Basement Wall Vapor Barrier for Block Walls

Basement Wall Vapor Barrier

Block foundation walls are common across southern Virginia and northern North Carolina, and block is porous. When red clay soil holds water against the outside of the wall, moisture moves through the block and shows up on the inside face. Sometimes it is a wet patch after a storm. More often it is a damp wall that never fully dries, a white powdery crust called efflorescence, and rust colored or dark staining where water has traveled the same path many times.

The interior perimeter drain handles the water that comes through the wall and reaches the floor. The wall vapor barrier handles the wall itself. It is a 20 mil poly liner applied to the inside face of the foundation walls, from the top of the wall down to the perimeter drain, so moisture that comes through the block is directed behind the liner and into the drain instead of evaporating into the basement or wicking up the wall.

That wicking is the part homeowners notice least and feel most. Damp block gives up moisture to the basement air all day, every day, so the basement stays humid even when there is no standing water, and the humidity carries upstairs. Covering the wall with a sealed liner stops that transfer. The efflorescence and staining go behind it, and the wall you see is clean and dry.

Twenty mil is a heavy material. It is much thicker than the thin sheeting sold for general use, it does not tear when a stored box leans against it, and it holds up over years in a damp environment. It is fastened along the top of the wall, sealed at the seams and penetrations, and it terminates at the drain so the water it collects has somewhere to go.

We recommend the wall barrier where the inspection shows water coming through the foundation walls rather than only at the joint where the wall meets the floor. If the wall is dry and the water arrives at the floor joint, the perimeter drain alone may be enough, and we will say so. If the wall shows efflorescence, staining or damp patches, the barrier is the difference between a basement that is drained and a basement that is dry.

The barrier is installed alongside the perimeter drain, not on its own. Applied to a wall without a drain beneath it, the liner would collect water with nowhere to send it. Together, the drain, the sump pump with battery backup and the 20 mil wall liner manage water that comes through the wall from outside, from the top of the block down to the footer.