What is the effect of low permeability vapor diffusion retarders in foundation walls?

Low permeability vapor diffusion retarders in foundation walls are designed to restrict the movement of moisture vapor. By having a low permeability, these materials create a barrier that limits the amount of moisture that can pass through the wall. This means that if moisture enters the wall assembly from the interior or the ground, it may not be able to escape easily. As a result, the vapor can become trapped within the wall. This trapped moisture can lead to various issues, such as mold growth, deterioration of building materials, and an overall increase in humidity levels within the foundation space. Hence, the effect of low permeability vapor diffusion retarders is that they tend to trap moisture vapor, which can create conditions favorable for moisture-related problems if not managed properly.