Waterproofing paint has a bad reputation among people who sell drainage systems and a very good one on the cans. Both are partly right, and the difference comes down to one question: is your basement wall damp, or is it wet? Paint can do something real for a damp wall. It cannot do much for a wall that water is actively pushing through.
This guide works from the manufacturers’ own data sheets for the two best-reviewed paints on Amazon, KILZ Basement and Masonry Waterproofing Paint and INSL-X WaterBlock Acrylic Masonry Waterproofer. It turns the “12 PSI” on both cans into something you can picture, shows how much paint a typical basement actually takes, and sorts the decision by what your wall is doing. If water is entering through cracks or the floor joint, start with the foundation waterproofing sealant guide instead, because that is a different problem with a different fix.
Damp or wet: decide this before you buy anything
Use this table first, then read the sections below for the numbers behind each row.
| What the wall is doing | Is paint the right tool? | What to do |
|---|---|---|
| Musty smell, cool damp feel, no visible water | Often yes | Test first (below), then a masonry waterproofer over bare, prepared masonry |
| White powder on the wall that dries and returns | Only after cleaning | Brush it off, deal with the moisture source, then coat |
| Cracks wider than 1/8 inch, holes, failed joints | Not alone | Fill with hydraulic cement first, then coat |
| Water visible after rain, trickling or pooling | No | Fix grading, gutters or drainage first; KILZ’s listing says not for walls with active leaks, and INSL-X says not to seal a crack that is visibly leaking |
| Basement floor | No | Both products say they are for vertical walls and not for floors |
A quick way to tell condensation from seepage
Before you paint, find out which kind of moisture you have. A common do-it-yourself check, described by home-inspection sources rather than by any standard, is to tape a square of plastic sheeting or aluminum foil tightly to the wall on all four edges and leave it for a day or two. Moisture that collects on the wall side of the sheet is coming out of the masonry. Moisture on the room side is condensation from indoor air meeting a cool wall. A cold basement can produce condensation even when the wall is perfectly sound, so repeat the test on a dry day, and run a dehumidifier for a few days if the result is unclear. If you can, test more than one spot, since a wall can be damp in one place and dry in another.
This matters because paint can help with the first kind of moisture only in a limited way and does nothing for the second. A coating on the inside of the wall does not change how much humid air is in the room.
What 12 PSI actually means
Both products advertise 12 PSI of hydrostatic resistance. The INSL-X data sheet says it meets ASTM D7088 at 12 PSI and puts that at greater than a wall of water 27 feet high. The arithmetic checks out: water exerts about 0.433 PSI for every foot of depth, so 12 PSI is roughly 27.7 feet of water. DRYLOK, a competing brand, makes the same kind of claim at 15 PSI and describes it as a wall of water 33 feet high, with a 15 year transferable warranty. That figure comes from its own blog, so read it as a manufacturer claim.
Now compare that to a real basement. An 8 foot wall that sits entirely in saturated soil carries about 3.5 PSI at its lowest point (8 times 0.433). The rating is far above the load, so pressure is not the weak point. The weak points are the things the rating does not cover. The test is run on a block panel, while your wall has cracks, a cove joint where the wall meets the floor, and tie holes. The coating also sits on the inside, so water pushes it off the wall instead of pressing it into the masonry. That is why the better data sheets spend more words on preparation than on the PSI number, and why a coating that is perfect on the test panel can blister on a wall with a drainage problem.
KILZ against INSL-X, from the data sheets
Both products are water-based acrylic masonry coatings for bare, uncoated masonry. I read the INSL-X data sheet (AMW-1000, revised 2025) and the KILZ data sheet (number 2390, a 2013 edition from its maker). The KILZ sheet does not state the 12 PSI figure. That number comes from the current Amazon listing, so check the label on the can you buy.
| KILZ Basement and Masonry Waterproofing Paint | INSL-X WaterBlock Acrylic Masonry Waterproofer | |
|---|---|---|
| Hydrostatic rating | 12 PSI on the listing | 12 PSI, ASTM D7088, on the data sheet |
| Coverage per gallon | 75 to 125 sq ft | 75 to 100 sq ft |
| Coats | Minimum of two | Two unthinned coats, a third if it still seeps |
| Touch dry and recoat | 1 hour and 4 hours | 1 hour and 4 hours at 77 F and 50 percent humidity |
| New concrete must age | At least 30 days | 14 days, or a surface pH of 13 or lower |
| Over old paint | Not on regular oil or latex paint; sound old paint may be coated but the maker will not guarantee the result | Apply directly to bare masonry |
| Damp surface | Slightly damp is fine; wipe off visible moisture first | Damp is allowed, dry gives the best results |
| Application temperature | 50 to 90 F, avoid direct sun | 50 to 90 F, not within 5 degrees of the dew point |
| Floors | Not for floors | Not for floors |
On paper they are close, so the choice comes down to the wall. For a new block or poured wall you are in a hurry to finish, INSL-X’s 14 day or pH 13 rule is the shorter wait, though a pH test strip is the only way to use the second option. For an older wall where you are not sure what is on it, KILZ is the one that says plainly that previously painted surfaces are at your own risk. Neither wins on spread rate in every case, and on rough block you should plan for the low end of each range.
