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?
  
• What is a vapor barrier?
  
• Will installing a vapor barrier make the walls sweat?
  

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.