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I live in India where most of the building roofs are made of concrete. In summer during night, the roof emits too much of heat that were absorbed during day time.

I was aware of the options like having good ventilation, insulation, trees etc. But I want to be very specific with concrete roof top.

The inside roof is painted with white cement.

I am looking for some kind of paint or any other material that can be applied on top (outside) of the roof so that heat is reflected back instead of concrete absorbing it

  • Welcome to Sustainable Living! Could you please describe the link between your question and sustainability? Do you somehow want to use the heat as energy? Or do you want to save on aircon costs? Or are you worried that the heat will affect the lifespan of your roof? – THelper Feb 23 '15 at 12:00
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    @THelper primary reason is to save aircon costs. I've updated my question. – Gopi Feb 23 '15 at 12:09
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    Is the roof painted white currently? – EnergyNumbers Feb 23 '15 at 17:07
  • @EnergyNumbers Yes, inside the home the roof is painted with white – Gopi Feb 24 '15 at 4:36
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    I think EnergyNumbers means the outside of the roof. White objects reflect much more solar radiation than black objects, so a white roof will be much cooler than a darker colored one. – THelper Feb 24 '15 at 8:12

19 Answers 19

13

Try the simple answer before looking for more complicated solutions.

Apply a white coating. They should be much more durable than a simple white paint, and a good quality coating will return nearly 90% of solar energy to the atmosphere. This should make a big difference.

Alternately, cover your roof with solar panels. Since they transform energy rather than returning it to the atmosphere, they have a slight geo-engineering benefit. Specifically they reduce atmospheric heat and they don't drive away clouds the way white roofs do.

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    Could you explain how white roofs drive away clouds? – Mutagon Apr 2 '15 at 23:16
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    Yes, here. – Sophit Apr 3 '15 at 0:43
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    Solar panels do not reduce the amount of heat in the atmosphere. This is because all electricity will eventually end up as heat in any case. – Uli Alskelung Von Hornbol May 29 '18 at 18:20
  • What if Tarpaulin is placed all over the concrete roof with elevation of 4 inches from all sides, making it a pool with 4 inches depth and then water is poured in? Overall weight of the water and evaporation rate is required to be calculated to make it more feasible. – bjan Apr 7 at 13:37
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@techjerk I too live in India and have the same problem. I don't want to use aircon because it consumes too much energy by local standards.

I have so far considered the following:

  1. Painting the concrete roof white
  2. Adding a layer of roof tiles (placed directly on floor of roof) which allow air to circulate under and over, but keep convectional heating of the roof slab to a minimum). This layer of roof tiles is fragile inherently and roof cannot be used for walking, etc
  3. Building another sloped tiled roof over current flat concrete roof supported by metal columns. [roof is usable] as shown here. Image shown is for a factory but imagine the same for a mid sized home.

Something needs to be done about west facing walls too. Mid-day to evening is when the house seems to absorb most heat.

I was also thinking about contacting The/Tata Institute of Fundamental Research (TERI) who have done a lot of studies/work in sustainable housing.

IMHO, the paint/material of inner side of roof will not make any difference. The heating must be managed on the outer side of roof.

10

Throw a bunch of large potted plants up there - maybe even something edible. They'll shade the roof and help feed you.

  • Even if this won't reflect sunlight, I like this solution. We could also put earth directly on the roof to plant! – J. Chomel May 12 '16 at 9:18
  • Putting earth directly on the roof might not be a great idea. There are all kinds of problems from weight to moisture retention that could damage the structure. – That Idiot Feb 13 '18 at 15:18
5

There are a lot of good answers here and shading the roof by whatever means to reduce the amount of heat from the sun hitting roof will all help.

As I understand your problem heat stored in the concrete roof is radiating into the rooms immediately below.

If the rooms immediately under the concrete roof have high ceilings, false ceilings can be constructed approximately 300 mm below the current ceiling.

Place insulation above the false ceiling but leave an air gap between the true ceiling and the insulated false ceiling. Then put ventilation holes in the walls at the height of the gap between the true and false ceilings. To aid ventilation a fan can be placed in the holes in the walls. Alternatively, if there is room outside the walls of the house tubing can be placed in the holes in the walls and a roof ventilator placed on the other end of the tube so that when their is a breeze it draws air out from the gap between the two ceilings.

Attic fan ventilator

attic fan ventilator

Roof ventilator roof ventilator

5

I live in Chennai, a place which has only three seasons, hot-hotter & hottest. I had tried this coolroof solution, where a white so called chemical solution is painted on the concrete roof. We did see some reduction in the temperature in the rooms, however, this is a short term solution and need to be repeated atleast every year and it does not come very cheap rates vary from Rs.12/- to 50/- per sqft.

I have learnt a couple of things from my previous experiments and would like to list it out for the benefit of all. So that you all can take informed decisions when it comes to looking for and investing in a durable and sustainable solutions.

