BILLS BASEMENT BLOG - Vapor barriers when finishing a basement

When finishing a basement the vapor barrier MUST go directly onto the foundation wall. A lot of basements we see that are experiencing problems with mold or discoloration within finished walls, are a result of either no vapor barrier at all or a vapor barrier that is attached to the outside of the studwork leaving the wood studs (organic material) exposed to the humidity from the vapor transmission through the concrete wall.

Concrete is a very porous material and allows a great deal of vapor to transmit through. Here is a quick experiment you can try at home to prove my point. Take a dry concrete block and set it in a couple inches of water. If no rain puddles are available, a pan will do. Let the block sit for half an hour of so. Note the moisture (discoloration) beginning to work its way up the block. This same thing happens in basements. Water vapor works its way through the block and into the basement space.

If the basement walls are framed, then insulated, then the vapor barrier is attached, all that material is trapped in an enclosed space along with whatever moisture comes through the wall. This can become a breeding ground for all sorts of nasty things. It's even worse if the foundation is prone to severe leaking during heavy rains. We've seen fiberglass insulation behind sheetrock soak up and hold water like a sponge. Not good.

In order to prevent these types of issues, the vapor barrier MUST be put on the wall before any finish work is done. If the foundation has known (or even suspected) water problems, installation of a good quality water proofing system is a must.