Dry basement key to successful finishing

Q: How can I tell if my concrete basement is really dry enough to finish? I had hoped to do the work this winter, but I think there might be a hidden water problem. There have been boxes stored on the basement floor since spring, and now that I’m moving them I see dark marks where they used to be. Is this anything to be concerned about? I’ve never seen any water running on the floor in the five years we’ve lived here, but I’m still concerned.

A: Since you’ve never seen a water leak, I suspect the marks are caused either by moisture wicking up through the concrete floor, or water droplets condensing on the cool floor during humid summer weather. Either way, it’s important to get things completely dry before finishing. Drylok is the best concrete waterproofing compound I know of, but it should be applied to bare concrete. A dehumidifier (and closed basement windows) is the only cure for summertime humidity problems. That’s because the source of summertime basement dampness is warm, humid outdoor air.