I had my greenhouse (sort of a 70's trombe wall thing) removed. It was built on the south side of my house and the floor was at the level of the house crawl space, poured cement bottom with cement block wall up to ground level where the glass portion then started.
Now, after tearing it down, the builder put joist hangers on the walls and then plywood to bring it up to level with the surrounding ground. The plan was to cover that with a waterproof barrier and then gravel, sand and pavers. The plywood portion is 7' x 36'.
After the winter, the plywood, walls, and joists are now covered with some kind of mold or moss INSIDE this pit. There is no ventilation and no way to install any. I am very uneasy about this. I am afraid that after some years the plywood will rot, my expensive terrace will sink, etc.

My builder assures me this will not happen. He thinks treating the space with moldex will solve the problem, but I think removing the plywood and filling the space with gravel would be the only sure thing. That is what I wanted from the start, but he said no, this would be better.
Expert opinions would be appreciated.