the best way to thermal insulate and water proofing of rcc slabs economical, may be some poly urethane material or whatever but should be very effective and light weight.
use a polythene sheet . i dont know about where u live but where i live we get very long and broad size thick polythene sheets to make shelter or the protect from rain . even poor ppl use it to make slums . they attach a sheet in bamboo and they bury half of that bamboo stick inside the ground and by arranging 4 of them like this in a square area they make a bamboo frame. then as i said they attach sheet and make temporary home but they live there for years.
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