Vapor Barrier Concrete Basement Walls

The vapour barrier is more properly called a vapour retarder.

Vapor barrier concrete basement walls.

No vb is perfect. It is designed to stop moisture in the form of vapour that is inside the house from moving into the walls and forming condensation or ice. Building codes usually require a vapor barrier 4 mil plastic sheeting on exterior basement walls if the framing is attached to masonry or concrete surfaces or if the wood framing butts up against the outer basement walls. Vapor barriers are used because while fresh concrete is poured wet it s not supposed to stay that way.

Or you can leave a 4 inch gap between the studs and concrete wall and probably get away with it. Basement insulation vapor barriers the trick with basement insulation jobs is in understanding where water vapor is being stored and where it s moving to. A concrete vapor barrier is any material that prevents moisture from entering a concrete slab. If you have more than one barrier in the thickness of the wall you will trap moisture in the middle.

Not to be confused with a vapor barrier which is placed on the warm side of the wall just in front of the insulation and behind the drywall a moisture barrier goes against the basement wall and. Concrete is not insulated let alone waterproof. Most jurisdictions require a 4 mil plastic sheet vapor barrier for basement walls against concrete. For a basement that vapor barrier should be the waterproof membrane or coating on the outside of the concrete wall.

If you have a framed wall that is adjacent to a concrete foundation wall where the vapor barrier is next to the drywall in front of the studs the fiberglass insulation gets wet and becomes a breeding ground for mold. Finishing basement walls with a vapor barrier will help make them waterproof. So there is enough gap for air to flow. That is if the studs are going to touch the concrete wall.

With fiber insulation and a vapor barrier moisture can get trapped in the insulation leading to moldy insulation and decay in the wood framing. It s not rocket science. As the wall dries the water vapor leaves the concrete and tries to penetrate the insulation and wall. When the mold grows it will spread to everything around it studs drywall etc.

You should then use fiberglass insulation that has a vapor barrier between the studs with the vapor barrier facing in toward the living space. Usually that is right behind the drywall. Hence it must always be located on the warm in winter side of any insulation. Of foam against the wall this should not be a problem in your climate.

The water vapor can come through the concrete from the soil or from indoor air that comes in contact with the concrete. Concrete is like a huge sponge which holds water and water vapor for years and years.