Putting a Plug in the Dike: Interior Basement Waterproofing


Protecting a basement from water can be done from the outside, as described in our blog post Holding Back the Waters: Exterior Basement Waterproofing, or from the inside. These are called “positive side” and “negative side” waterproofing, respectively. These two approaches are complementary, and for best results, a combination of both should be used.

Negative side waterproofing can be thought of as like the boy in the legend of the boy and the dike: pushing against a water pressure coming from outside. By itself, this approach might be effective, but ultimately the best fix is the external, “positive side” fix. Nonetheless, newer technologies and materials make interior basement waterproofing quite effective as well.

Interior Basement Waterproofing

Interior basement waterproofing takes numerous forms, depending in part on the source of the water or moisture. Flooding is a separate issue than chronic moisture entry, and flooding prevention is handled in blog posts on backflow check valves and Toronto’s 2013 flooding. So the first step is understanding the source of basement wetness.

Hints on the source of the moisture are found in the evidence of water: a musty smell, or wet spots on basement floors or walls, are typical signs. Efflorescence (a white salt powder) on concrete walls or concrete blocks is another sign: as water penetrates the concrete it dissolves salts in the concrete itself. When it reaches the surface, evaporation occurs and the salts are left behind on the surface of the concrete.


One way to check on the source of moisture is to tape a letter-sized piece of foil on the foundation wall and to leave it for a few days. If it is collecting wetness on the room side of the foil, then the air is the source of the moisture; if it is collecting wetness between the foil and the foundation wall, then the wall itself is the source. Both are possible, unfortunately.

Treatments for moisture depend on the source of moisture, and there are several. Usual causes of basement moisture include cracks in foundation walls or floors, improperly handled water runoff from gutters, a high water table, warm air entering a cool basement through the rim joist (the board above the foundation wall that supports the floor joists and the sill plate), or water condensing on cold pipes.


Cracks in floors or walls

Cracks in foundation walls or floors are handled with a concrete sealer, epoxy sealer, and a waterproof masonry paint. Usually that treatment needs to be complemented by methods that reduce the amount of water surrounding the foundation in the first place. If moisture is finding its way into your basement through cracks, you can be pretty sure that there is a surrounding environment of water or moisture that also needs to be addressed.


Improperly handled water runoff

This is really a matter of considering improvements to exterior waterproofing, but needs to be dealt with as a source of problems.

High water table (Groundwater swelling)

If you are dealing with groundwater swelling outside the walls and floors, a system of draining that leads to a sump pump is desirable. Such a system is not unlike the drainage system described for exterior basement waterproofing: a weeping tile (or “French drain”) is installed that collects water, and removes it from interaction with the basement walls and floor.


In this case, an interior system can also be installed. This is typically a job for professionals who will break the concrete slab along interior wall, excavate to insert weeping tile, connect the weeping tile to a storm line or sump pump, wrap the weeping tile in filter cloth, cover the weeping tile with gravel, cover the foundation wall with a dimple board membrane, and re-cement the slab excavation.

A “capillary break”—a sheet of polyethylene or asphalt mastic applied to the top of the footing—is another good tool in fighting moisture arriving through capillary draw, but realistically, this method is best used in new construction as access is difficult once the foundation work has been done.

Moist air entry

The entry of moist air is managed by sealing gaps against air intrusion. A mortar of low-shrink sealant is used to seal gaps along the rim joist, as well as any pipes or water lines that penetrate the basement wall. Some condensation issues also can be managed by improved insulation and dehumidification systems, to reduce the temperature differential that causes condensation, and to actively remove moisture from the air itself. Cold water pipes can also be insulated to prevent condensation.


A basement vapor barrier can also help. Such vapor barriers are usually polyurethane sheeting that is stapled to to framing being installed on a basement wall, before the wall is finished with plaster or drywall.

Interior basement waterproofing is a good solution to wetness in a basement, and often is the most cost effective means when access is easy, as in an unfinished basement.

Master Drain