Quick Comparison
| Factor | Interior Waterproofing | Exterior Waterproofing |
|---|---|---|
| Average cost | $3,000-15,000 | $10,000-30,000 |
| How it works | Catches water that enters | Prevents water from entering |
| Excavation needed | No | Yes, around foundation |
| Disruption to yard | None | Significant |
| Walls stay dry? | No | Yes |
| Typical warranty | Lifetime transferable | 10-25 years |
What Interior Waterproofing Actually Does
Interior systems don't keep water out. They manage water that's already in. That's an important distinction nobody made clear to Frank.
The Drain Tile System
They jackhammer a channel around the perimeter of your basement floor, lay perforated pipe, fill with gravel, and pour new concrete over it. Water that seeps through walls or floor hits the channel and flows to a sump pump.
The sump pump kicks on, pumps water outside, and your floor stays dry. It works. But the water still came through the walls.
Why It's Popular
Interior systems are cheaper. No excavation outside. Can be installed in winter. Lifetime warranties are standard. Contractors love them because they're profitable and don't depend on weather or yard access.
Frank's contractor pushed interior hard. In retrospect, Frank thinks the contractor just didn't do exterior work.
The Hidden Problem
Your walls stay damp. The concrete absorbs water, releases humidity into the air, and can still develop efflorescence and that musty smell. You're managing symptoms, not solving the problem.
Frank's basement doesn't flood anymore. But he runs a dehumidifier constantly, and the walls still show water staining.
What Exterior Waterproofing Actually Does
Exterior waterproofing stops water before it reaches the wall. Different philosophy entirely.
The Process
They dig a trench around your foundation, all the way down to the footing. Apply waterproof membrane to the exterior wall surface. Install drainage board to direct water down. Lay drain tile at the footing level. Backfill and restore landscaping.
Water hits the membrane, runs down to the drain, flows away from the house. The wall itself never gets wet.
Why It's More Expensive
Excavation is labor intensive. You're essentially unburying your entire foundation. If you have a deck, porch, or landscaping along the house, it's getting destroyed and rebuilt. If you have underground utilities, they need to be worked around.
My cousin got quotes for exterior waterproofing. Ranged from $22,000 to $31,000 for a standard two-story house. The cheapest interior quote he got was $5,500.
When It Makes Sense
If your walls are actively wet, not just leaking at the floor. If you want to finish your basement and need completely dry walls. If you're already doing excavation for another reason. If humidity control matters for health reasons.
Frank's Lesson
What Frank should have done differently.
The Symptom vs Cause Problem
Frank had water on his floor. The interior system fixed that. But his actual problem was water coming through the walls. The interior system doesn't address that. It just catches the water lower down.
Now Frank is looking at exterior waterproofing on top of what he already spent. He could have done exterior only for less than both combined.
The Right Question
Before choosing a system, ask: Do I need to keep water off my floor, or do I need dry walls? If you just need a dry floor and don't care about wall moisture, interior is fine and cheaper. If you need actual dry walls, interior won't get you there.
When Interior Makes Sense
Interior waterproofing isn't wrong. It's just not right for every situation.
High Water Table
If water is coming up through your floor because the water table is high, exterior waterproofing won't help. You can't waterproof against water pressure from below. Interior drain tile and a sump pump are the correct solution.
Budget Constraints
If exterior is $25,000 and interior is $8,000, and you can't afford exterior, interior is still better than nothing. You'll have a dry floor even if walls are damp.
No Exterior Access
If your house is three feet from the property line, or there's a shared driveway along the wall, excavation may not be possible. Interior becomes your only option.
When Exterior Makes Sense
Exterior is the more complete solution when feasible.
Wall Cracks Leaking
If water is coming through cracks in your walls, exterior stops it at the source. Interior catches it after it's already inside. For wall leaks, exterior is the better investment.
Planning to Finish the Basement
If you're putting up walls and flooring, you need actually dry walls. Moisture behind finished walls leads to mold. Exterior waterproofing gives you walls you can finish against.
Already Excavating
If you're doing other foundation work that requires excavation, adding waterproofing at the same time makes sense. The excavation is the expensive part. The membrane itself isn't that much extra.
What I'd Do
Having watched Frank's situation play out.
Diagnose First
Figure out exactly where water is entering. Floor? Walls? Wall-floor joint? The answer determines the solution. Don't let a contractor sell you their preferred system before understanding your specific problem.
Consider Both
For severe water problems, some houses need both. Exterior to keep walls dry, interior to handle high water table or floor seepage. One or the other isn't always enough.
Get Quotes for Both
Even if you think you know what you need, get quotes for both approaches. The price difference in your specific situation might surprise you. And hearing contractors explain why they recommend what they recommend helps you understand the trade-offs.