Principle :-
Black surface absorb more Heat Energy where white reflect solar radiation. Roof is major part to store the Heat and release it during night time.
My House:
I am living in Madurai city located in Tamilnadu state in India where the temperature raises maximum 42 c – 44 c. My house faces East direction. So we face more heat radiation from morning 8.00am ie above 34 c. At 3.00pm the ceiling temperature is 36 c. After sun set 7.00 pm, the ceiling temperature is 38-39 c. So the room is hot even running fan. (it brought hot air from ceiling).
Drops by used PVC flux banner sheet in front of Door and window location would block morning radiation effectively.
Making the surface by white colour, the solar radiation is reduced on roof top by 20 c. and inside room ceiling heat reduced from 39c to 31c.
After white paint, the temperature is constant such as 31c. It is enough cool with fan for normal condition.
5 kg bag good quality lime powder @ Rs 45 to 55/-
250 ml Fevical DDL binder @ Rs 23/-
It cover 600 sft approximately for one coat. Use two coat.
Myself and my two daughter’s paint the surface within one hour duration for 600sft single coat. Next coat require 24hr curing. Best time to paint Night after 7.00pm. After painted cure it with water sprinkled thrice a day for 2 days. Total cost Rs 200/- (without labour cost). Now we feel enough cool without A/C. (labour cost may Rs 500/-) Life time to be tested. Anyway it is equal to monthly mobile recharge cost

Alternative method
Fixing of White tiles (ceramic) cost Rs 6000/100sft approx. for 10-15 year life span
Trade summer cool Paint cost Rs 5000/100sft – 3 coat – 4-5 year life span

Heartful Thanks to V.Balasubramanian, Coimbatore And Dr.S.P.Periaswamy

For more detailes visit his web site