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A cool idea

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Oct 15, 2012 | From the print edition

Roofs that reflect sunlight save electricity

Delhi is set to become more energy efficient. Chief Minister 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’).

roof

“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 Laboratory (LBNL) in the US and International Institute of Information Technology in Hyderabad (IIIT-H), commercial buildings can annually save 13 to 14 kWh/m2 of energy by switching to cool roof. A commercial building with 150m2 roof area can save up to Rs 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 ponds 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’).

image

Delhi is the third city after Mumbai and Hyderabad to promote 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 temperature, 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 bricks in lime mortar are better. A semi open space with a temporary roof can help in humid climates like that of Chennai.

Manu Gopalan, principal architect, Earthauz, an architecture firm based in Auroville, warns, “One should be cautious while choosing a 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.”

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Definitely an appreciable cool idea which never struck my mind before. It's vernacular and easy to implement. 30 degrees Celsius difference in temperature is magical, admirable, although an  unacceptable figure. I will personally favour the idea of vegetation on roof which can act as a replacement to the vegetation harmed by the concrete structure. This cool idea can be extended to the walls.

1 October 2012
Posted by
Madhav Bhardwaj

Dear Madhav,

I am glad that the article added to your knowledge base. As for 30 degree Celsius temperature difference, the point to be noted is that Khatri is referring to the temperature of the roof surface, not that of the air; and here in Delhi, temperature of the roof surface can shoot up to 90 degrees Celsius in peak summer. The reduction which Khatri claims is on the higher side but it is not magical. Scientific studies by LBNL and IIIT-H have recorded 15-20 degree Celsius on average by converting a black roof to white roof in Hyderabad. Reflective coatings used in ITC-Gurgaon building are of way higher SRI value then used in the said studies, which sort of support Khatri's claim and I have just quoted him (haven't personally verified his claim). You can read about these studies online. Green or cool roof is your personal choice and project need, I will be happy if you are using anything except dark uncool roof, do refer to white v green roof box on the option section.

Regards,

Avikal

10 October 2012


Posted by
Avikal Somvanshi

Hey...As per me, not only in Delhi, but even in case of Ahmedabad the problem is similar--people prefer to avoid buying property on top floor, considering the amount of heat the slabs transmit to the flat below, converting it into an oven. In general, I would prefer to replace the top most slab with filler slab technology to create a buffer and minimise the temperature inside the room. Also, on the top of that I will suggest to create mosaic of white reflective ceramic tiles, which as Manu Gopalan said, would not be toxic and would be a permanent solution for the cause; plus it adds to the waterproofing requirements. Recycling of reflective tiles out of their broken pieces, easily available with tile manufacturing companies, is cost efficient, too. In corporate offices where AC consumes much of energy during day time, it will work for sure and also in residential buildings. Iin this write up, u mentioned about commercial buildings; so for instance, if say there is a mall, then on the large atrium of the malls, would painting the sheets covering them minimise the heat below? Or will these TATA coated sheets available  act similarly???  Ar.Rohit Tak Pune

2 October 2012
Posted by
Rohit Tak

 Dear Rohit,

Good to know you are already thinking about cool roofs, honestly it is all common sense and most architects are well aware of it. As for the atrium of malls and similar structures, yes, applying cool roof technology will surely bring down the cooling energy need. But painting the sheeting covering of the atrium skylight won't be a very wise step as it will cut off the natural light from it. I'll recommend using sheets which are of inherently reflective material or cover the skylight with vinyl-coated polyester mesh fabric or similar products. Tata Bluescope roofing sheets which are colour coated are good option compared to a dark roof. Any metal roof with lighter colour coating shall reduce the heat gain but they normally need to be coupled with some kind of insulation underneath to cut down the heat transfer due to their high thermal emissivity and low thermal mass. Tata Bluescope website claims some high end sheets to be of lower thermal mass and higher heat reflectivity, but they do not give any numbers regarding SRI, which is the real indicator of roofs' cooling ability, so I am a bit sceptical about their actual performance. You can read more about SRI and roof cooling options at www.coolrooftoolkit.org

Regards,

Avikal

10 October 2012


Posted by
Avikal Somvanshi

Dear Avikal

Impressed by the simple solutions of creating a cool roof.

