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.
