CHAPTER 3: RESILIENCE STRATEGIES: ROOF COOLING AND PASSIVE VENTILATION
3.1 Introduction
UHI mitigation strategies vary from providing natural vegetation to cool roofs and cool pavements.
Different strategies are suggested by different experts in the ield of urban planning, architecture,
natural resource management and transportation. These strategies have an impact on both local and
global climates.
In addition to environmental beneits, these strategies also help in reducing the energy
consumption for cooling and increase the thermal comfort in buildings and slums during summers. This
chapter discusses different such strategies both for indoor and outdoor environment of a building, like
cool roof and cool pavements, passive ventilation, insulation, etc. Local climate should be considered
before deciding the suitable mitigation strategy or combination of complementary strategies for
dealing with UHI.
3.2 Trees and Vegetation
Trees and vegetation are simplest way to reduce UHI effects. Trees help in increasing the albedo of
the surfaces. Planting of trees and vegetation has both direct reduce CO
2
from atmosphere and indirect reduce energy consumption contribution
in reducing CO
2
from the atmosphere [32]. Other study revealed that amount of CO
2
reduced due to indirect effect is considerably greater than the
amount consumed directly in the photosynthesis of plants [33].
Trees and vegetation improve the air quality and also lower surface and air temperatures by providing
shade and through evapotranspiration. They are useful as a mitigation strategy when planted in
strategic locations and use of appropriate species of plant around buildings and on the streets.
.
3.3 Green Roofs
A green roof is a vegetative layer grown on a rooftop. Green roof reduces the surface temperature and
provides shading, like trees and vegetation and reducing the air temperature by Evapotranspiration
mechanism. Green roof temperatures depend on the roof’s composition, moisture content of the growing
medium, geographic location, solar exposure, and
other site-speciic factors [34]. Green roofs can be installed on various buildings like industrial, public
and private. There are two kinds of green roof based on plant density and vegetation: 1. Extensive and
2. Intensive.
In Germany, Green space is used as a premium in urban growth to limit the urban sprawl and
developers are often required to provide green roofs as a compensation of lost open space [35].
Green roofs ilter particulate matter from the air and a study estimated that 93 m2 green roof can remove
about 40 pounds 18.15 kgs. of PM from the air in a year, while also producing oxygen and removing
carbon dioxide CO
2
from the atmosphere [36].
3.4 Cool Roofs and Cool Pavements
In most urban areas, roofs and pavements constitute almost 60 of the land surface area
[37]. Most of the solar energy gets absorbed by these paved surfaces and roofs, increasing the
roof and ambient air temperatures. A dark colour roof absorbs more solar energy in comparison to
the light colour roof. The higher the relectivity and emissivity of the roof material, the less likely it is to
store the heat and radiate it back into the building through the walls and roof [38] [39]. The observed
maximum 50
O
K temperature difference between surface and ambient air for low albedo surfaces
while for high albedo surfaces, this temperature difference was found to be only 1
O
K [40]. High absorptive and less emissive surfaces
contribute in keeping the ambient air temperature high and thus indirectly increasing the cooling
demands of the buildings. It was estimated
that retroitting with cool roof options, 100 m
2
of roof offsets 10 tons of CO
2
emission. They also estimated that permanently retroitting urban roofs
and pavements in the tropical and temperate regions of the world with solar-relective materials
would offset 44 billion tons of emitted CO
2
, worth 1.1 trillion at 25ton [37].
3.5 Passive Ventilation
Passive ventilation is the most cost effective means for achieving thermal comfort. Before airtight houses
and mechanical ventilation systems, people used to keep their houses cool by regulating the air low
inside the building with the help of appropriately positioned window openings and thermal mass.
These methods were traditionally part of the building design.
Passive cooling systems use the minimum or no mechanical energy to keep the buildings cool.
Passive ventilation can be deined in two ways 1.Pressure system and 2.Buoyancy system.
Pressure systems rely on positive pressure on the windward side of the building causing a lower
pressure on the leeward side, in turn creating air movement through the building. Hot air becomes
light and rises up. The buoyancy system works on principle of removing this light hot air through
ventilators in buildings.
