Temperature differen?als and wind loading from dense cold air are the leading forces that induce the convey-ance of contaminant soil vapors from the ground into homes and buildings. The lower the outdoor tempera-ture, the grater the building stack effect and the greater resultant indoor negative pressure that draws in soil gas.
The function of a Vapor Intrusion Mitigation System (VIMS) is to place a greater level of vacuum on the soil below the slab than the combined environmental and building forces can place on the top of the slab. The problem is that weather- induced forces by themselves can create a seasonal “in order of magnitude pressure differen?al” between the underlying soil and the interior of the building, even with a VIMS with a constant speed motor running (see graph on right). Many VI systems that func?on well during the summer months do not achieve their design objec?ves during hea?ng season simply because they do not have the reserve capacity to overcome these forces.