If you’re trying to figure out how to cure a wet or musty basement, you’re probably curious about advertisements for products that claim to waterproof basement walls. So you wonder: Is it really possible to dry out a basement simply by sealing the walls?
Yes, it is possible — but to make sure you’re choosing the right option, you need to figure out if the moisture is coming from the outside, or if it’s actually high humidity that’s condensing on the cool walls of your basement.
How to Find Out What’s Causing the Moisture
Tape a 1-foot-square piece of aluminum foil to the inside of your basement walls, and leave it in place for 24 hours.
If there’s condensation on the outside of the foil, you have high humidity in your basement. Fix it with a portable room dehumidifier or a whole-house humidifier system instead of waterproofing products.
If the foil has condensation on the inside surface (next to the wall), it may be the soil around your house is naturally damp from a high water table or poor soil drainage. In that case, waterproofing your basement walls can be useful.
You can waterproof just your interior walls, which may solve the problem. Or you can waterproof your exterior walls, which is a better bet — but more costly.
Here’s the scoop on the different types:
4 Types of Interior Waterproofing
Concrete waterproofing coatings: These thick coatings are cement-like; once dry, they adhere permanently to concrete and masonry walls. You apply the coating with a heavy brush made with tampico bristles — a natural fiber. Swirl the brush at the final stage of application to give the wall an attractive, finished look.
Concrete waterproof coatings can’t be applied to previously painted surfaces; check the label. A 5- gallon bucket (you add water to the dry mix) is $30-$40 and covers 100 sq. ft.
Silicate-based concrete sealers, also known as densifiers, are also suitable only for walls that haven’t been painted or sealed. The sealers soak in and chemically react with ingredients in the concrete or brick, forming a hard, waterproof surface.
Because these are penetrating sealers, they can’t flake off or peel, and you can paint over them (but check the label first). Applying a silicate-based sealer with a brush, roller, or sprayer is easy enough for a DIY project. A 1-gallon can is $40-$50 and covers 200 sq. ft., but you’ll need at least two coats.
Waterproofing paint is an acrylic formula, not all that different from ordinary wall paint. But you brush, roll, or spray it on much more thickly — one gallon covers just 75 square feet, not the 300 square feet typical with standard paint.
Waterproof paint is fine for DIY application. You can apply it over painted surfaces, and paint over it once it’s cured (1 gal./$35).
Plastic sheets and panels are suitable as wall waterproofing only in combination with interior basement drainage systems. They don’t stop water from getting through the wall, but they do stop it from ruining things in the basement.
Water that gets through the wall runs down the back of the plastic, into a drainage system in the floor. A sump pump moves water out of your basement. The entire system is $3,000-$5,000 for a 20-by-20-foot basement room.
TIP: None of these products will work unless cracks and gaps are properly sealed. So make sure you fill cracks and gaps less than 1/8-inch wide with polyurethane caulk made for masonry ($5/10-oz. tube). Patch wider cracks with epoxy filler.
Your Best Bet: Exterior Waterproofing
The surest way to waterproof your basement walls is a full-scale exterior waterproofing solution. It’s also the most expensive — often $15,000 to $30,000.
Exterior waterproofing involves excavating all around the house to the full depth of the foundation walls, then installing a waterproof coating or membrane topped by drainage panels. The panels provide an easy path for water to flow down to an exterior French drain at the bottom of your foundation. From there, water flows by gravity — or with the aid of a sump pump — away from your foundation to another part of your property, or into a storm drain.