Q: We have a bathroom on the ground floor, and we’re replacing parts of the subfloor because they have dry rot. The subfloor is above concrete. Do you recommend putting insulation between the concrete and subfloor to make the floor warmer? If so, what kind? I know I don’t want the kind of insulation that attracts mice.

Also, the current subfloor is made of plywood. Should we replace the rotten parts with plywood or some type of water-resistant material? If so, what do you recommend?

A: Although we’re a little unclear about the structure of your bathroom floor, we recommend that you insulate regardless of how it’s built. Use rigid foam insulation and replace the damaged wood members with pressure-treated material to minimize the possibility of future damage.

From your description, we believe the plywood subfloor is fastened either to floor joists that are suspended above the concrete slab or to “sleepers” that are attached directly to the slab. In either case, a layer of insulation will help keep your feet warm when using the commode.

Insulating the floor is important, but the first thing you have to do is eliminate the source of the moisture that feeds the dry rot.

Dry rot is a misnomer. It’s not dry at all. What’s really at work here is a fungus that requires a moist environment to live. Remove the moisture and you’ll have no more “fungus among us.”

Whether it’s a leaky toilet, seepage from a tub or shower, or a leak in one of the supply pipes that is the moisture source, you must identify it and eliminate it.

After eliminating the moisture source, assess the structure itself. Is it likely that the condition that created the rot will repeat itself?

If so, you must change the structure to prevent the damage. For example, if the subfloor is attached to sleepers and it appears that the rot is the result of moisture from the concrete, replace the sleepers with new ones using pressure-treated material.

If the problem is only a leaky seal on the toilet, replacing the seal should do the job without any structural changes. In any case, isolate the source of the moisture and eliminate it.

As far as replacing the damaged material, use pressure-treated material whenever possible. Pressure-treated plywood is available, but you might have to make some phone calls to lumberyards to find it. An acceptable and expensive alternative is marine plywood.

For insulation, we recommend you use rigid foam insulation. Insulation board is available at lumberyards. Rigid insulation comes in varying thicknesses and the thinner material can be cut with a utility knife. A table saw, if you have one, works well with thicker material. If you don’t have a table saw, sandwiching two layers of thinner boards works equally as well.

If your floor structure consists of sleepers on a slab, cut the board to fit between the sleepers and try to shim the boards so that the top of the insulation is even with the top of the sleepers. A little air space between the slab and the bottom of the insulation is preferable.

If the floor joists are suspended over the slab, install ledgers on the bottom of the joists for the board to rest on. Again, install the insulation so that it is even with the top of the joist.

Once the insulation boards are in place, run a bead of caulk around the seam between the insulation board and the floor joists. If you’d rather not caulk, a strip of duct tape will also work. Install the subfloor and then the floor covering of your choice.

We’d suggest you consider installing tile over cement backer board for the finished flooring. In our experience, this flooring treatment is the best defense against moisture penetration.

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