Cool Roofs: Lime-wash Or Heat Reflective Coating?

From Alternative.in, an inexpensive lime-wash mixed with fevicol cools roofs by 5 degrees centrigrade.

My friend Dr. S.P. Periaswamy and I explored several options to reduce the heat in our house. Many options like the sun-reflective white paints were very expensive. After a long search on the Internet and several enquiries with paint companies, we found a cost-effective option for painting our roof white – the finely ground (microfine) lime powder. It is sold by commercial names like Janata Cool or Summer Cool. I bought the Janata Cool sold in 5-kg bags with a small packet of translucent crystal inside. The instruction says it has to be dissolved in 9 liters of water, further adding the lime powder and mixing it well, and leaving to dissolve for 10 minutes.

We used the following procedure:

We took 8 liters of water, dissolved the packet of crystals in it, added 500 ml of Fevicol (adhesive) and then mixed in it the 5 kg of lime powder well.

We let it for 10 minutes, then mixed the solution well and painted the roof with a wide brush. It covered about 600 square feet (60 square meters) of roof surface. We have to paint the roof either early in the morning (06:00 to 10:00 am) or in the evening (04:00 to 08:00 pm) to avoid inhaling the pungent fumes that emanate from the solution when exposed to direct sunlight and high temperature. You can water the painted surface 12 h later for curing.  A second coating is applied 24 hours after the first application. We can water the painted roof for 3-4 times in two days for proper curing. After drying, we can see the bright white roof. We use the solar panels to get solar power for our home use.

We measured the temperature inside the house before and after painting the roof white with the lime powder: 35 oC  before and 30 oC after painting the roof white, a reduction of 5 oC in room temperature, under Coimbatore conditions (16-17 April 2013). We can now comfortably sleep in the bedroom with a fan only in the night. We have avoided the use of AC even during the peak of this summer, with a simple solution like painting the roof white with lime. The cost in Indian Rupees (Rs) of lime painting for 600 square feet (60 square meters) of roof surface is as follows:

Ingredients         Price (in INR)
Janata Cool lime powder (two bags 5 kg each) 360 (180X2)
Fevicol Adhesive(one litre)   80
Labour Charges 500
TOTAL 940 per 60 Square Meter

You can also use this simple solution to beat the heat this summer at an affordable cost.

………

The coating of lime solution mixed with fevicol adhesive will last 3+ years on sloping roof and 2-3 years on flat roof, if there is not too much trampling. If you walk over lime-coated terrace every day for drying clothes, etc., you may have to lime-coat the terrace once in two years. Alternatively, you can place a wooden plank on the terrace to walk over and dry the clothes or do other activities. Minimum trampling will keep the lime coating for longer.

Unlike some of the enamel paints that will come off like flakes, lime coating will not peel off (especially if the surface is a bit rough before lime coating). It is better to keep the lime-coated surface clean by periodic sweeping with a smooth broom (It is easy to clean with a smooth broom). The maximum you may need is an annual lime coating to get maximum reflection of the sunlight and thus an effective cooling of the rooms inside the building……”

[Further details on costs, materials and suppliers can be found at the Alternative.in link]

In the UAE, scientists are recommending the use of a reflective paint developed by Watergy International Group which reduces temperatures by 20%, they claim.

But at Quora.com one contributor claims that lime-wash doesn’t do much and passes on a suggestion from a civil engineer:

I had the same problem and was trying to find a solution to this few months back. If you are living in the top floor, the best way is to use a heat reflective coating. I live in Bangalore and it gets around36C during noon and my home used to get too hot. Thermometer in my room used to read1C-2C higher than outside temperature always. Roof does not cool soon and it stars radiating the heat to lower floors when the outer temperature drops. Due to this, even during night, my room temperature usedto be around30C when it was25C outside. It was almost impossible to stay inside home during noon even after having good ventilation. One of my friend who is a Civil engineer and is working in a constriction company told me many of the common techniques like lime wash and growing plants are not much effective as they do not cover whole of your roof.The idea is to cover your roof with something so that it does not absorb any heat at all. He suggested me to use something called as Heat Reflective Coating(Insulative paint) and it worked wonders. It was the cheapest solution I found (Costed me around 4.5k Rs). There are many companies out there who has these products like Fosroc, Pidilite, Monarchindia, etc. I got one from ebay India and applied it on my roof. To my surprise, my home temperature dropped to 29C-30C during noon when it was 36C-37C outside. Fan was more than enough for cooling and this saved me lot of money as I had earlier though of investing in multiple AC’s in the house. Before applying the coating, I was able to feel the heat on the roof while walking during evening. After applying it, it is literally cool even during noon. Heat Reflective Coating are mainly used in industries and are not well known or marketed. I fell these can be used for cooling house roofs as well.”

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