responds: The answer is simple. Or,
more accurately, the question is irrelevant since
codes in most states with mild climates
require a vapor barrier on the side which
is warm in winter — the inside. However,
it is important to remember that walls built in
this climate must have the potential to dry.
Mild (or "mixed") climate zones are those areas
requiring both heating and cooling for several
months of the year. During hot, humid, summer
months, the predominant vapor drive will be from
the outside in during the sunny daytime. At night,
the wall should dry to the outside, if given the
An exterior sheathing of plywood or OSB will act
as a mild vapor retarder, but will still allow the
wall to dry to the outside at night.
To fully understand why this is important, you
must understand what a vapor barrier does. First,
let’s be technically accurate and call
this a vapor diffusion retarder! A vapor shield,
such as polyethylene. sheeting, can only
slow the flow of vapor diffusion
— it can’t stop it.
Vapor retarders are intended (at least in theory
— probably not too well in practice) to
limit the amount of moisture — in vapor
form — that passes through the building.
The rate of vapor diffusion is determined by the
permeabilityof the building material and the
driving force, which is air pressure. The higher
the pressure or the lower the permeability, the
greater the vapor diffusion will be.
Vapor diffusion is also a function of surface
area. A vapor retarder that has 30% holes would
only be 70% effective. However, this may not be as
critical as you think.
Diffusion is only one way moisture migrates
through a wall, and the least important. The
others, in order of importance, are bulk moisture
(rain and snow), capillary action (wicking), and
air-transported moisture (leakage of humid air).
Thus, having a few small tears in the vapor
retarder of a wall is less important than sealing
all the wall penetrations, such as electrical
outlets, light switches, and gaps between the
interior wall board and the framing.
If all this seems confusing, remember, given the
opportunity, most conventional wall systems are
somewhat forgiving. Control the moisture in the
order of importance, and you should have no
— Frank Vigil is senior project
manager with the North Carolina Alternative Energy
Corporation, currently developing a voluntary,
statewide performance standard for new