Continuity is critical to the design of vapor barrier systems. Without a vapor barrier/ retarder, condensation can occur in building roofs, walls, and floors. This can result in rot, corrosion, staining, and expense for the building owner.

A vapor barrier is placed on the moist side of the wall (the side with the highest vapor pressure). If the highest vapor pressure is on the exterior, moisture will flow towards the interior spaces, so the vapor barrier should be placed on the outside. In all instances, the vapor barrier should be placed on the moist side of the wall assembly to prevent damage.

The introduction of a vapor barrier within a wall assembly must be carefully studied to avoid trapping moisture in an undesirable location. Each design condition must be analyzed individually to determine the need and location for a vapor barrier within the wall assembly.

To be considered a vapor barrier, the material generally must have a permeation rating of 1.0 or less. In general, the vapor barrier must be continuous such that all joints, penetrations, and other holes must be sealed. Common ordinary duct tape is not suitable for long-term vapor sealing.

Vapor barrier material should have good tear and puncture resistance, in addition to low permeability.

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