I am working on a water tank problem... I pretty much know what I am going to do, but I got to wondering how the water tank lid failed in the first place.

The original lid was 1/2 plywood, glassed on one side and with a gel coat finish... that went on the built-in-place water tank. The tank has a wide lip all around that was used as a gasket area... filled with a silicon sealer about 1/8 thick.

Assuming that the tank lid never leaked (big assumption, actually) what caused the plywood to delaminate and get all moldy... I am wondering if it was condensation... which would form typically in the plywood and under the pad used for berthing, which had a thick vinyl bottom.

So do any of you see this sort of situation on your berthing areas over water tanks... is condensation an issue? If so, how do you combat this?