  1. Painting and trying out planting green cover for your roofs is only trying to reflect or partially absorb the heat after it has reached your roof. Painting is expensive depending on which material you use and green cover maintenance is not an easy job, especially in Chennai like climate.
  2. Idea should be to prevent sun rays from hitting your roofs directly - the best solutions is to have a raised coated aluminium sheet cover on preferably metal stilts OR pillars above the concrete roof. There should be enough space between the roof and the sheets for the space to be useful for walking around, drying clothes or to hold any family functions even during rainy season. The air in the space between the roof and the sheet would act as a insulation and since the sides would be open the cross breeze would take away any heat being radiated to the roof. Keeping the interior of the house very comfortable, reducing your energy bills by a substantial amount. Point to be noted is this solution does not come cheap, it is expensive but a one time solution and it gives you return for long time. Also, there are very aesthetic looking sheets and could give a nice look to the house or roof tops.

Folks, this is my personal opinion and would suggest you all to check all options before investing in any specific solution to the issue.

Thanks, Wilson

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Method 1.

Ideally you want a material with a low taber number: You want a material that is very reflective in the visible spectrum, but close to black in the infrared. Try for paints that use titanium dioxide for the pigment.

Method 2:

If it's a flat roof, and water is cheap, put a pond on top. Even 2 inches of water will absorb a lot of heat, and evaporate off. It's possible that this would make a mosquito problem.

Method 3.

Put mist nozzles and a timer that turns them on on a regular basis, or uses a photocell that can tell the difference between a dry roof and a wet one. Make sure your cement roof can handle the extra weight of tile.

  • Taber number? I know the concept in relation to the wear resistance of laminated floorboards, but surely that is not what you mean. – THelper Mar 30 '15 at 11:51
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    Taber number = emissivity at 300K divided by e at visible wavelengths. A perfect black body has a Taber number of 1. Titanium dioxide, the white pigment has a taber number of about 1/16 -- it's essentially black in the IR. TiO2 paint is used on astronmy domes. It doesn't heat up from sunlight, and it cools off in infrared quickly. Less heat shimmers. A semi-transparent layer of gold on glass has a taber number in the teens. It is fairly absorbent in visible, but shiny in the IR, so the object heats to almost double it's normal blackbody temperature under solar illumination. – Sherwood Botsford Mar 31 '15 at 13:57
  • I assume that methods 2 and 3 require a climate with low humidity in order to be most effective? – Mutagon Apr 2 '15 at 23:17
  • Low humidity helps, but it is effective in high humidity climates. Consider: The vapour pressure of water rises about 8% per degree C increase, so as soon as the water temp gets above the average air temp, it will start evaporating at a signfiicant rate. It's hard to get a shallow open water basin much above ambient air temp. While this is less effective at active cooling, at least it prevents the roof from getting as hot. Also, it will cool some at night, and this will slow the heating during the day. – Sherwood Botsford Apr 3 '15 at 1:42
  • But check the static strength first! You don't want it to collapse. – RedSonja Jul 10 '17 at 12:40
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I have the same issue over 2 flats under the same roof top. However 1 unit has concrete pavers laid approx. 50mm above the roof top. It's unbelievable the difference in temperature by having the pavers with an air lock. That unit under the paver roof is always comfortable but the one directly under the concrete is always uncomfortable.

I will try the suggestion above and plant some pot plants with a large cove, maybe even some vines.

3

It is fairly common to add a layer of gravel on top of a slab roof in many commercial buildings around the world (I guess 20-30cm). A friend (engineer) explained to me that the slab would then absorb much less heat during the day, because there was only little contact between the small stones and the slab through which heat could be conducted.

Another way that is increasingly used these days around the world is a layer of dirt with plants in them (the so-called green roof - either edible plants or meadow or lawn-type plants) which also both isolate the slab, shade it somewhat, and provide evaporative cooling. Of course this needs to be watered regularly, or you should try to find out which plants in your locale survive wild easily with only natural rainfall (they may still benefit from additional water, growing lusher). I think this is more complex to install, as ideally one would still need a gravel layer underneath for drainage, a root barrier inbetween to keep drainage and roots/soil separated, then the growing soil. But I doubt many plants have roots going down much further than 30-40cm. In the long run this may however be more "green" as it works against the "urban heat island" effect, and you can use this for extra gardening and recreational space.

In a pinch, I would try this with a couple of large (water tight) plastic containers in which the plants are planted. One would obviously need to leave walking paths between them so that all could be reached (I guess one could reach about 50-60cm, so no more than 1m-1.2m between paths), and work out some way that would provide sufficient drainage. This would be a lot less effective, but might still do the trick sufficiently for your needs.

Optionally an isolating layer between the slab and the cover may be necessary (I know of dense polystyrene sheets used for this). Can't really be more detailed due to not knowing the local circumstances.

In all cases you should be sure that the roof is able to carry the additional weight. I guess the fee for a structural engineer may be worth the cost of knowing you won't lose your house and all occupants...

In all cases one would also make sure that the roof is sufficiently waterproofed. A naked roof will have the water run off and dry off quickly, but those layers I suggested will retain water for longer, which can lead to all sorts of weakening to the slab (and insulation layer, if added).

You should also be sure that the layer is retained in some way and can not slide off or be washed off especially during heavy rainfall.

