Before any of the numbers, one fact that reframes this whole project: your basement was probably never waterproofed, and that was entirely legal. Under IRC R406.1, foundation walls enclosing below-grade space need only be damp-proofed — a coating designed to stop water vapour, explicitly not designed for hydrostatic pressure. Actual waterproofing is required only where a high water table or severe soil-water conditions are known to exist.
So a basement that stayed dry for twenty years and then started leaking usually has not failed. It was built to a lower standard than the one you now need, and the ground around it changed.
What it costs — and why the sources are 2.6× apart
| Source | Figure |
|---|---|
| Angi | Average $5,239; most pay $2,461–$8,201 |
| HomeGuide | $6,000 – $18,000 |
| This Old House | $13,640 for a 1,000 sq ft basement |
This Old House’s average is more than two and a half times Angi’s. The gap is definitional, not sloppy: This Old House prices a full system by basement area, while Angi’s distribution includes small crack-and-seal jobs that never touch the perimeter. Which number applies to you depends entirely on whether you are fixing one crack or rebuilding the drainage.
Interior versus exterior
| Source | Interior | Exterior |
|---|---|---|
| This Old House | $2,000 – $6,000 | $7,000 – $15,000 |
| Angi | ~$3,000 | ~$7,000 |
| HomeGuide | $6,000 – $12,000 | $10,000 – $18,000 |
The three disagree on absolute cost but agree on the multiple: exterior waterproofing runs about two to two and a half times interior. That is because exterior work means excavating to the footing — $50–$200 per cubic yard before anyone applies a membrane.
The individual pieces
| Item | Cost | Source |
|---|---|---|
| Interior drain tile / French drain | $4,000 – $17,000 installed | HomeGuide |
| Interior drainage, per linear ft | $60 – $120 | This Old House |
| Sump pump system | $600 – $2,500 | HomeGuide, Angi |
| Crack / epoxy injection | $250–$800 or $800–$1,500 per crack | HomeGuide vs Angi & TOH |
| Exterior membrane | $4 – $7 per sq ft installed | HomeGuide |
| Excavation | $50 – $200 per cubic yard | Angi, This Old House |
| Exterior regrading | $1,000 – $5,000 | HomeGuide |
| Mold remediation | $500 – $15,000 | Angi, HomeGuide |
Crack injection is the sharpest conflict in this guide — HomeGuide says $250–$800 per crack, Angi and This Old House both say $800–$1,500. Two of three point to the higher figure, but get it quoted rather than assumed.
And notice the regrading row. At $1,000–$5,000 it addresses the same water as $10,000–$18,000 of exterior waterproofing, by keeping it away from the wall in the first place. It is the cheapest genuine fix on the board and the one contractors selling systems are least likely to lead with.
Is a dehumidifier enough?
Sometimes, and we are not going to pretend there is a rule. No source we could find defines a threshold separating a humidity problem from a water-intrusion problem. What we can tell you is the price gap: a portable dehumidifier is $130–$300 and a whole-house installed unit $1,500–$3,500, against $6,000–$18,000 for waterproofing.
Given that spread, it is worth asking an independent inspector — not a waterproofing contractor — whether you have condensation or intrusion, before committing to a system.
What “lifetime transferable warranty” actually means
Every waterproofing company advertises one. Two publish terms specific enough to quote, and both reveal the same two limits.
Coverage is bounded to the treated area. Olshan’s published terms cover water coming through the perimeter system it installed, and state the guarantee “applies only to areas where the Water Lock system is installed.” If water later enters through a wall the crew never touched, that is not a claim. This is the single most common reason a lifetime warranty does not pay.
“Lifetime” is not the same as free. Olshan’s terms make the work “subject to an agreed adjustment fee”, and transferability is conditional on no major structural changes having been made. Other companies differ — Armored Basement states its warranty transfers to the next owner at no cost — so the transfer terms are worth reading before you choose on warranty alone.
We looked for a standard list of exclusions and there is not one. Every warranty is a private contract with no industry-standard form, so the only reliable approach is to read the actual document before signing.
Does it add resale value?
Waterproofing is not in the Remodeling Cost vs. Value report. Be careful here, because Basement Remodel is one of the 28 tracked projects and the two get conflated constantly. Finishing a basement is tracked; keeping water out of it is not.
The honest framing is the same as for a chimney or a septic system: a wet basement does not lower your price, it narrows your buyer pool and hands the ones who remain a discount to argue for. You are removing a deduction, not earning a return.
Working out your own number
- Get an independent diagnosis first — condensation and intrusion look identical and cost twenty times apart.
- Price regrading ($1,000–$5,000) before pricing exterior waterproofing ($10,000–$18,000). Same water.
- One crack? That is $250–$1,500, not a system.
- If you are quoted a system, ask what area the warranty covers — not whether there is one.
- Remember your house was likely only ever damp-proofed. That is a starting point, not a defect.
A sump pump is one part of a wider system. Here is what a sump pump installation costs on its own.
Figures from Angi, HomeGuide and This Old House’s 2026 guides. Code distinction from IRC R406.1 and R406.2. Warranty terms quoted from Olshan and Armored Basement’s published pages.