Q: The water tank behind my toilet sweats a great deal in the summer, causing much water to drip onto the floor. I had read somewhere that this can be stopped by lining the inside of the tank with polyurethane so as to insulate it against the cold water that enters when the exterior is warm. But several plumbing stores tell me this substance is no longer available. Can you recommend a solution for this problem?

A: You are right that lining the inside of the tank with an insulating material will usually solve the problem of condensation forming on the outside of the toilet tank. The easiest material to use for this purpose is Styrofoam. You can buy it in sheets from many plastic supply houses, as well as from some craft shops.

Shut off the water supply to your tank, then flush to drain it. Wipe the walls on the insides dry, then cut pieces of the Styrofoam for a snug fit around the sides. In many cases no adhesive will be needed, but you may want to glue the foam in place.

Questions about home repair problems should be addressed to Bernard Gladstone in care of The New York Times Syndication Sales Corp., 200 Park Ave., New York, N.Y. 10017. Questions of general interest will be answered in this column.