heat exchange in buildings

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Environmental
Factors
Personal
Factors
THERMAL
COMFORT
Graphics source: https://www.linkedin.com/pulse/role-cfd-evaluating-occupant-thermal-comfort-sandip-jadhav/

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HIGHER
TEMPERATURE
LOWER
TEMPERATURE

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HIGHER
TEMPERATURE
LOWER
TEMPERATURE

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1
2
3
1
2
3
ROOF
WALL
FENESTRATION
Heat transfer through envelope components by
Conduction

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Short wave radiation transmitted
directly through glass
Heat transfer through Radiation

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LONG WAVE RADIATION
Heat transfer through Radiation

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Heat transfer through Convection

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Air circulation through
fenestration
Heat transfer through Convection

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External heat exchange
Short wave
radiation
Long wave
radiation
Convection
Conduction

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Internal heat gains
Occupants Artificial lighting Equipment / appliances

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SENSIBLE HEAT
(Heat that can be felt)
LATENT HEAT
(Heat absorbed or released
when there is a change in
phase.)
1. Radiation
1. Convection
1. Conduction
(For human body: mostly radiation and
convection from skin and convection
from respiration)
1. Evaporation of moisture
(For human body: evaporation of sweat
from skin and evaporation in lungs during
respiration)

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Lighting Heat Gains
Lighting use factor: the ratio of the time the lights will be in use. Typically 1 for most spaces
Lighting Special allowance: takes into account the heat from ballasts.
Fluorescent lights: Typically 1.2
Incandescent lights: 1.0 (due to the lack of ballasts in incandescent lights)

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Building Area Method Space Function Method
Lighting Power Density (LPD): ECBC 2017

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Short wave
radiation
Long wave
radiation
Convection
Conduction

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Short wave
radiation
Long wave
radiation
Convection
Conduction

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Short wave
radiation
Long wave
radiation
Convection
Conduction

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Modes of heat transfer
Conduction
(heat transfer in stationary medium;
through a solid or through solid objects in
physical contact with each other)
Convection
(heat transfer by the movement of fluids)
Radiation
(heat transfer in the form of
electromagnetic waves and does not
require any medium)
https://physicsabout.com/conductionconvection-and-radiation/

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Short wave
radiation
Long wave radiation
Convection
Conduction
External heat exchange

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Solar
radiation
Solar
radiation
Radiation
reflected
Radiation reflected
How much heat is reflected and
emitted is influenced by the Solar
Reflective Index (SRI) of the surface

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Heat transferred from outside surface
to inside surface through conduction
Outside surface temperature
increases
The amount of heat flow from outside
surface to inside surface is influenced by
thermal transmittance and thermal mass of
the wall and roof assembly

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Heat exchange now happens with the
inside surface and the human body
through radiation and convection
Radiation
from internal
surfaces
Convection

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Thermal properties of wall & roof
Reflectance & Emissivity, combined as Solar Reflective Index (SRI)
Thermal conductivity & transmittance (ability to conduct heat)
Thermal mass (ability to absorb and store heat)

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Solar Reflective Index
https://continuingeducation.bnpmedia.com/article_print.php?C=790&L=68

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The Solar Reflectance Index (SRI) is a measure of the solar reflectance and
emissivity of materials that can be used as an indicator of how hot they are likely to
become when solar radiation is incident on their surface. The lower the SRI, hotter
a material is likely to become in the sunshine
It is defined so that:
Standard black surface (reflectance 0.05, emittance 0.90): SRI = 0
Standard white surface (reflectance 0.80, emittance 0.90): SRI = 100
The dark coloured, conventional roofing finishes have SRI varying from 5 - 20
Solar Reflective Index

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https://continuingeducation.bnpmedia.com/article_print.php?C=790&L=68
High solar reflectance + high thermal
emittance = low surface temperature
Black roof
Surface 60°C - 70°C
White roof
Surface 40°C - 45°C
Air temp 37°C

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High SRI products
High SRI roof tiles High SRI paints China mosaic finish

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High SRI finish useful in in predominantly hot climates
High SRI finish mainly on exposed roof as roof gets the highest
radiation
Utilising SRI
High SRI = low surface temperature
less heat conducted inside during the day & high heat lost at night

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Heat transferred
in unit time,
through unit area (1 m2) of homogenous
material,
of unit thickness (1 m) of the material,
the two surfaces of the material differing
by one unit of temperature (1°C)
Thermal conductivity (κ or λ)
Measured in watts per metre-kelvin
W/(m.K)

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Thermal conductivity of different materials
Type of material Density (kg/m3) Thermal conductivity
(W/m.K)
Specific heat capacity
(kJ/kg.K)
Solid concrete block 25/50 2427 1.396 NA
Dense concrete 2410 1.740 0.88
Reinforced concrete cement (RCC) 2288 1.580 0.88
Solid burnt clay brick 1920 0.81–0.98 0.80
Perforated burnt clay brick 1520 0.631 0.99
Fly ash brick 1650 0.856 0.93
Aerated autoclaved concrete (AAC) block 642 0.184 0.79
Expanded polystyrene 24.0 0.035 1.34
Foam concrete 400.0 0.084 0.92
Rock wool (unbonded) 150.0 0.043 0.84
Mineral wool (unbonded) 73.5 0.030 0.92
Glass wool (unbonded) 189.0 0.040 0.92
Resin bonded mineral wool 64.0 0.038 1.00
Exfoliated vermiculite (loose) 264.0 0.069 0.88
For more materials see ECBC 2017 or Eco-Niwas Samhita 2018

