Maintaining any interior environment that is different than the exterior environment presents costs, both financially and environmentally. The other side of weather barriers is their pivotal role in achieving energy efficiency for a building. Weather Barriers are Essential for Energy Efficiency Whether from the structural or health hazards, a brand new building can become inhabitable exceptionally quickly without proper weather protection. When moisture gets into the building envelope, wooden materials rot, steel materials corrode, and mold begins to grow. Improper installation or an absence of weather barriers allows for the elements to enter a wall or roof assembly and cause the materials within to deteriorate. Water (whether vapor, rain, or drainage) and air are driven to seams and leakage points by wind, gravity, and natural properties of surface adhesion. Weather barriers provide the most important line of defense protecting the building interior from the elements on its exterior. It may seem logical to want to prevent air or moisture from entering and exiting a building, but what makes this protection essential? Weather Barriers are Essential for Durability The impermeability of a vapor barrier is often measured in perms (1.0 US perm = 1.0 grain of water vapor/square foot/hour/inch of mercury), where anything lower than 1 perm is viewed as impermeable. In warmer, more humid climates, the barrier should be installed on the exterior side of the assembly to prevent exterior moisture from entering the assembly from the outside. In cooler climates where the building is heated more often, the vapor barrier should be installed on the interior side of a wall or roof to prevent moisture from entering the assembly from the inside.

This can end up trapping moisture inside the building enclosure and causing more harm than good.ĭepending on the climate, a vapor barrier ought to be placed on different sides of an assembly.

This is why vapor barriers are never placed on both interior and exterior sides of an assembly. Since at least some degree of moisture will inevitably find its way into a building, it’s important to have a structure that can “breathe,” allowing for moisture to dissipate on its own. The word control is important because complete impermeability is not necessarily helpful. Vapor barriers are intended to control and slow the rate at which moisture can flow through building materials.

For that reason, fluid-applied air barrier membranes are typically the best choice, as they can be applied over rough surfaces, openings, penetrations and they dry as a single, monolithic layer. Standard sheathing materials like gypsum or plywood partially function as air barriers, but have vulnerability at the seams between individual boards, making their role as an air barrier incomplete.Ī wall assembly can contain multiple air barriers, but to stop air intrusion and air leakage, one single, continuous layer with no seams or gaps is critical. While other layers in the building enclosure may have varying degrees of permeability, the air barrier should be impermeable to the passage of air.Īn air barrier can take many forms, including mechanically fastened building wraps, adhesive membranes, fluid-applied materials, rigid insulation or concrete. This is especially important in the age of air conditioning, where it is essential for energy efficiency to keep conditioned air inside a building’s walls and unconditioned air outside of them.Īn air barrier serves as the shield between the interior and exterior of the building. Here are some key basics to understanding air barriers and vapor barriers and their role in providing weather resistance.Īir barriers are designed to prevent the flow of air between the interior and exterior of buildings. Weather barriers, put simply, are layers in a roof or wall assembly that are designed to keep exterior elements out and keep conditioned air in. The phrase “weather barrier” most often refers to either air barriers, vapor barriers, or a combination of the two. Weather barriers, put simply, are layers in a roof or wall assembly that are designed to keep exterior elements out and keep conditioned air in.

Moisture barrier