BUILDING ENVELOPE: Vapor Barrier
Questions:
Will a wall with a five-eighths-inch
thick, foil-faced sheathing on the outside and a 6-mil plastic vapor barrier
on the inside have moisture problems?
There is the potential for a moisture problem, but the likelihood of this
depends on the quality of the installation.
If warm, moisture-laden air from inside a home gets into the wall, and the
inside face of the foil is cool enough, condensation could result. If the
inside vapor barrier is carefully installed and sealed to prevent air leaks,
this potential is significantly reduced.
The other factor affecting the potential for moisture problems is the temperature
of the inner foil face. Because the sheathing has a high R-value, there's
less chance the foil face will be cold enough to cause condensation.
What is a vapor barrier?
A vapor barrier is an impermeable material, typically plastic or asphalt paper,
attached to insulation.
The purpose of a vapor barrier is to prevent moisture from passing through
the insulation and condensing on the cold outer surfaces. A vapor barrier
has two main functions: keeping moisture inside a home, and preventing it
from condensing in the insulation.
In new construction, a sheet of polyethylene film is applied to the studs
before installing the drywall. Always apply the vapor barrier on the warm
side of the wall, ceiling or floor.
If insulation is to be blown into an attic, lay down the sheet of polyethylene
film first, or attach it before the sheetrock is added.
Everyday household tasks such as washing, cooking and bathing release moisture
inside the home. A vapor barrier slows the movement of this moisture from
the home's interior to the outside, raising indoor humidity levels and preventing
condensation in the wall or attic.
Will installing a vapor barrier make
the walls sweat?
No, but it's easy to confuse the installation of vapor barriers with moisture
problems because vapor barriers do effect indoor relative humidity.
The purpose of a continuous vapor barrier is to prevent moisture from entering
wall cavities and attics, where it can condense on cold surfaces and cause
structural damage.
The vapor barrier also reduces air leakage. Moisture produced by household
activities accumulates quicker because of the reduced airflow, resulting in
a higher relative humidity. If the humidity gets high enough, windows and
other cold surfaces begin to sweat, or condense moisture.
Condensation problems can be more serious during a new home's first winter.
This is due to extra moisture stored in drywall from joint compound and paint.
Use of exhaust fans during periods of peak moisture production, such as while
showering, bathing, cooking and wet cleaning can prevent or control moisture
problems. Construction-related moisture problems will diminish with time as
finish coatings cure. However, additional ventilation may be necessary during
a new home's first winter.
