if
it is more than zero, the temperature in the building will increase.
Conduction heat flow rate through a wall of a given area can be
described by the equation:
Qc= A x U x ∆T
Where, Qc= conduction heat flow rate, inW,
A = surface area, in m²,
U = transmittance value inW/m² degC,
∆T = temperature difference in degC
Convection heat flow rate between the interior of a building and the open air depends on
the rate of ventilation, i.e. air exchange.The rate of ventilation can be given in m³/s.
The rate of ventilation heat flow is described by the equation:
Qv= 1300 xV x ∆T
Where, Qv = ventilation heat flow rate, inW,
1300 = volumetric specific heat of air, in J/m³ degC,
V = ventilation rate in m³/s,
∆T = temperature difference in degC
If the number of air changes per hour (N) is given the ventilation rate can be found as:
V = (N x room volume) / 3600
where 3600 is the number of seconds in an hour.
Radiation through windows
The solar heat flow through windows is given by the equation:
Qs = A x l x θ,
Where, A= area of the window in m²,
l = radiation heat flow density inW/m²,
θ = solar gain factor of window glass.
periodic heat flow
All the equations and calculation methods seen so far are valid if and
only if, both out-door and indoor temperatures are constant.
As perfectly static conditions do not occur in nature, the basis of the
above methods is the assumption of steady state conditions.
In nature the variation of climatic conditions produces a non-steady
state. Diurnal variations produce an approximately repetitive 24-hour
cycle of increasing and decreasing temperatures.
The effect of this on a building is that in the hot period heat flows from
the environment into the building, where some of it is stored, and at
night during the cool period, the heat flow is reversed: from the
building to the environment.
As the cycle is repetitive, it can be described as periodic heat flow.
time-lag & decrement factor
The two quantities characterizing this periodic change are the time-lag (or phase
shift θ) and the decrement factor (or amplitude attenuation µ).
The decrement factor is the ratio of the maximum outer and inner surface
temperature amplitudes taken from the daily mean.
▪ MICRO-CLIMATE CONTROL
▪ STRUCTURAL CONTROL
▪ MECHANICAL CONTROL
The environment immediately outside and between buildings can be influenced by the
design of a settlement and by the grouping of buildings to a minor extent.
Structural (passive) means of control can provide a further leveling out of the climatic
variations, and often even comfort conditions can be achieved by such means.
Precisely controlled indoor climate can only be achieved by mechanical (active)
controls (the straight line in the figure), but this may not be our aim, and even if it is,
with adequate structural controls, the task of mechanical controls is radically reduced
and it becomes more economical.
Heat absorbing glass
On opaque surfaces the incident radiation is partly absorbed and
a + r = 1
with transparent bodies, it may be absorbed, reflected or transmitted.
a + r + t = 1
An ordinary window glass transmits a large proportion of all radiation
between 300 and 3000 nm, i.e. both visible light and short-wave infra-
red, but very little around and outside the 300 to 3000 nm range. Its
transmittance is selective.
This selective transmittance can be modified by varying the
composition of the glass to reduce substantially the infra-red
transmission, whilst only slightly affecting the light transmission.
Such a product is referred to as heat absorbing glass.
Other special glasses
Whilst the heat absorbing glasses achieve a selective transmittance by
selectivity in absorption, the heat reflecting glass achieves a similar selective
transmittance by selectivity in reflection.
The glass is coated by a thin film of metal (usually nickel or gold), applied by
Such glasses absorb very little heat, therefore the improvement in reducing the
total solar gain is far greater, but unfortunately they are still rather expensive.
Recently, several types of photo chromatic or light-sensitive glasses have been
developed, containing submicroscopic halide crystals, which turn dark when
exposed to strong light and regain their transparency when the light source is
Their transmittance may thus vary between 74 and 1%. When the technique is
more developed and more economical, these glasses may have a future in