Vapor retarders are materials that restrict or reduce the rate and volume of water vapor diffusion through a building’s ceilings, walls, and floors. Available in a variety of materials, they are commonly made of polyethylene sheets, treated papers, and metallic foils. Historically, they have been called vapor barriers but are now called vapor diffusion retarders or simply vapor retarders. Although the familiar term vapor barrier implies that the material halts all moisture transfer, this is incorrect. Vapor retarders actually only reduce the rate of moisture transfer, they do not stop moisture flow completely. As you will discover in this chapter, the main reason to retard the transmission of gaseous water vapor through building envelopes is to prevent it from condensing to liquid water within the structure or insulation.
Thermal insulation has only been prevalent in residential construction during the last 50 years. As originally designed, older structures did not need to restrict the flow of airborne moisture because there was little within the affected cavities to hold it. Older homes tend to have less insulation and more air gaps in their thermal envelope. The wall cavities, if wetted, dried quickly because of the leaky construction methods employed. Windows and doors were not sealed tightly, and most construction materials were not vapor-tight. Simply stated, water vapor was able to pass through the building envelope unobstructed, thereby not becoming trapped within the structural assembly. As residential design became more energy conscious during the second half of the twentieth century, the homes became more airtight.
Storm windows, caulking, weatherstripping, and thermal insulation interfered with the movement of water vapor. It soon became evident that moisture movement was of prime concern if insulation was to remain effective and structures were to remain sound.