why back fill sand into a crawl space

The house was built in 1969. In 1994 (20 ) years ago a two story addition was added to the house. the crawl space is 32' x18' and the footing is 12" inside the 8" block foundation. thus making it a minimum 20" footing. and 48" in depth. The soil is clay based that extends about 6" above footing approximately 18" of sand on top of that. Why back fill sand into the crawl space? they thus lowered the space. I Want to remove the sand and the clay down to the footing height only. (for now) its a heated crawl space. then spray foam the walls al lthe way to the footings and the underside of the floor. and all the duct work as well.

there is no mould and there is currently pink baton insulation in the joists with vapour barrier stapled to the joists. air cavities between the floor and the insulation. Mice were kind enough to pull some down so i have to buy more insulation and fill in the holes where it fell. all in all still 98% still in between the joists.

I suspect the sand was put down as fill and to act as a counter force against the earth on the exterior walls. But why? is a crawl space not just a really short basement. it has footings just the same.

The sand may also be acting as a filter as the clay is damp and reducing mould and raising the floor requires less area to heat. there is 6 ml. plastic played down onto of the sand as well.

I Don't have 50 to 100 grand to have the crawl space dug out to a basement so i would like to do this in stages. dig to the footing (only) and get about 48" of crawl space. maybe get tired at that point and leave it there.

there is a window on the exterior to gain entry to the crawl space. no interior entry point leading into the crawl space from the house.

i want to insulate the walls (Spray foam) and stop the heat loss and energy cost.

Then learn everything i can from the structural engineer and build it right.

I don't see why you'd do anything much beyond insulating the floor (ceiling) and ductwork. Regardless of the numb-nut liars out there frauding for the Foam Industry, there's no appreciable heat nor cold loss by the crawlspace floor or walls and there's no good to come from trapping even more moisture within the footing zone. GET RID OF the plastic on the sand, that would and should only ever be for Radon control...unless you have a Radon problem.

- I realize there's a lot of bad information out there/here, but reconsider what you're listening to. No crawlspace is heated nor cooled past osmosis, there's no return duct vent down there. And by insulating the ducts, you'll kill the heating or cooling of the crawlspace...not AT ALL a bad thing.

- So, I'd say your best, most permanent and correct efficiency and health course of action is to insulate the ceiling and ductwork and change that window to a screened and louvered vent and add another vent to the opposite wall to give the crawlspace the ability to breathe and dry much more effectively. This isn't new nor wrong nor improper and only proven to be THE BEST.