Wet Basements

Published on November 14th, 2012 | by Henri

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Moisture-proofing a concrete-block basement

Q.    I would like to moisture-proof the basement to eliminate dampness and make the basement humidity the same as in the rest of the house. The house is 10 years old and has an unfinished basement six feet below grade. There are no signs of water seepage or wall cracks.

My plan is to apply two coats of Drylok, a rubber roofing membrane, and two layers of 6-mil plastic before studding, insulating and putting up drywall. Is this overkill? What is the best approach?

A.    Yes, I would say this is overkill. Since you mention that there are no wall cracks, I assume that your foundation is built with concrete blocks. Just in case you haven’t seen my oft-repeated discussions about the problems with waterproofing a block foundation from inside, be aware that doing so invites water to fill the blocks because it cannot leak into the basement. This water evaporates into the building and creates a very unhealthy environment that includes molds, mildew, a bad smell and potential deterioration of the framing and finishes. Years ago, I worked on an apartment building that had to be vacated for that very reason.

I realize that you haven’t had any moisture problems or leakage yet but that does not mean that you won’t. Once the water is in the blocks’ cores you won’t be able to get rid of it easily and you won’t even know it’s there until too late. You must also keep in mind that insulating the basement walls may mean that the ensuing deeper frost penetration will cause the walls to crack.

Insulating any foundation walls safely requires an effective drainage system (for protection if the proper grading fails or the water table rises), backfill of the foundation with coarse material that will drain efficiently and not expand in freezing temperatures; and positive grading to drain water away from the foundation. In the case of a 10-year-old house where you may not know if there is a functioning foundation drain and what kind of backfill there is, at least make sure the grade slopes away from the house.

As to the best way to finish the basement, tape 6-mil plastic to the walls no higher than 2 feet below the grade line and drape it onto the floor by 5 inches; this will ensure that if you ever have leakage at the base of the walls (where it usually occurs), water will not wet the studs and will seep under the plastic. If water seeps, this will tell that you have a grade problem. Nail a pressure-treated plate to the concrete floor through the plastic. Stud the walls, but only insulate them from the top to no more than two feet below grade; the heat loss through the bottom of the walls will help keep deep frost at bay. Keep the bottom of the drywall an inch from the floor and install a wood baseboard with screws for possible removal.

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