I am putting in a new tub in the bathroom. When I took the old one out there was slightly moldy insulation which I am going to replace. When I started to take the old insulation out I found frost on the plywood. I know I have to dry this out before I put the new stuff in but what could I put in there to prevent the moisture from forming? It is a second story bathroom, vinyl siding, house was built in 1975. This is the first time the wall has been opened up in that area. Thanks in advance. Ax.

The insulation should have a vapor barrier that faces the inside (warm side) of the wall - this keeps humidity from seeping through the insualtion to the cold outside wall. Consider using plastic sheeting as a vapor barrier - use unfaced insulation batting and then staple mylar sheeting over top of it. Then comes your concrete board and tile.

I agree with ebaines. The plastic barrier is very important since this is a wet area.