How to seal a basement wall
A sealer treats the surface of the wall. Whether that is enough depends on where the water comes from, and you can check that before you buy anything.
You seal a basement wall by cleaning the masonry, filling any crack or hole with a patching product made for it. And then applying a coating over bare, sound concrete or block. That is the short answer. The longer one is that sealing only holds when the water problem is small.
If water arrives from the soil outside under pressure, no surface product will win. This page helps you decide which case you have before you spend a weekend on it.
Check the cause before you seal anything
Start with a simple test. Tape a square of plastic sheet, about a foot across, tight to the bare wall and leave it for a couple of days. If moisture collects under the plastic, it is coming through the wall. If it collects on the outside of the plastic, it is condensation from room air, and sealing the wall will do nothing.
Then look at when it happens. Dampness that shows up only after heavy rain points at the yard. Read water in the basement after rain first, because the cheapest fix is often gutters and grading, not the wall.
What a seal can handle
Sealing suits a few narrow cases. Be honest about whether yours is one of them.
| What you see | Is sealing a reasonable first step? | Why |
|---|---|---|
| Faint dampness in humid weather, no liquid water | Often yes | It is vapor moving through the masonry |
| A small, hairline vertical crack that weeps a little | Often yes, after you track it | A patch can close a stable crack |
| A small hole where a tie or pipe once passed | Yes | A fast-setting patch is made for this |
| Water at the floor and wall joint after rain | No | Pressure from outside, so drainage is the answer |
| A horizontal crack, a stepped crack or a leaning wall | No | The wall is moving; see below |
The steps, in the order that works
Prep decides whether a seal lasts. Most failures are a prep problem, not a product problem.
First, remove everything that is not masonry: old paint, loose flaking material and white crust. Efflorescence is a salt left by moving water, so it tells you where moisture travels. See efflorescence on basement walls for what it means. Wire-brush the wall back to a firm surface and vacuum the dust.
Second, deal with any crack or hole. Cut a small crack open slightly so the patch has something to grip, then fill it with the product the manufacturer says is meant for that use. A fast-setting cement-based patch is the usual type. Press it in, hold it, and let it cure before you go further.
Third, apply the coating to a wall that is clean, sound and at the dampness level the label allows. Products differ, so follow the instructions on yours. Work in the stated number of coats and give each time to dry.
When sealing is the wrong move
Sealing from the inside puts a barrier on the wrong side of the wall. Water pressing from the soil will blister the coating, push it off or find another route. That is why the same product gets praise from one owner and complaints from the next. The page on waterproof paint on basement walls explains the pattern in more detail.
If the water keeps coming back, the next step is drainage, not another coat. Compare the approaches in interior vs exterior waterproofing.
The wall itself may be the problem
Do not patch a crack and paint over it if the wall is bowing, if a crack runs horizontally along the wall. Or if it steps through the mortar joints. Those signs point to movement, and a seal would hide the evidence. Photograph them, measure them and date the notes.
A licensed structural engineer, ideally one who does not sell repairs, is who you call for that. Never dig beside a wall that may already be loaded. Rules for permits and repairs are set locally, so your building department is the place to ask what applies.
Frequently asked questions
Can I seal a basement wall from the inside?
You can, and it works for faint dampness and small stable cracks. It does not stop water pushing from the soil outside, because the barrier sits on the wrong side of the wall.
What should I use to seal a crack in a basement wall?
Use a patching product made for masonry and follow its label. Small stable cracks are the only ones worth patching yourself. A growing, horizontal or stepped crack needs an engineer, not a patch.
How do I know if moisture is coming through the wall?
Tape a plastic square tight to the bare wall for a couple of days. Moisture under the plastic is coming through the wall, and moisture on the outside of it is condensation.
Why did the sealer peel off my basement wall?
Usually because water was pushing from the other side, or the wall was not clean and bare before the coating went on. Peeling is a sign of pressure, so look at drainage next.
Should I seal a wall that is bowing inward?
No. A bowing wall is moving, and sealing hides the evidence. Have a licensed structural engineer look at it first.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.