Rain in the Basement
Insulation can stop that annoying drip
Under the concrete stoop at our daughter's home there is a little storeroom that opens into the basement. During our cold winters, condensation drips from the storeroom ceiling. Can this be prevented?
— Ed, Omaha, NE
Tom Silva replies: Sure, as long as you raise the surface temperature of the underside of the porch, or reduce the amount of water vapor in the basement air, or both.
When the storeroom ceiling is dry — probably this fall — paint it with a cementitious masonry sealer like Drylok or Thoroseal. When it's dry, stick about 2 inches of rigid, foil-faced foam insulation board to the surface, using a construction adhesive that's compatible with foam. Some codes require foam to be covered with drywall for fire safety. If that's the case where you live, then you'll need to anchor a gridwork of 1x strapping to the ceiling after the foam is in place. This will provide a firm base for fastening the drywall.
The insulation will help to isolate the cold slab from the relatively warmer air in the basement, and that should reduce or eliminate condensation. If it doesn't, you could insulate the storeroom walls, but first try parking a dehumidifier down there. All that insulation won't be much help if the moisture level in the basement and the rest of the house is over 50 percent.