Thermal qualities like temperature, heat, and heat transfer mechanisms are important for building design. Temperature is measured in degrees Celsius and heat is measured in joules. Heat transfers between objects through conduction, convection, and radiation based on temperature differences. A building's heat transfer is analyzed using factors like U-value, solar gain, ventilation rate, and time-lag. Maintaining a building's thermal balance requires considering all heat exchange between the interior and exterior environments.

1. THERMALQUALITIES FOR
BUILDING DESIGN
ArPa Ravi

2. TEMPERATURE
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
appearsto bewarmer.
▪ Temperatureismeasured by theCelsius scale.
▪ A position on this scale, i.e. the temperature of an
object is donated as: o
C but an interval or
differencein temperatureis: deg C.

3. HEAT
▪ HEAT
▪ SPECIFIC HEAT
▪ LATENT HEAT
▪ THERMAL CAPACITY

4. Heat
Heat is aform 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). Assuch, it ismeasured in general
energy units: joules (J).
Specific heat of a substance is the amount of heat energy necessary to
causeunit temperatureincreaseof aunit massof thesubstance.
It ismeasured 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 changein temperature. It ismeasured 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
causeunit temperatureincreaseof thebody, in unitsof J/degC.

5. Heat Flow:
▪ CONDUCTION
▪ CONVECTION
▪ RADIATION
▪ Unit

6. Heat flow
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 temperaturezones, by any or all of thefollowing ways:
▪ Conduction
▪ Convection
▪ Radiation
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
temperaturedifference, thefaster therateof 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)

7. Heat exchange processes between a building and the external environment
Heat exchange processes between a human body and the indoor

8. Conduction
Conduction heat flow rate through a wall of a given area can be
described by theequation:
Qc= A x U x ∆T
Where, Qc= conduction heat flow rate, in W,
A = surfacearea, in m²,
U = transmittancevaluein W/m² degC,
∆T= temperaturedifferencein degC

9. Convection
Convection heat flow ratebetween theinterior of abuilding and the
open air depends on the rate of ventilation, i.e. air exchange. The
rateof ventilation can begiven in m³/s.
In convection, heat is transferred by the bodily movement of a
carrying medium, usually agasor aliquid.
Heat transfer by convection takesplaceat thesurfacesof walls,
floorsand roofs.
Therateof heat transfer in convection dependson threefactors:
temperature difference (difference in temperature of the medium
at thewarmer and cooler points)
the rate of movement of the carrying medium in terms of kg/s or
m3
/s
the specific heat of the carrying medium in J/kg degC or J/m3
degC
These quantities will be used in ventilation heat loss or cooling

10. Radiation
In radiation heat transfer, the rate of heat flow depends on the
temperatures of the emitting and receiving surfaces and on certain
qualitiesof thesesurfaces: theemittance and absorbance.
Radiation received by a surface can be partly absorbed and partly
reflected: the proportion of these two components is expressed by the
coefficientsabsorbance (a) and reflectance (r).
Thesum of thesetwo coefficientsisalwaysone:
a + r= 1
Light coloured, smooth and shiny surfaces tend to have a higher
reflectance.
For theperfect reflectivetheoretical whitesurface: r = 1, a= O.
The perfect absorber, the theoretical 'black body', would have the
coefficients: r = 0, a= 1.

11. • CONDUCTIVITY
• RESISTIVITY
• CONDUCTANCE
• RESISTANCE

12. Thermal conductivity (or 'k-value') is defined as the rate
of heat flow through unit area of unit thickness of the
material, when thereisaunit temperaturedifferencebetween
thetwo sides.
Theunit of measurement isW/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, thebetter insulator amaterial is.
Resistivity isthereciprocal of thisquantity (1 /k)
measured in unitsof: m degC/W.
Better insulatorswill havehigher resistivity values.

