Prevent excess basement moisture

Q: I have a small basement — 20 by 15 feet — that has no windows and is very damp. Since I directed rainwater away from the house, the cement block walls are pretty dry. We also wrapped all of the water pipes and they don’t sweat as much. I was hoping you could advise me as to the best sealer for the walls and the concrete floor. — Thomas D., via e-mail

A: Sealers will only work as long as the water pressure on the other side of the walls remains low. The concrete blocks facing the “dirt” side of the house are unsealed and therefore still vulnerable to water seeping through them. After a while, you’ll have a wet basement again.

You have already taken steps to reduce this possibility by redirecting the rainwater drainage. The next step is to make sure the grading outside the house isn’t directing groundwater back toward the walls. If the ground slopes toward the house, rather than away from it, then groundwater is building up against the foundation and basement walls.

Keep dirt from piling up around the mudsill (sill plate) that runs between the foundation and house walls. As a rule, a 6-inch clearance should be maintained between the ground and the mudsill. This also applies to any ground-level (basement) windows and vents you install.

Additionally, find out if any drainage options were built into the basement. For example, some basement floors are created by pouring the concrete over a bed of crushed rock that contains drainage tiles slanted toward a drain point (sump). These are called “floating slabs” and are often built in areas prone to groundwater flooding, like the Northeast. It’s possible to modify the basement floor to increase drainage (or seal it further), but I’d recommend consulting a professional before taking more drastic measures.

Once the groundwater situation is addressed, you can look into sealing the walls and improving ventilation. Installing windows will help bring air and sunlight into the room, but they need to be properly installed and sealed to keep moisture from entering and condensation from forming. Check vents leading from appliances like the dryer and dishwasher too. If they ventilate into the basement, they could be bringing additional moisture into that space. Redirect those vents to the outside of the house.

If moisture is only an occasional problem (such as during damp spring months), bring in a dehumidifier to take care of the excess.