Everyone knows what to do about cold water pipes that sweat during the hot, humid summer: A quick wrap with pipe insulation solves the problem. But when a toilet tank sweats, the solution isn't quite so simple.

Sometimes you can get by with one of those cloth tank covers, but after a while the cover may become saturated, so sweating continues. Mildew can form on the tank cover, and water can drip to the floor beneath the tank, lifting floor tiles and rotting wood so that the whole section of floor and subfloor around the toilet needs replacing.

The best way to avoid all these problems is to insulate the toilet tank from the inside. You can buy special kits to do the job at some plumbing supply houses, but you can also make your own easily using sheets of foam rubber about half an inch thick. You can buy this at most department store or, if necessary, from an upholstery shop.

To start, shut off the valve in the line feeding the toilet. It should be visible right beneath the tank. Then flush the toilet. This will drain most of the water out of the tank.

Remove the remaining water by sponging it up and then let the tank warm up for about half an hour. Then wipe the inside dry with paper towels. Don't try drying the tank right after draining -- it may be cool enough to condense moisture out of the air and immediately become wet again.

Once the tank is dry, cut four pieces of the foam rubber to fit the four inner surfaces of the tank. Work carefully to achieve a good fit. The pieces needn't go all the way to the top of the tank, just an inch or so above the water level. You can usually tell where the water line is by examining the tank for stains. If no stains are visible, just make sure the inner extends an inch above the level of the overflow pipe.

The piece of rubber for the front wall of the tank will have to be cut out to provide a hole for the flush handle. After all parts are cut, test fit them. Make sure they fit well with no large gaps, and don't interfere with the operation of the flush mechanism.

When you're satisfied with the fit, you're ready to glue the rubber in place. Use silicone rubber caulk or sealant. Squeeze it out onto the surfaces of the tank and spread it around with a piece of cardboard. Then press the pieces of rubber into place. Let the adhesive set up for about 12 hours. Then you can reopen the valve serving the tank and put the toilet back in operation. Q: My home needs storm windows and I want to build my own to save money. I'll save on heating costs if I make them with two layers of glass instead of just one, but I'm afraid I will get condensation between the panes. Is there a simple way to build a double-glazed storm window and still avoid fogging? A: There's a simple means of construction that should do the job. Start by building the frame from clear, dry pine. Size the frame to fit inside the exterior trim of the window. Simple butt joints should be strong enough if you use waterproof glue (epoxy or resorcinol) and a pair of four-inch screws at each corner.

The two sheets of glass to fit the frame should be 1/4" shorter and narrower than the inside dimensions of that frame. This will produce a gap all around the glass to provide room for thermal expansion. Clean the inner surfaces of both panels of glass. Then apply a thin layer of silicone caulk to the edges of the spacer strips, and place the strips between the two sheets. The strips should be placed around the perimiter of the glass assembly, and set in slightly from the edges of the glass. Setting the strips in from the edge is important, for it creates a channel all around the two pieces of glass. Using a putty knife, fill that channel completely with caulk. This will provide a moisture-tight seal all around the edges of the glass.

Set the glass aside for a day to let the silicone set. Meanwhile, use waterproof glue and nails to fasten one set of face strips to the frame.

When the caulk has set, place the frame on a flat surface, with the side already fitted with face strips facing down. Shoot a thin bead of caulk onto those face strips. Then place the glass in the frame, positioning it so there is an even 1/8" gap all around it. Work silicone into this gap and nail on the other set of face strips.

Let everything set for another day. Then finish off the frame with a coat of exterior primer, then one or two coats of trim paint.

That should produce a fog-free window, but one extra bit of insurance might be in order: When cementing the two panes of glass together, put a tablespoon of silica gel between the sheets. Silica gel, a desiccant sold at many nurseries and flower shops, looks like salt and will fall down out of sight at the bottom of the frame. Meanwhile, it will soak up any moisture in the air between the two sheets of glass to help prevent condensation in very cold weather.