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Is Basement Moisture a Leak, or Humidity?

Q: My basement stays fairly dry, but in the spring and during other really rainy periods I can see the concrete walls are wet. Is it condensation or water leaking inside? I haven’t seen any puddles of water on the floor, but it is really humid at these times. — Bill in upstate New York

A: There’s an easy way to test whether the moisture on the basement walls is seepage (water leaking inside) or condensation. Get a few large squares of aluminum foil and tape them to random spots on the basement walls and floor. Use water-resistant tape and completely cover all four edges of the foil so that no air can get underneath it.

Leave them there for a couple of days, and then carefully peel away the tape and check both sides of the foil. If the side facing the wall (the inner side) is wet, then seepage is your issue. If the outer side is wet, condensation is the problem.

If both sides are wet, seepage is the most likely issue, with condensation possibly a result of the seepage.

To reduce seepage, check all of your gutters and downspouts to make sure they aren’t damaged and that rainwater is being directed well away from the foundation. The drain trough under the downspout also should be checked; make sure water isn’t pooling there.

Next, check the foundation and basement walls for small cracks, holes or gaps. Repair these with the appropriate type of cement. Finally, paint the interior basement walls with a waterproofing paint specifically formulated for this task, which adheres well to concrete or masonry.

To reduce condensation issues, install a dehumidifier in the basement, or install a fan that will pull air outside and help the basement air circulate. Wrap insulation around cold-water pipes so condensation is less likely to occur in the basement space. Outside, try and increase the amount of air and sunlight that reach the basement by trimming back plants and hedges, especially around basement windows.

HOME TIP: Debris and leaf dams in your gutters can damage them and lead to other problems around your home’s exterior. Clean your roof gutters twice a year.

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