The short answer is no. At this point we have what amounts to a piece of thin plastic swiss cheese behind our interior finishes that likely does not provide an adequate barrier to moist air infiltration. As described below, as the air moves through the wall construction from inside to outside in winter, and outside to inside in summer, it cools. Because warm air can carry more moisture than cool air, the moisture is left behind and collects or condenses on absorbent building materials as the air moves through the wall. The effects of moisture condensation on building materials range from severe to negligible because they depend on a number of variables. For example, the amount of moisture generated by nearby environmental conditions influences the severity of effect; this is why the greatest amount of moisture damage inside walls is usually found around bathrooms and kitchens.
|Damage due to water and air infiltration|
In addition to wood rot and rust, fiberglass batt insulation has the capacity to retain moisture. When the batts become wet enough, they tend to compress downwards due to the effects of gravity. This downward movement usually creates large gaps in what should be a continuous thermal barrier, which in turn allows a more efficient exchange between warmer and cooler air, and therefore a better environment for condensation and crack loss occur. Even worse, the thermal effectiveness of fiberglass batts depends upon the ratio of air trapped between the fibers to the thickness of the batt, or how fluffy it is; wet, compressed fiberglass batts lose most of their value as a thermal barrier, and instead become very efficient generators of the type of environment that organic growth thrives in. Mold and fungus, for example, thrive in this type of environment. Most people are familiar with time lapse video. Imagine a time lapse video shot from inside a wood framed bathroom wall that's susceptible to significant air and moisture infiltration over a period of ten years; the final scene shows a mound of moldy potting soil replete with happy insects. That's all it takes, I've witnessed the phenomenon.
That is a sample of why I experience such chagrin when an apparently experienced contractor announces that a polyethylene vapor barrier has been installed on every project they've ever worked on.