For a 20 year old concrete roof in Kolkata, would 'smooth coating' make sense (if i understood the 'options' correctly). Does it mean painting the roof white with a commercially available white exterior paint. Is there a non-toxic paint available? Would lime-wash suffice?

Too many queries...

8 October 2012
Posted by
Tautik

 Dear Tautik,

Thank you for your kind words. Yes a smooth white coat will be effective to reduce the heat gain. While choosing the paint check for that the paint is free of lead and Volatile Organic Compounds (VOC). Normally these are mention on the product itself. Lime wash will be an excellent option as its both non toxic and light on pocket. Only issue with lime wash is that you will have to redo it annually after every monsoon. If that is fine with you, I'll recommend lime wash over commercial paints (they will last for 3-4 years). If you are looking for a more permanent solution, laying white bathroom tiles or a terrazzo work with broken white tiles or chips on your roof is most recommended. Be free to ask any other queries if you have, I will be happy to answer.

Regards,

Avikal

 

10 October 2012


Posted by
Avikal Somvanshi

Black body is a good absorber of the radiant heat and is a good emitter too.
White bodies are different from this.
Now a days,one can get white coloured cement paints in the market ,that can seal minute cracks and pores as well as it gives this added effect in thermal properties also.
One can have two birds in one shot too.
I have tried recently.
It is effective.
Why not you too ?

11 October 2012
Posted by
M.Ravindranathan

I have done an experiment at our organization(on 12,13 March 2013).
We have a 2 floor building in vijayawada(A.P), the roof is openly exposed to solar radiation. To protect from the heat being conducted(as our office is airconditioned), we have insulated the roof from the bottom with thermacoal sheets. Still my assumption was that, heat may be getting transfered through pillars and walls(as pillars/walls are in contact with roof). For this to understand, I have filled up the roof with water(all through the day) and observed if there was any reduction in power consumption. But there was no reduction in power consumption. I have repeated the expt, but still no decrease in power consumption was observed.....so finally i concluded that, power saving by stoping radiation will not work in our organization....
Please let me know your feedback on this...

28 March 2013
Posted by
Mallikarjun Nalluri

Dear Mallikarjun Nalluri

Heartening to read about your experiment with roof insulation, it is sad you felt lost at the end of it. What I can figure out by your description of the 2 experiments is that they were very short duration, and you gave up too soon. Let me explain.

The first one with thermacol sheets was flawed to start with. Putting insulation under the roof is wrong practice for hot climate like ours, as this allows the roof to get heated by the sun and transfers it via other elements in building like you observed (walls and pillars). Under the roof works for cold climate where there is a need to keep the heat inside. In hot climate it should be on the top of the roof stopping it from gaining heat. Like your second experiment did by creating a water pool on your roof. And it is strange that you could not feel the difference in the thermal performance of your office space. One of the reasons for this has to be that your office is completely air conditioned, thus you and your staff might not have experienced dropping of temperature. Second reason for this is your quantification of saving via monitoring power consumption. There won’t be any power saving if you continue using air-conditioning at same settings as before. Advantage of have a well insulated roof is that it brings down the room temperature by 2-5 degree centigrade, allowing the thermostat of the AC to be set at higher temperature around 24-26 degree centigrade. If you didn’t do that your power meter won’t register any change. And in longer run this also translates to lesser use of AC as there will be more days in the year when thermal conditions will be naturally pleasant. You should give these passive measures more time to kick in, over a day difference do happen but then your building should have been un-condition to really feel the change.

Permanent water pool on roof is not best of the option, as it is very water and resource intensive as you will have to regularly replace the evaporated water. Additionally there may be water seepage problems. Like discussed in the article a white wash or a vegetative garden will offer more durable solution.

Keep me posted about your experiments, and keep experimenting. Who knows you may discover something even better.

Thank you.

