The design of energy-efficient, comfortable and long-lasting buildings must include restrictive barriers that protect the building envelope from air, water, condensation, movement and energy loss.
Achieving these functions in new construction and renovation projects requires a clear understanding of the differences between air, moisture, vapor and weather barriers. Here are four simple comparisons between barrier types and materials:
Tremco Construction Products Group offers a wide range of air, moisture, vapor and weather barriers that builders and architects can use to ensure moisture-resistant, vapor-resistant and airtight building envelopes. Read on to learn more about these specific building materials.
Air barriers form a continuous plane around a structure to stop uncontrolled air movement in and out of the building envelope. Air leakage accounts for up to 100 times more moisture infiltration than diffusion.
A high-quality air barrier limits heat losses and gains through convection, conduction and radiation.
When tested according to ASTM E 2178, an air barrier's air permeance should not surpass 0.02 L/(s·m²) at a pressure difference of 75 Pa (or 0.004 cfm/ft2 at a pressure difference of 1.56 lb/ft2).
Air permeance represents the amount of air that moves through a material. In contrast, air leakage flows through holes and gaps.
Air barriers regulate the indoor climate by stopping airflow between the exterior and interior of a building. In addition, air barriers prevent air-transported moisture from flowing to the interior of a wall system. Controlling air and preventing moisture infiltration:
Builders often install air barriers on the external side of the wall assembly to keep unconditioned air out of the wall cavity, which is the area between the inner and outer wall. A quality air barrier should provide continuity, structural support, air impermeability and durability regardless of the type. Air barrier types include:
Air Barrier Standards for Commercial Buildings
Section C402.5.1 of the 2021 International Building Code (IBC) demands a continuous air barrier for commercial structures, except climate zone 2B, throughout the building envelope.
Air Barrier Standards for Residential Structures
Section R402.4.1.1 of the 2021 International Residential Code (IRC) instructs to apply a continuous air barrier in a building envelope and sealing breaks for joints in the air barrier. IRCs apply to detached one-and-two family townhouses and dwellings not exceeding three stories above grade plane in height. Additionally, they must provide a separate means of egress with accessory structures not exceeding three stories above grade in height.
Moisture barriers, or water/weather resistant barriers, in residential construction are applied over the exterior sheathing and are usually vapor permeable to prevent trapping moisture vapor in the wall cavity. Waterproofing membranes are used in below grade construction to protect the foundation from moisture.
Above-grade moisture barriers seal against water infiltration to protect the building envelope's integrity and improve the safety and comfort of the occupants. Specifically, building envelopes need moisture barriers because they:
Below-grade moisture barriers typically involve applying a sheet or liquid-applied membrane to the below-grade walls as positive, negative or blind side waterproofing:
Manufacturers produce commercial waterproofing membranes in many types, sizes and thicknesses. However, builders typically use liquid-applied waterproofing membranes or pre-formed sheets.
The 2021 IRC R406 and the 2021 IBC 1805 specify the conditions requiring waterproofing or damp proofing:
Vapor barriers dampproof a structure to combat moisture and prevent water vapor inside the building from moving throughout the structure and permeating the walls and insulation.
The American Concrete Institute (ACI 515.1R-85) defines waterproofing as treating a structure or surface to resist water intrusion under hydrostatic pressure. In contrast, they define dampproofing as treating a surface or structure to resist water intrusion without hydrostatic pressure.
Specifically, vapor barriers stop vapor diffusion, which happens when moisture flows from an area of higher moisture concentration towards a space of lower concentration. It can also occur when moisture flows from a hotter to a cooler place within a building material, like insulation. Vapor barriers stop diffusion, while vapor retarders only slow vapor diffusion.
The ASTM E96 desiccant method determines a building material's ability to limit moisture from passing through it, giving it a vapor retarder (barrier) class:
Buildings prone to direct contact with water require a vapor barrier. However, your local building inspection offices can provide guidance on using vapor barriers. Building envelopes need vapor barriers to:
Most building experts recommend applying vapor barriers on the side of the wall experiencing the more hot and moist conditions — the inner surface in cooler climates and the outer surface in humid, hot climates. Examples of vapor barrier applications include:
Manufacturers commonly make vapor barriers out of water-resistant materials, including:
The need for a vapor barrier on a building's interior or exterior depends on the climate zone. The 2021 IRC R702.7 and the 2021 IBC 1404.3 mandate Class I or II vapor barriers and retarders on the interior of frame walls in climate zones Marine 4 and 5, 6, 7 and 8. However, climate zones 1, 2 and 3 do not require vapor retarders and barriers.
The AAMA (American Architectural Manufacturers Association) defines a weather barrier as a surface or a wall responsible for stopping air and water infiltration to the building's interior, protecting the building and its occupants from damaging and unhealthy mold and rot.
Placing a weather barrier over the sheathing and behind the siding allows moisture that infiltrates the wall system to dry out quickly. Vital components of a high-quality WRB include high tear strength, durability, ultraviolet resistance and straightforward installation.
Today’s building envelope designs require permeable weather barriers. Weather barriers protect a building against moisture-laden air and water infiltration which shields the structure and its occupants from unhealthy mold and rot.
Permeable weather barriers also protect against condensation because they allow moisture that accumulates in the wall system to escape. High-quality permeable weather barriers resist air, moisture and vapor by creating energy-efficient, healthy, long-lasting and comfortable buildings.
The WRB must also comply with the 2021 IBC 1402.2 requirements for water resistance and vapor permeability and the 2021 International Residential Code for water resistance (IRC R703.1.1).
Properly applied air, moisture, vapor and weather barriers help contractors and architects create energy-efficient, comfortable, safe and long-lasting buildings. Furthermore, air, vapor and moisture-resistant systems that feature high-quality products can contribute toward code-compliant, sustainable construction.