Waterproofing materials

The single most useful thing to understand: a coating is not a membrane and a membrane is not a drain. Interior waterproof paint fills pores in concrete. It can help with damp-feeling walls and condensation. It cannot resist water pushed by hydrostatic pressure, and if it is applied over a wall with active pressure behind it, it will blister off. That is not a defect — it is the wrong product.

Exterior barriers go on the outside face of the wall after excavation. Sheet and liquid-applied membranes form a continuous skin. Sodium bentonite is clay that swells on contact with water to seal itself, priced at $2-$5 per square foot (Fixr). Dimple board (a plastic sheet with raised bumps) is often installed over the membrane; it creates an air gap so water runs down to the footing drain instead of sitting against the wall.

Drainage materials are the perforated pipe, filter fabric and washed gravel that make a footing drain or interior perimeter drain work. Fabric matters: pipe laid in soil without a filter silts up and the system quietly stops draining after a few years. Ask what the pipe is wrapped in and what gravel size is being used.

Interior vapor barriers ($1,500-$4,000, Fixr) are plastic sheeting that keeps wall moisture off finished surfaces and directs it into the drainage channel. They are appropriate as part of an interior drainage system. Sold alone as 'waterproofing,' they hide a wet wall rather than fix it.

How to choose

Match the material to the water pressure

Damp walls and condensation: dehumidification and possibly a coating. Water under pressure: drainage. No coating solves a pressure problem.

Ask for the product data sheet

Any legitimate membrane, pipe or pump has a published spec sheet. If a contractor cannot or will not name the product, that is a real warning sign.

Filter fabric is not optional

Drain pipe without proper fabric and washed stone silts up. This is a common source of systems that fail five to ten years in.

Be skeptical of proprietary 'systems'

Many branded basement systems are standard drain pipe, dimple board and a pump with a trademark on them. That is fine — but price them against a generic equivalent.

Discharge matters as much as collection

Where does the sump pump water go? Discharging next to the foundation just recycles the problem. Confirm the discharge line length and destination in writing.