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I put DE in the places that they dust bathe (mixed in with the dirt). I also put it in on the coop floor/corners/etc when I clean, but before I put down fresh litter. If they are nearly feathered out (usually around 6 weeks) they can take temps around 70 degrees without much issue. You can tell if they are too cold if they huddle super close versus being next to one another. That said, it is typical to wait until they are feathered out to take them all the way off the heat lamp. Sliding panels would make for some good options.Sorry, don't know what DEFG is.
Regarding sand. I live in middle GA - hot, humid summers. We have sand in run, coop and in nest boxes - they ate anything else I tried in the nests.. I really like the sand, easy to keep things smelling nice.