Preparation that decides whether the paint holds
- Strip anything that is not bare masonry. Old paint and sealers are the most common reason a waterproofer peels, and both sheets prefer bare, uncoated masonry. If you scrape or sand old paint, the data sheets warn that it may release lead dust, so wear a respirator and use wet methods.
- Remove efflorescence and mildew. INSL-X says the surface must be free of dirt, white powdery deposits and mildew. It suggests a diluted muriatic acid solution for efflorescence and a mix of 1 part household bleach to 3 parts water for active mildew, rinsed well, and tells you to follow the safety directions on both. Never mix the two.
- Patch before you paint. INSL-X says to fill holes, joints and cracks wider than 1/8 inch with hydraulic cement, and to seal hairlines narrower than that with concrete sealant or caulk. KILZ calls for fast-setting hydraulic cement on every hole and crack for below-grade walls. For the technique, see the concrete crack filler guide.
- Test the surface. KILZ’s sheet describes pressing masking tape on the wall in several places and pulling it off sharply. If concrete comes away with the tape, remove it down to sound material before coating.
- Brush it in. INSL-X says to work the first coat into the pores with a stiff synthetic brush or a thick-nap roller and to back brush if you roll. Do not thin either product. If seepage is still showing after two coats, both sheets say open pores or pinholes remain, so recoat those spots instead of spreading the paint thinner.
KILZ Basement and Masonry Waterproofing Paint
This is the better-reviewed of the two on Amazon: 4.7 stars from about 1,260 ratings, an Amazon’s Choice badge, and Amazon itself as the seller when I checked. It is a styrene acrylic, covers 75 to 125 square feet a gallon, and the listing says it is not for floors or for walls with active water leaks. One reviewer with a rough, divoted block wall needed three coats to hide the gray, which fits the advice to plan on the low end of the coverage range. One reviewer also mentioned a strong smell while painting, so ventilate well.
Check price on Amazon: KILZ Basement and Masonry Waterproofing Paint (1 Gallon). This is an affiliate link (see our disclosure for details).
INSL-X WaterBlock Acrylic Masonry Waterproofer
WaterBlock is an acrylic masonry waterproofer made by Benjamin Moore and rated 4.6 stars from about 300 ratings, also with an Amazon’s Choice badge and shipping by Amazon. Its data sheet is the more detailed of the two on preparation. It is alkali resistant and has a mildew inhibitor in the dry film. It can be tinted with up to 2 fluid ounces of colorant per gallon, and its VOC content is under 100 grams per liter.
Check price on Amazon: INSL-X WaterBlock Acrylic Masonry Waterproofer (1 Gallon). This is an affiliate link (see our disclosure for details).
How much to buy: a 20 by 30 foot basement
The perimeter of a 20 by 30 foot room is 100 feet. With 8 foot walls that is 800 square feet, and taking off about 20 square feet for two small windows leaves 780. Two coats means 1,560 square feet of paint film.
| Coverage per gallon, per coat | Working | Gallons for two coats |
|---|---|---|
| 75 sq ft (rough block, the low end of both) | 1,560 divided by 75 | 20.8, so 21 |
| 100 sq ft (the high end of INSL-X) | 1,560 divided by 100 | 15.6, so 16 |
| 125 sq ft (the high end of KILZ, smooth walls only) | 1,560 divided by 125 | 12.5, so 13 |
I read each spread rate as per coat. The INSL-X sheet supports that: its dry film of 7.4 to 9.8 mils at the stated spread, doubled for two coats, gives the 14.5 to 20 mils it asks for in total. The KILZ sheet gives film thickness at each end of its range without saying so directly, so confirm on the can. For this room, expect to buy 16 to 21 gallons of either one if the block is rough, and whether a larger pail suits the job is worth checking before you add up one-gallon cans. Allow about 24 hours before heavy traffic along the wall and about two weeks before washing it.
When to skip the paint and call someone
Skip it when you can see water enter the basement, when the paint on an earlier attempt has blistered or flaked off in sheets, or when white deposits return soon after cleaning. Those point to water moving through the wall, and a coating on the inside will eventually be pushed off by it. Companies that sell interior drainage and sump systems make the same point louder, and they have a reason to, so weigh their claims accordingly. Still, the paperwork backs up the core of it: KILZ’s listing says not for walls with active leaks, and INSL-X’s sheet says not to seal a crack that is visibly leaking. Start outside. Make sure the soil slopes away from the foundation, gutters are clean and downspouts discharge well away from the house. If water still gets in, a foundation contractor or waterproofing specialist can tell you whether you need drainage work, and that is a job for a professional rather than a coat of paint. Angi’s foundation pros are one way to find one. This is an affiliate link (see our disclosure for details).
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.