Regards,
Avikal

29 March 2013


Posted by
Avikal Somvanshi

we have done couple of experiments at our R&D building near TADA AP, a very hot place in Summer. An east facing SW development center with complete glass panels in R&D used to keep the AC ( 2.5 ton ) in 18 to 20 deg C during summer till 2012. Even then it used to be uncomfortable. We developed a thick wall of local creepers 5 feet away from the glass panels in 2012-13. From 2013 summer on wards till date the AC is being used at 26 deg C. For cross ventilation when required Remi Wall mounted DC fans ( 12V, 22W ) are being used which consumes no electricity or battery power, runs only on solar panels from 8 am to 5.00 pm. Also my own room facing the wrath of south sun and other engineers facing the west sun also had this problem. First we have put bamboo chicks on the outside ( 10 deg C temp benefit we got ) and now we are growing green cover also. I have't switched on my AC since two years as I have a nice USB fan and also DC fan with a container to hold ice cubes with tiny holes mounted firmly on the backside of the fan. Also I have built a 100 % eco friendly house at Sulurpeta near Sriharikota where I have done thermal, water, energy and waste management. Can post the article if interested. Ramkumar Senior VP R&D, ReGen Powertech

23 May 2014
Posted by
M P Ramkumar

We are planning to make fix solar water heating panel as a canopy on our terrace.

Besides the cost benefit of having hot water , there will also be benefit of having heat reflected from the terrace surface.

Does one need any special permission from BMC to install these panels over roof top in Mumbai city?

3 April 2013
Posted by
Viren

Hi Viren,

You don't need any kind of special permission from BMC to install solar water heating panels on your roof top. In fact you can avail government subsidy or soft loan for these under Jawaharlal Nehru National Solar Mission (JNNSM) of the Ministry of New and Renewable Energy (MNRE). You may also refer to MNRE official website at “www.mnre.gov.in” for more details.

As for cool roof benefits, they will be limited. I'm assuming your panels wont completely cover the whole of your roof top, thus portions of it wont be shaded from the sun. This will lead to heating up of the roof. My advice would be to apply one of the cooling techniques mentioned in the article for the portions of your roof which wont be shaded by the solar heater panels. This should improve your buildings performance.

Thank you.

Regards,
Avikal

4 April 2013


Posted by
Avikal Somvanshi

Many thanks Avikal, will go thru mnre website along with BMC guidelines also .

4 April 2013
Posted by
Viren

Will white reflective bathroom tiles perform better than white glossy paint that is used to paint metal structures?

25 April 2013
Posted by
MySchizoBuddy

Another question regarding residential roofs. Should i just opt for a cool roof made of white ceramic tiles and forget about the thermocol in the roof itself or should i do both of them.

26 April 2013
Posted by
MySchizoBuddy

Dear MySchizoBuddy,

Old/broken white bathroom tiles over any synthetic paint anytime. Tiles will be both more durable and non-toxic. And as for your second question, you need insulation for sure in your roof, cool roof will add to that. Just cool roof wont be as effective. So go for both.

Cheers,

Avikal

26 April 2013


Posted by
Avikal Somvanshi

In 70s itself I designed a cooling system. Usually in South India people use mud pots for water storage. These mud pots have a cover(concave shaped). It has about 1foot diameter and i cm thickness. When inverted it will have convex shape. On a flat roof these CLAY COVERS will be arranged side by side with gaps filled with mud. These are white washed(calcium coated). One gets on an average 5 degrees Celsius drop from ambient temperature. The principle is white colour reflects then the sunrays have to pass through mud and air trapped between clay cover and roof. Still air acts as INSULATOR. These clay covers can be made locally.

Yet another method is to spread a polythene sheet on the roof and covering with thin layer of soil. One can grow fast growing plants like Coriander seeds and mustard seed. these plants germinate quickly and the roots spread horizontally. One can spray water. This way the temperature inside the building will come down drastically. After the Summer season, the polythene sheet can be rolled back.
Dr.A.Jagadeesh Nellore(AP),India

18 May 2013
Posted by
Dr.A.Jagadeesh

I live in a flat on the 8th floor which is also the top floor/ I have three windows facing west. I want to reduce the heat from the roof top. Although nobody walks on the roof but I am not very clear whether thermocol will withstand the pressure if somebody walks on it even if it is covered with white tiles. Please suggest the best option.

18 May 2013
Posted by
G C Saxena

Dear Saxena ji,

Thank you for inquiry.

West facing windows can be a big source of heat, first measure which you can explore is to add a chhajja (sun shade) on the window if you don't have one already. West sun will also require some vertical shading device too. Explore options of having vertical fins on the side of the windows.