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I had a new roof put on my house that is metal and coated with a white material (sort of a plastic/rubber coating). They installed it on a support system so it's a bit raised off the old roof. This made it amazingly cooler. That was not why I had it done - the old roof leaked. So painting the roof white will help. But putting on another roof - metal panels for instance - with spacers so air can flow a bit between it and the concrete would help too. You would not have to use expensive materials as all you are trying to accomplish is shade for the roof you already have, not provide a water-tight new roof. A space between your roof and ceiling, perhaps installing a new ceiling in the process - with vents to the outside in this space could work too, but this may be too expensive.

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Simple , cheap , safe and effective solution , please try and share with others :Paint the roof top ( exterior surface facing sunlight) You can simply use any lime powder available in 10 kg bags at Rs. 120 per bag (Rs. 12 per kg). Use the proper ratio of lime, water and pevicol. For every 5 kg of lime powder, add 8 liters of water and mix well, then add 0.5 liter of pevicol (adhesive) and mix well everything and let it stand for 5-10 minutes before applying the lime + pevicol solution on the terrace. Clean the terrace, dry it, and then apply the lime + pevicol solution with a brush. Leave it overnight, cure it by watering morning and evening the next day. Apply a 2nd coating of 5 kg lime in 8 l of water mixed with 0.5 l of pevicol on 3rd day and repeat curing with water for the next two days. That is it. Check this link for details: http://www.thealternative.in/lifestyle/beat-the-heat-with-a-white-roof/

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A very nice and eco-friendly way to keep our cement roof cool, is by planting grass or any green small plants on it. Just make a square block with bricks and cover it with a plastic sheet to avoid leakage issues. Just spread soil and some manure on it and grow anything like grass or small plants. It looks so awesome on my roof.

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A plastic rubber coating will help you to not loose as much heat during the evening. Painting the roof white will not help during the evening after the sun goes down. The way I see it you have 2 options that will help: 1) put a rubber coating on the top of the roof (rubber is an excellent insulator). 2)add a lower ceiling with insulation material in between the existing roof and new ceiling.

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I am thinking the option of laying plastic tubes at regular intervals along the length of building roof (after the concrete slab is casted) and a common header pipe at the ends of all tubes. The tubes on both sides will be connected to header. The one side header will act as inlet and another header as outlet. keep provision of water inlet and outlet in the headers.

After placing the headers and tubes at over the slab, pave the roof slab (ensure all tubes are placed well and compact with concrete mixture, so that after paving the roof will look like a normal concrete floor.

This will help in two ways...

  1. The air gap in the tubes will act a insulation, since heat travel through air will be less. Though direct contact happens at the places where tubes are not placed, this will give a 60-70% reduction in heat conduction.

  2. We can let the water in to the tubes when required, may be for half an hour to cool the roof. The water can be use it for household purposes, if the piping is done accordingly. If you have surplus water use it for gardening on need basis.

  • Welcome to Sustainable Living! Interesting idea. Do you know if this has been tried somewhere? And if so how effective it is? – THelper Nov 4 '16 at 8:01
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We have the same problem for our house in India. For the roof, we are considering a combination of greenhouse rooftop, solar panels and heat reflective coating. There's a green-colored tarp type material that's cheap (few thousands in Rs.) that seems effective too. There are other things you can do on the inside and the walls too.

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Get some woven mats which sell for Rs. 150 for about 30 sq ft and put it on roof top. These are traditional mats made of hemp or plastic. The plastic ones are also washable. Just use some weights to hold them on the roof. The reason you want the mats to be temporary is because you can remove them during winter. The same hot roofs will help in reducing the heating bills in winter.

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The above answers were too complicate. You can buy a tarpaulin sheets which is available in 9X12ft and larger sizes also available. If you have any doubt please see the velai illa pattathari (vip) Tamil film. On that film he used to make temporary building at the top of building. Or you can easily see the song (amma amma song from vip).

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    Tarpaulins wear out much faster than paint, though. They're also not very nice to look and and make noise when it's windy. – Móż Mar 30 '16 at 2:46
  • @Móż what if Tarpaulin is placed all over the roof with elevation of 4 inches from all sides and then water is poured in? – bjan Apr 7 at 13:29
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I coated with Asian heat and water proof paint on that surface spreed plastic mat then on that one feet thickness of coconut coir spray water above all use green colour green house plastic shade put a c it will be very comfort and less electricity try and reply tan q

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    Hello and welcome! I don't quite understand what you did. Could you explain in a little more detail? Thank you. – Earthliŋ May 15 '16 at 13:50
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The best solution for houses that are already built is to put black rubber mats with holes in them - you can also get anti-fatigue rubber mats in big rolls. These can easily be found in the market. People use small pieces as doormats too. Lay them on the roof. The rubber mats have holes in them which makes for great insulations. Place thin interlocking concrete or ceramic tiles over them. Sleep easy after that.

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    Welcome to Sustainable Living! Why black rubber mats? Wouldn't white ones be more effective? Also I have some doubts if rubber mats are really the most effective solution. – THelper Dec 26 '16 at 9:44
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A cheap method to reduce direct sunlight is covering the roof with large size paper cartons.

You need some weight on that to protect from high winds.

protected by THelper Feb 12 '18 at 7:36

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