13. CONDUCTANCE &
RESISTANCE
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(R).
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. Theunit of measurement is W/m² degC.
Resistance of abody istheproduct of itsthickness(b) and the
resistivity of itsmaterial:
R= b x 1/k = b/k
It ismeasured in m² degC/W.

14. MULTILAYER BODY
If a body consists of several layers of different materials,
its total resistance will be the sum of the resistances of
theindividual layers.
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
= b/kΣ
Cb = 1/Rb = 1/ b/kΣ
Note that the conductances are not additive, only the
resistances.

15. SURFACE CONDUCTANCE
In addition to theresistanceof abody to theflow
of heat, a resistance will be offered by its
surfaces, where a thin layer of air film
separates the body from the surrounding air.
Thisisthesurface orfilm-resistance.
It isdenoted as1/f (m² degC/W),
f being the surface or film-conductance (W/m²
degC).

16. OVERALL AIR-TO-AIR RESISTANCE
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
Where,
1/fi= internal surfaceresistance,
Rb = resistanceof thebody,
1/fo = external surfaceresistance,
all resistancevaluesin m² degC/W.

17. transmittance(u-value)
The reciprocal of the overall air-to-air resistance (Ra) is
theair-to-airtransmittance or U-value.
A U value is a measure of heat loss in a building element
such as a wall, floor or roof. It can also be referred to as
an ‘overall heat transfer co-efficient’ and measures how
well parts of a building transfer heat. This means that
the higher the U value the worse the thermal
performanceof thebuilding envelope.
U = 1 /Ra
Itsunit of measurement isthesameasthat of conductance-
W/m² degC.
This is the quantity most often used in building heat loss
and heat gain problems.

18. CAVITIES
If an air space or cavity is enclosed within a
body, through which the heat transfer is
considered, this will offer another barrier to
thepassageof heat.
It is measured as the cavity resistance (Rc)
which can be added to the other resistances
described above.

19. Sol-air temperature
For building design purposes, it is useful to combine the
heating effect of radiation incident on abuilding with the
effect of warm air. This can be done by using the sol-air
temperatureconcept.
Ts = To + [(l x a)/fo]
whereTs = sol-air temperaturein ˚C
To = outsideair temperaturein ˚C
l = radiation intensity in W/m²
a= absorbanceof thesurface
fo = surfaceconductance(outside), W/m2
degC.

20. 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
compositeclimates, when ventilation isreduced.
solargain factor = (a x U) /fθ o

21. Heat exchangein 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
beexamined.
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.

22. Convection
Convection heat flow rate between the interior of a building and the open air
dependson therateof ventilation, i.e. air exchange. Therate of ventilation can
begiven in m³/s.
Therateof ventilation heat flow isdescribed by theequation:
Qv = 1300 x V x ∆T
Where, Qv = ventilation heat flow rate, in W,
1300 = volumetric specific heat of air, in J/m³ degC,
V = ventilation ratein m³/s,
∆T= temperaturedifferencein degC
If thenumber of air changesper hour (N) isgiven theventilation ratecan be
found as:
V = (N x room volume) /3600
where3600 isthenumber of secondsin an hour.

23. Radiation through windows
Thesolar heat flow through windowsisgiven by theequation:
Qs = A x l x ,θ
Where, A= areaof thewindow in m²,
l = radiation heat flow density in W/m²,
θ = solar gain factor of window glass.

24. Periodic heat flow
All theequations and calculation methods seen so far arevalid if and only
if, both out-door and indoor temperaturesareconstant.
As perfectly static conditions do not occur in nature, the basis of the
abovemethodsistheassumption of steady stateconditions.
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 theenvironment.
Asthecycleisrepetitive, it can bedescribed asperiodic heat flow.

25. Time-lag & Decrement factor
The two quantities characterizing this periodic change are the time-lag (or phase
shift )θ and thedecrement factor(or amplitude attenuation µ).
The decrement factor is the ratio of the maximum outer and inner surface
temperatureamplitudestaken from thedaily mean.