Sealing a Mud Floor
Q: "What should I do about my soggy basement mud floor?"
I have an 1850 farmhouse in northern Vermont with a stone foundation and a mud floor. I have put in a network of drainpipes, but the basement stays wet and the humidity moves up into the house, causing mildew and other headaches. Have you ever heard of putting down a one-inch layer of lime on a wet mud floor before putting down a vapor seal membrane? I am worried that, after I lay down the membrane, the water will eventually get stinky and the scent will permeate the house. To avert this, a friend suggested that I put down about an inch of limestone first, then the membrane, then the pea stone. Is the smell problem a real concern or not? If it is, and lime is not the way, how would you care for it?
— Andrew, Vermont
Tom Silva replies: To make sure you have adequate cross-ventilation in the basement, put in an electric exhaust fan at one end connected to an electronic damper at the other. The fan will pull air out when it gets humid. When it goes on, it will trigger the damper, which will open to let air into the basement. This system pulls air across the basement, insuring that air gets in and out on a regular basis and moist air and smells don't get a chance to enter the rest of the house.
I've never heard of using lime. But if your basement floor is muddy, pea stone is a good idea. I would run a plastic vapor barrier (in large pieces so there are few joints for moisture to seep through) over the floor and up the wall covering the stone foundation. Before you reach the sill, run a thick bead of polyurethane or butyl caulking along it. Bring the vapor barrier up over the caulk and staple it to the sill. The combination of caulking and stapling will insure a tight seal that will keep moisture out. On top of the floor, I'd put two inches of sand and then pour a mud slab or concrete floor at least three inches thick.
Finally, I would insulate the ceiling of the basement. If you use fiberglass, put another vapor barrier up underneath the insulation (so you can see the insulation up through it). Also, weatherstrip the basement door to make it tight. These steps will keep any moisture that does occur from penetrating upstairs. Having heat in the basement in the winter will also help combat your moisture problem.