SCHEMATIC DIAGRAM OF URBAN HEAT ISLAND MITIGATION STRATEGIES IN BUILDINGS
Planting of Vegetation
Shading Devices
Building Envelope Energy Eficiency
Alternatives to air conditioning
Building
Protect from solar radiation
Minimize heat iniltration
Reduce anthropogenic heat
Maintain comfortable thermal environment
Movable Optimum insulation
Energy Eficient lighting
Ground coupled heat exchanger
Natural and controlled mechanical ventilation
Radiant cooling system
Solar cooling
Energy eficient household appliances
Energy eficient ofice equipments
Air tightness
Relective materials
Thermal inertia Fixed
Green Roofs
Green Walls
Vegetation around buildings
Adopted from Melissa Giguere [32] F5
Solar Relectance
Solar Emittance
Solar Relective Index
Thermal Comfort Life Expectancy
Types of Slope Elastomeric Acrylic Coating
water based Elastomeric Acrylic Coating
solvent based Cementitious Coating
Spray Polyurethane Foam Heat Insulation Tiles
Clay and Concrete Tiles Thermoset Membrane
Thermoplastic Membrane Modiied Bitumen
Metal Cool Rooing Built-up Rooing
Extensive Green Roof Intensive Green Roof
Roof Pond Cooling System Roof Mist Cooling System
Thatched Roof
0.7 - 0.85 0.7 - 0.85
0.7 - 0.85 0.7 - 0.85
0.77 0.7
0.7 - 0.8 0.7 - 0.8 white
0.4 - 0.6 colour 0.6 - 0.75
0.5 - 0.7 0.5 - 0.8
Low Low
Low High
Low 0.8 - 0.9
0.8 - 0.9 0.8 - 0.9
0.8 - 0.9 0.94
0.8 - 0.9 0.8 - 0.9
0.8 - 0.9 0.8 - 0.9
0.8 - 0.9 0.8 - 0.9
Low Low
Low Medium
Low 100 - 112
100 - 112 100 - 112
100 - 112 96
92 - 100 92 - 100
92 - 100 69 - 93
55 - 85 55 - 100
Low Low
Low High
Low 3 - 8
o
C 3 - 8
o
C 3 - 6
o
C 3 - 6
o
C Good
Good Good
Good Medium
Low to High Medium to High
High High
2 - 4
o
C 2 - 5
o
C Medium to High
6- 7 years 7 - 10 years
7 - 10 years 5 - 7 years
50 years 50 years
10 - 20 years 20 - 30 years
10 - 30 years 20 - 50 years
10 - 30 years 5 years
10 - 15 years 5 years
5 years 5 years
Low Steep Low Steep
Low Steep Low Steep
Steep Steep
Low Steep Low
Low Low Steep
- Up to 40
Up to 4 Low
Low
ROOF COOLING OPTIONS
T1
Steep
Earth Air Tubes Wind Tower
Solar Chimney Passive Downdraft Evaporative
Cooling
Orientation of the Building Attic Ventilation: Whole House Fan
False Ceiling Earth Berming
Rat Trap Bond Masonry Building Insulation
Rammed Earth Walls Exterior Shading Devices
Shading: Landscaping ADVANTAGES
LIMITATIONS
Most effective in dry and hot climate Not effective during early morning and late evening
Provide comfort by cooling the building structure at night
Not effective during summer day time Beneicial in hot and dry weather
Not effective in very humid region Most economical solution
Site constraints in high density urban areas Most effective during early morning and late evening
Noisy if not properly installed Reduces the heat load on HVAC system, thereby
reduces the energy costs Very effective natural heat sink
Water seepage problems can occur Cost effective alternative to conventional brickwork
for better thermal and sound insulation Building services like electrical, plumbing and
drainage have to be planned in advance Also improves weather prooing and sound
resistance. Makes it dificult to ix heavy items on the insulated
walls Very effective thermal mass - cool in summer and
warm in winter Not feasible for high rise buildings
Least cost solution for cutting heat gain into the building
Applicable in all weather conditions Dense vegetation can make the air warmer near the
building Ensures better indoor air quality and fresh air
circulation for effective year round comfort Needs access to additional land for running
underground tubes
PASSIVE COOLING TECHNIQUES: ADVANTAGES AND LIMITATIONS
T2