Lining water tank,

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You know how intense this summer has been, particularly in Lahore. It's very hard to take a bath even in the evening. I am thinking of making an additional wall outside the water tank at a distance of a couple of inches from tank's wall. Also covering the roof with concrete slabs.

Please guide whether this will help keeping the water cool. Is it required to insert something like thermopore in the space. What would the most economical solution like cheap wooden planks etc.