Your second question regarding roof insulation using thermacol. Thermocol can be used, but it will absolutely need a good protection. This protection is generally a layer of 3''-thick PCC (plain concrete) with joints every 6 feet, and that entails using a very strong, high-density thermocal, capable of supporting permanently the load of the PCC. Ordinary thermocal won't do. Then this PCC can be covered with white tiles. It's a relatively advanced solution, which addresses both roof insulation and cool roof. It is best if you can do this.

Regards,
Avikal

22 May 2013


Posted by
Avikal Somvanshi

An interesting article indeed. I am building my own house and had come to the conclusion myself by googling on this over the last few months that white ceramic tiles would be the best solution for cooling the roof, with the next best solution being white paint. The two issues that bugs me regarding ceramic tiles are:

1) Whether the joints between the tiles will crack/come off over time due to heat and UV exposure leading to water seepage.

2) Ceramic being a poor conductor of heat, the tiles will also trap the heat well, thus not letting the heat escape during night.

I live in Kolkata.

6 July 2013
Posted by
Pinaki

Dear Pinaki,

You have some genuine concerns. Water seepage from the joints is a serious problem as the building ages, I would recommend that you use broken ceramic tiles to do mosaic work on your roof instead of laying the tiles in the regular formal fashion. Make sure the spacing between the tile pieces around one centimeter, and finishing coat is in white cement. This will drastically reduce any future seepage problem and will also take care of your heat trap concern.
Do send me pictures of your roof once its done.

Cheers,
Avikal

20 July 2013


Posted by
Avikal Somvanshi

1cm spacing of cement(even if it is white cement) will turn dirty in few months.
Another problem the 1 cm spacing with broken tiles will end up in having 10-20% of the total area being covered in cement exposed to Sun thereby reducing the effect of cool roof.
And then of course I will have to buy good tiles and break them to make it, as I am unaware of a source of broken tiles here.
Cost of installing broken tiles with 1 cm gap between pieces will also be higher.

I have seen old buildings with roof covered with broken china pieces just the way you suggested, I am not sure of the source of materials and cost of laying it.

I will keep looking for solutions.

20 July 2013
Posted by
Pinaki

Found a product called Thermatek. Do you have any feedback on this?
Called them up and it sounds very costly though. Not sure how effective it is and how long will it last.

27 July 2013
Posted by
Pinaki

Hello,

I need some advice regarding the roof top of my house that we are moving into. I stay in Guragaon and have this roof top wherein 300-400 fot area is covered in tin sheets, in hut type structure. I would like to know what kind of insulation can be done as this is also the area of indoor games for my three kids. Would appreciate a response.

31 August 2013
Posted by
omana s arora

Dear Omana,

First thing to start with will be painting the tin-shed white or in any other cool-colour which have high albedo. This should be readily available in any hardware store, I am assuming you are not harvesting rain from this roof, if you are then look for lead free toxic paints. This measure will drastically reduce the heat which the roof absorbs. To cut it further down, you can add some insulation in form of padding under the roof from inside. I suggest using grass or bamboo mats as padding, as these are organic and non-toxic. One inch thick layer will be enough. You can use lime plaster seal cover it in order to stop insects and rodents from taking home in it.
Keep us updated on your little project. Do share your experiences and pictures for benefit of everyone.

Thank you.

Very Best,
Avikal

5 September 2013


Posted by
Avikal Somvanshi

best cool terrace solutions :

1. Use a tin/asbestos shading roof on terrace.

2. If you like stargazing & prefer a open roof, put a XPS board (Extruded polystyrene) layer on terrace and cover it with white coloured tiles or heat resistance tiles

Low Budget ideas :
3. Source some old bathroom ceramic tiles and put a layer on your terrace, paint them white with waterproof paint or heat reflective paints which are available plenty in market viz.. SURYA COOL,THERMATEK PAINT (google these keywords to get more details)

4. Fix a shade net/tarpaulin which is commonly available in hardware stores its usually green coloured (http://www.kasturiagronet.com/), Fix the net/tarapaulin across the terrace walls with a gap of 5-10ft .