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In most high insulating materials we know today, most of the volume the material
is air or, in a few cases, other gases or even vacuum. Thermal insulation
materials are therefore all lightweight.
In building materials, locked air is the main insulation material

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Heat transmission,
in unit time
through unit area of a material or
construction and the boundary air films,
induced by unit temperature difference
between the environments on either side.
Unit of U value is W/m2.K.
Thermal transmittance (U value)

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Thermal transmittance (U value)
Wall Roof
All Climatic
Zones
Composite Climate, Hot-Dry
Climate, Warm-Humid Climate,
and Temperate Climate
Cold
Climate
0.13 0.17 0.10
0.04 0.04 0.04
Source: Eco Niwas Samhita 2018

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For U-value of 0.4 W/m2.K
RCC
3.7m thick
AAC
0.42m thick
Brick
1.8m thick
XPS
0.08m

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• Property of the mass of a building which enables it to store
heat, providing "inertia" against temperature fluctuations
• Scientifically, thermal mass is equivalent to heat capacity-
amount of heat to be supplied to a given mass of a material to
produce a unit change in its temperature.
• Measured in Joule per Kelvin (J/K)
Thermal mass
Heat stored in an object
Q= m x cp x ΔT
m: mass
cp: specific heat
ΔT: temperature change

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Steady-state temperature Dynamic temperature variation

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Incident solar
radiation
Transmitted
Reflected
Absorbed
Re-emitted
Re-emitted
How much heat is transmitted and
emitted inside is influenced by the
Solar Heat Gain Coefficient
(SHGC) of glass

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Conducted
Conducted
Convection
Infiltration
Infiltration
How much heat is transferred inside
from the outside surface of glass to
the inside surface is influenced by the
thermal transmittance (U value) of
glass
Heat will also be conducted via the
fenestration frame is influenced by U
value of the frame
Some heat transfer will also happen
through infiltration

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Thermal properties of windows
• Solar Heat Gain Coefficient or SHGC (fraction of solar radiation
radiated inside through the glass)
• Thermal conductivity & transmittance of frames and glass
(ability to conduct heat)

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Solar Heat Gain Coefficient (SHGC)
Solar heat gain coefficient (SHGC) is
the fraction of incident solar radiation
admitted through a fenestration, both
directly transmitted, and absorbed and
subsequently released inward through
conduction, convection and radiation.
Incident solar
radiation
Transmitted
Re-emitted

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External shading devices impact the SHGC of
a fenestration by impacting the solar radiation
incident on
The impact of the shading device on the
unshaded SHGC results in SHGC equivalent
Effect of shading on SHGC

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Overhang
Side fin Side fin
SHGC Equivalent is the SHGC of a
fenestration with a permanent external
shading projection (overhang and side
fins)
SHGC equivalent

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1. Calculate projection factor (PF)
Calculation of SHGC equivalent
Overhang Right-side fin
Left-side fin

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2. Select External Shading Factor (ESF) value for each shading
element from Tables in Eco-Niwas Samhita 2018, corresponding
to the PF and the orientation
Calculation of SHGC equivalent
• Overhang (ESFoverhang): Refer Table 10 and Table 11
• Side fin-right (ESFright): Refer Table 12 and Table 13
• Side fin-left (ESFleft): Refer Table 14 and Table 15

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2 tables for shading element
corresponding to the site location
For locations with LAT≥23.5˚N
For locations with LAT<23.5˚N

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3. Calculate the total external shading factor (ESFtotal)
Calculation of SHGC equivalent
ESFtotal = ESFoverhang × ESFsidefin
where,
ESFsidefin = 1- [(1- ESFright) + (1- ESFleft)]

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4. Calculate the equivalent SHGC of the fenestration (SHGCeq)
Calculation of SHGC equivalent
SHGCeq = SHGCunshaded × ESFtotal

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Annual graph of shaded, un-shaded and difference in
SHGC for a south facing orientation with an awning in
New Delhi, India.
Kohler,C., Shukla, Y., Rawal, R. (2017). Calculating the Effect of External Shading on the Solar Heat Gain Coefficient of Windows

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Radiation
(2/3 of heat transmission in
conventional DGU)
Convection
Conduction
(1/3 of heat
transmission in
conventional
DGU)
Heat transmission through
radiation can be nearly
eliminated by a coating
Having argon instead of an
air gap reduces conduction
Convection can be reduced
by optimization of the gap
Low emissivity
coating
Double Glazed Unit (DGU)

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Properties of different glazing types
Glazing type Glass pane
thickness (mm)
U value
W/(m²K)
SHGC VLT
Single clear glazing 6 6 0.81 0.89
Double glazing (clear) 6 2.7 0.70 0.79
Double glazing (low-e) 3 1.8 0.71 0.75
Triple glazing (clear) 3 2 0.67 0.74
Double glazing, argon filled
(low-e)
6 1.4 0.57 0.73
Source: www.wbdg.org/resources/windows.php, Whole Building Design Guide
Double glazing (low-e)
SKN Envision
6 1.5 0.33 0.55
Source: Saint Gobain

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Thermal transmittance of different frames
https://www.vinyltek.com/about/green-commitment/efficiency-redefined/