History of Vapor Retarders

Historically, vapor accumulation has been remedied by a number of strategies. Prior to the construction of energy-efficient homes, less vapor-resistant materials were used in outer walls. Lowering the indoor relative humidity can be accomplished either by lowering the rate at which water vapor is added to the indoor air or by increasing the ventilation of the house indoor air to the exterior. With today’s lifestyle and indoor activities such as clothes washing and drying, showers, baths, hot tubs and spas, dishwashers, and humidifiers that tend to raise the indoor relative humidity, it has become necessary to increase the vapor resistance on the warm side of the insulation. These behaviors and design preferences have led to the scientific study of the proper placement of a vapor retarder.

Historically, the implementation of an insulating material with an asphalt/kraft paper or aluminum foil backing resulted in one of the first simple means of achieving a relatively effective “barrier” to the transmission of water vapor through exterior walls. The attachment flanges were configured to require the material to be installed with the backing material on the warm interior side of the walls in an effort to make the installation foolproof. The method was not entirely perfect, however, because water vapor could pass readily through the studs themselves and through the significant crack length where the insula­tion was attached to the studs.

The relative success of this first step led to further consideration of the problem. Before long, the application of 6-mil-thick sheets of poly­ethylene film on the interior sides of exterior walls, under the dry wall, or other finish, lapping and taping all joints, became standard prac­tice. It is one of the most effective available means of inhibiting the transmission of water vapor through exterior walls that is currently employed.5 Even this concept is now being scrutinized by many experts.