8 January 2014
Posted by
JR

Avikal Somvanshi,

Hi Avikal, Bangalore temperatures are touching 40 degrees...unbearable...chennai is still 35 degrees...
Any how read all your above articles and I am so educated to beat the heat. Before I could read your article...I had ventured into purchasing 3 mm thermocol sheets and had insulated it on my roof top although it was hassle free now...going forward dont know what all issues I may face as we move to rainy days because water may start clogging...but I can remove the Celo-tape which I used to join these sheets and fold it and store it until next summer much effortlessly...

My current problem is my roof top is 1000 sq ft and close to 900 sqft is exposed to hot sun for 10-12 hours...in my current experiment I have covered just 500 sqft of my roof top with thermocol (i.e just two bedrooms) sheets rest is still exposed to sun and I have not seen any reduce in temperature in thes bed rooms instead during aternoons the temperatures in these bedrooms have risen...and it is more suffocating compared to the other areas of my house... Is this because I have covered only half portion and the heat starts percolating to these covered interiors ?

Also I had an Idea to fill some buckets of water under this thermocol so that it will reduce the temperatures and keep the interiors cool and as the water is under the thermocol it may not get evaporated too fast and weekly once 4-5 buckets of water should help me give the AC effect as the thermocoal also will hold the water from running away and going into the drain, pls share your knowledge in this regard.

3 April 2014
Posted by
Vinay

Avikal Somvanshi,

My experiment went waste....can u delete my above post.

The fact is I bought the thremocal sheet of 3mm..and Tried to stick it with a adhesive tape...but it did not with stand the air and it flew in the air ....so from that day till today ...their was no change in the temperature at all.

My current problem is I am unable to stick the thermocal sheet to the roof-Floor...because of the dust and hot temperature the tape does not hold to the roof floor...

What I have decided is to Stick the corners of the Thermocal sheet with adhesive tape and then put Fevicol on the Roof Floor and also on the adhesive tape part and then try to stick to the roof floor...

so that Adhesive tape which is already STRONGLY binding/sticking to the thermocal it will also help me stick to the rooffloor much stronger...

If u did not get what I explained here..let me know I will post few photos for you to understand what I am facing.

16 April 2014
Posted by
vinay

You are doing good job. I have problem. I want to make cattle shed 500 Sq feet and i have idea to save cost because i cannot bear it. I have two ideas number one is to make Iron roof structure and bind 8*4 thermocol 3mm sheet to cover the whole roof .i face rain average two to three times in a year but temperature is almost 50 some time above 50 and also more then 9 month of long summer season Can it work under hot sun light for long time . the fixing problem i can solve to bind each sheet with frame at various points . waiting for kind suggestion.

12 May 2014
Posted by
arshad

Can i make whole shed roof with thermocol can it bear sun light . the aera is 5000 sq feet the sheets are fix on the steel frame . i have idea to use 3 inch thick sheet lenth of 8 *4 feet , give me right suggestion and the how long these sheets can work i have more then 9 month hot summer but rain is average 2 to 3 times a year

12 May 2014
Posted by
arshad

Hello Avikal

I stay at the top floor of a two-floor house in Faridabad and like many others I am distressed due to extreme weather conditions in summers and winters.
I read quite a few comments on this page but honestly I am confused which is the cheapest and best way to cover the roof. The end objective is that the temperature inside the top floor should be 6-7 degrees less than the outside temperature in the summers and should be 6-7 degrees more than the outside temperature in winters. So that the overall energy consumption to heat or cool the house goes down and it becomes more comfortable to stay inside the house.

Kind Regards,
Hitesh Batra.

Could you please suggest which is the most optimum solution.

9 June 2014
Posted by
Hitesh

Dear Hitesh,

Cool roof or painting the roof white might help reduce heat gain in your rooms by 3-5 degrees but to get a more effective and all season fix to heating/cooling problem, you should try insulating the roof. You can read about insulation here http://www.downtoearth.org.in/content/what-s-good-insulation

Regards,
Avikal

9 June 2014


Posted by
Avikal Somvanshi

Dear Mr.Avikkal,
Nice posts for cool roof. I live in chennai and experience the torture of 40* C. I am planning to build a house . I have found out that the cool roof tiles with white cement, dolomite, pp fibre and nano acrylate will serve best
Vaidy

16 July 2014
Posted by
Vaidy

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