The humid air is indoors.
Plastic vapor barrier basement walls.
A vapor barrier is also helpful to keeping a basement insulated and warm.
You should then use fiberglass insulation that has a vapor barrier between the studs with the vapor barrier facing in toward the living space.
In winter the dew point of the interior air is typically about 50 f assuming an air temperature of 70 f and a relative humidity of 50.
The cool surface is the sheathing assuming no exterior insulation.
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.
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The material is stapled to the frame of a basement wall before it is finished with plaster or drywall.
Polyurethane sheeting is a typical vapor barrier that contractors use to prevent moisture from penetrating the walls of a basement.
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If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface moisture problems can occur.
The construction of the wall for instance is vital to the use of a vapor barrier.
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.
The main concerns are condensation and drying potential.
When the mold grows it will spread to everything around it studs drywall etc.
Plastic on the outer surface of a wall in cold weather could cause problems.