Vapor Barrier

A vapor barrier (or vapor retarder) is an airtight membrane installed on the warm side of the building envelope — typically between the interior finish and the insulation — that prevents moisture-laden indoor air from migrating into the wall, floor, or roof structure. In Finnish construction, the vapor barrier is usually a polyethylene sheet (0.2 mm thickness) sealed at all joints and penetrations with specialized tape.

The vapor barrier is one of the most critical components for preventing moisture damage in insulated structures. During cold weather, indoor air is significantly warmer and more humid than outdoor air. Without a vapor barrier, this moist indoor air passes through the interior finish into the insulation, where it encounters progressively colder temperatures. At the dew point, the moisture condenses into liquid water, saturating the insulation and wetting the structural members — creating ideal conditions for mold growth and wood rot.

The effectiveness of a vapor barrier depends entirely on its continuity. Even small holes or gaps — from nails, screws, electrical boxes, pipe penetrations, or poorly sealed seams — allow concentrated streams of moist air to enter the structure, causing localized moisture accumulation that can be worse than having no vapor barrier at all. Every penetration must be carefully sealed with appropriate tape, gaskets, or sealant compounds.

During renovations, particular care must be taken not to damage the existing vapor barrier. If the vapor barrier is found to be damaged or missing during renovation work, it should be repaired or installed as part of the project. In some retrofit situations, a vapor-retarding paint can be applied to the interior surface as an alternative to a sheet membrane. Consult a building physics professional if you are unsure about the best approach for your specific situation.