A cool idea - Centre for Science and Environment

CLASSROOM
AVIKAL SOMVANSHI
Delhi is set to become more
energy efficient. Chief Mini -
ster Sheila Dikshit has asked
the Bureau of Energy Efficiency and
municipal bodies to build cool roofs in
new buildings, especially the government
ones. She gave the directions in
mid-September. Such roofs reflect
sunlight and heat unlike the conventional
dark ones that absorb heat (see
‘How cool roof mechanism works’).
“Cool roof at ITC Green Centre in
Gurgaon reduces surface temperature
by 30 ° C and saves energy of air conditioning
in the top floor by 10 to 15 per
cent,” says Niranjan Khatri, general
manager, environment unit at the
company. According to a study by
Lawrence Berkeley National Labo ra -
tory (LBNL) in the US and
Inter na tional Institute of Information
Techn ology in Hyderabad (IIIT-H) ,
commercial buildings can annually
save 13 to 14 kWh/m 2 of energy by
swit ching to cool roof. A commercial
building with 150m 2 roof area can
save up to `15,000 annually in the
electricity bill.
Switching to cool roofs is no
rocket science. “A white wash of a dark
roof can cut indoor temperature by
5 ° C,” says Chitra Vishwanath,
an architect in Bengaluru. The concept
is not new. Almost all the houses in
Jaisalmer have their rooftops painted
white. Likewise, the common practice
of lime coating and roof po nds are the
ingenuous ways of reducing
roof temperature.
Present day architects have many
choices. They can either use cool roofing
material during construction or
retrofit the existing roofs with cool
roof technologies. One can also opt for
green roofs with vegetation cover,
which do not reflect sunlight but have
cooling benefits (see ‘The options’).
Delhi is the third city after
Mumbai and Hyderabad to promote
54
Down To Earth October 1-15, 2012
A cool idea
Roofs that reflect sunlight save electricity
this concept. “Cool roofs can reduce
the need for thermal insulation,” says
Vishal Garg, assistant professor at
IIIT-H. They also mitigate the heat
released from roofs during night. It
lowers air temperatures in cities that
contributes to formation of harmful
ozone gas. Besides reducing tempera-
THE OPTIONS
Use these while
building a house
COOL MATERIAL
ture, cool roofs last longer.
But one type of roof may not suit
all cities. Vishwanath says climate
decides the nature of application. For
instance, reflective coating is suitable
in the moderate climate of Bengaluru.
In Delhi, reflective coatings in combination
with earthen pots with broken
PHOTOGGRAPHS COURTESY: NEERAJ KAPOOR
How cool roof mechanism works
The Sun’s radiation
hits the roof surface
These are materials that are inherently reflective and
include white vinyl, white cement, white ceramic tiles,
aluminium and even recycled tetrapak sheets. In
picture: white ceramic tiles on rooftop of Institute of
Rural Research and Development in Gurgaon
Cool roofs reflect the
solar energy
Source - Lawrence Berkeley National Labo ra tory
Normal roofs absorb
the solar energy and
transfer heat to
buildings below
Normal roofs also release
some of the absorbed heat
back at night. This
contributes to temperature
rise in cities

How rooftops of two cities differ
Dark rooftops in Delhi White rooftops in Jaisalmer
bricks in lime mortar are better. A
semi open space with a temporary roof
can help in humid climates like that
of Chennai.
These consist of a layer of vegetation on the rooftop that
helps reduce heat flow through evapo-transpiration. One can
place potted plants on rooftops, have manicured grass or let
wild grass grow. In picture: use of white gravel with grass at
Institute of Rural Research and Development in Gurgaon
White v green roofs
● Green roofs are costlier than
white roofs. Green roofs have
high installation and maintenance
cost, while white roofs need
periodic refurbishing
● Green roofs have a longer life
● Green roofs improve storm water
run-off management whereas
white roofs optimise rain water
harvesting
● White roofs are three times more
effective than green roofs in
cooling down the neighbourhood
Manu Gopalan, principal architect,
Earthauz, an architecture firm
based in Auroville, warns, “One
should be cautious while choosing a
GREEN ROOFS COOL PAINTS
CLASSROOM
cool roof. Most paints are toxic and do
not last more than four years.
Instead, white bathroom tiles on roofs
increase the reflective properties and
minimise dust.”
Other experts point out that cool
roofs are effective mostly in low-rise
buildings where maximum heat gain is
via roofs instead of walls.
But is the city ready to turn cool?
A M Athle, chief architect with New
Delhi Municipal Corporation, is
optimistic, “People took time to
understand the gains of CFL. But it
eventually happened as savings were
difficult to ignore. Same will happen
with cool roofs.” ■
These paints, like white enamel and lime wash, have the
ability to reflect sunlight. Though they are economical, they
may not be the best as they wear away quickly. In picture: a
house with painted roof designed by Chitra Vishwanath
in Bengaluru
TRY THESE IN HOUSES ALREADY BUILT
Roof types Conventional Cool roof option Cool roof SRI * SRI*
Pay back**
Built-Up roof 37-50 Try white gravel 79 3-4 years
or smooth coating 100
Clay tile