So what does crack loss have to do with vapor barriers? Crack loss renders conventionally installed polyethylene vapor barriers ineffective. The principal function of a vapor barrier installed on the inside face of an exterior wall is to limit airborne moisture from migrating from inside to outside. There are a number of reasons for limiting this particular migration, but principal among them are:
1. To limit condensation from forming within the exterior wall construction
2. To reduce heat exchange from inside to outside via crack loss; moist air transfers more heat than dry air.
In order for a polyethylene vapor barrier to function adequately for the purposes enumerated above, it must be virtually air-tight, but in practice, this is impossible. For example, the average fastening pattern for interior gypsum wallboard one encounters uses 50 screws per 48" X 96" sheet; therefore, a wall that is 16'-0" long and 8'-0" high will require 4 sheets of wallboard to cover and will perforate the vapor barrier 200 times. Cut-outs for piping, electrical outlets and other penetrations generally add even larger gaps and holes, and seams and average installation tolerances only make discontinuity an even bigger problem.
Moving from inside to outside, heated relatively moist air passes through the porous plastic sheeting and usually encounters fiberglass batt insulation which has been installed between the studs. It's worth noting here that fiberglass insulation has the capacity to trap liquid moisture. After passing through the fiberglass batts, the moist air encounters the exterior rigid sheathing which is also liquid absorptive and porous due to the same fastening, cut-outs and penetrations the plastic sheeting suffers from. The moist air runs into the spun-bonded polyethylene house wrap. In winter the moist air has usually been cooled and dried considerably by the time it reaches the back side of the house wrap, and because the house wrap has been purposefully perforated by the manufacturer to allow air to pass through it, it does and finally encounters the back side of the exterior vinyl clapboards.