bathroom subfloor

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Our bathroom subfloor has black mildew/mould on it. It is dry and sound for the most part. This mildew is do to former sweating of the toilet tank during humid days in the summer. When we first built the house in 1983, we were not aware of the potential damage this sweat could cause and we just allowed it to take place. But, as we noticed the baseboard behind the toilet getting black, we decided to try some things. We put a terry jacket on the tank. Then we tried a pan underneath the tank and finally, we began adding a gallon of hot tap water to the tank each time it filled after flushing in order to temper the cold water from the well which attracts moisture from the warm humid air causing it to condense on the cold tank.

This may seem like a lot of trouble to go through but this mixing of hot water with the cold has done the trick.

Even though the subfloor is black with mildew stain, we are confident that this is old stain since the underlayment and subfloor are bone dry as we removed the old underlayment.

We are now replacing the original linoleum with ceramic tile and while the linoleum had only one layer of 1/4" lauan underlayment, we are replacing with 2 layers of 1/4" fir underlayment on top of the 3/4" t&g fir subfloor along with Schluter Ditra Mat between the tile and the top layer of 1/4" fir.

Do we need to treat the old dry mildewed area of the subfloor with anything before laying down the first layer of subfloor? We don't want to give any possible future water around the toilet a way to get into the wood layers and then get trapped in there. Could we run a bead of caulk around the hole where the pipe comes through, close to the edge? The original builder/plumber ran a bead of caulk around the base of the toilet causing a sort of dam which, we believe, is how some of the water was able to get into the space between the old underlayment and subfloor. On second thought, maybe it is best to leave everything open, not caulk anything, so that any future wetness has a way to dry out.

We imagine that the dripping condensation from toilet tanks during the summer months is much more of a problem rotting out floors than the occasional overflow of a toilet.