Can I insulate flat roof using this method?

Question

I just wonder if I can insulate my flat roof in northern India by following steps as under:

1. Apply a cover of Bitumen
   2. Cover it with polythene (for water proofing)
2. Lay used cement bags on polythene (LDPE bags will give thermal insulation.)
3. Cover with soil.
4. Lay tiles over it.

The traditional method used here is

I just wonder if I can insulate my roof by following steps as under:

1. Apply a cover of Bitumen
2. Cover with soil.
3. Lay tiles over it.

Answer 1

Certainly seems like it should be some improvement over the traditional method - how much is not clear, since the bags will presumably pack down and lose most of the insulating air spaces under soil pressure, but even "more or less solid" LDPE would be a slight thermal improvement over soil, if you have access to a lot of cement bags.

A rigid foam insulation (or foamcrete) would be more effective in this application, but presumably expensive or hard to source locally.

Answer 2

Instead of simply putting those bags there empty, you should fill them with an insulating material, like rice husks, lava rock, or any other lightweight, air-trapping material you can find. For that matter, you don't even really need to put them in bags.