Air barriers are designed to prevent the flow of air, and the moisture attached to it, from entering a building envelope. A vapor barrier only aims to prevent the transport of moisture by vapor diffusion into a home’s building envelope. Notably, the amount of moisture carried by airflow is 50 to 100 times greater than that carried by vapor diffusion, which makes the need of a high-quality air barrier, like Barricade® Building Wrap, more essential than a vapor barrier.
Furthermore, impermeable vapor barriers may cause a build-up of mold and rot, while permeable air barriers, like Barricade® Building Wrap, ensure evaporation of moisture within a home’s wall system.
The 2018 International Energy Conservation Code (IECC®) defines an air barrier as one or more materials joined in a continuous manner to restrict or prevent the passage of air through the building thermal envelope and its assemblies. An air barrier material must also have an air permeance that is not greater than 0.02 L/(s·m²) at a pressure difference of 75 Pa (0.004 cfm/ft2 at a pressure difference of 1.56 lb/ft2) when tested in accordance with ASTM E 2178. Air permeance is the quantity of air that permeates through a product, while air leakage is the air that passes through gaps and holes.
The purpose of an effective air barrier is to regulate the indoor climate by stopping the transfer of air, and the moisture associated with, between the interior and exterior of a home. An air barrier must also resist air pressure differences that act on them. Stopping air transporting moisture to the interior of a wall assembly is critical because when the warm vapor touches the cool interior walls, the vapor turns to liquid by condensation. Essentially, air barriers minimize or restrict heat loss and gains through conduction, convection, and radiation.
The 2018 IRC (Table R402.4.1.1) states a continuous air barrier shall be installed in a building envelope, the exterior thermal envelope contains a continuous barrier, and breaks for joints in the air barrier shall be sealed.
The 2018 IBC, section C402.5.1, air barrier criteria for commercial buildings (required for all climate zones but 2B) require a continuous air barrier throughout the building thermal envelope. Furthermore, the air barriers shall be permitted to be located on the inside or outside of the building envelope, situated within the assemblies composing the envelope, or any combination thereof. In addition, the air barrier shall comply with Sections C402.5.1.1 and C418.104.22.168.
A vapor barrier prevents vapor diffusion through building materials. In building science, the second law of thermodynamics governs vapor diffusion. Simply put, moisture flows from an area of higher concentration towards an area of lower concentration of moisture, or from a warmer to a cooler space within a building material like gypsum and insulation.
It is important not to confuse a vapor barrier with vapor retarders. Vapor barriers stop vapor diffusion, while vapor retarders only slow vapor diffusion. Importantly, the desiccant method of ASTM E 96 is used to determine a material’s ability to restrict the amount of moisture that passes through it, which determines its vapor retarder (barrier) class.
Historically, placement of vapor barriers (typically polyethylene) was on the interior wall and ceiling insulation to prevent vapor division into the wall systems during the winter months when the interior of a home is warmer than the air within the wall system.
Vapor Diffusion – a Minor Player in Moisture Intrusion to a Wall System
A 2018 study* out of Denmark looked at the influence of driving rain and vapor diffusion on the movement of moisture and heat through a hygroscopic and permeable building envelope. A hygroscopic building envelope can absorb and store moisture from the surrounding air. A permeable building envelope allows for vapor diffusion.
The study concluded that the presence of a vapor barrier did not produce significant changes in the moisture content of the wall assembly. Furthermore, of the four mechanisms for moisture transfer into the wall system, liquid flow, capillary suction, air movement, and vapor diffusion, vapor diffusion represented the smallest amount and so is less likely to cause severe damage to a home.
Not only do vapor barriers not significantly help a wall system stay dry, but they may also damage the integrity of the home. If moisture infiltrates a wall system, the low permeance of a vapor barrier may stop a wall system from drying. Insufficient drying within the building enclosure can lead to mold and rot, which is unhealthy to the occupants of a home and can damage the integrity of the house.
The use of a vapor barrier on the interior or exterior of a building is dependent on the climate zone. The 2018 International Building Code (IBC) 1404.3 and the 2018 International Residential Code (IRC) R702.7 instruct the use of Class I or II vapor barriers and retarders on the inside of frame wall in climate zones 5,6,7,8, and Marine 4. Southern climate zones 1, 2, and 3 do not mandate vapor barriers and retarder.
Barricade Building Wrap is a continuous air barrier over the entire home’s building envelope. Barricade Wrap is also impermeable to airflow, durable over the expected lifetime of the home, and has the stiffness and strength to resist the forces that act on it during and after construction.
Barricade Wrap is an effective air barrier that is continuous, permeable, durable, and strong. Unlike impermeable vapor barriers, the Barricade Wrap can resist moisture while allowing humidity to escape from exterior wall cavities, which is of particular importance in hot and humid climates. Please visit Barricade® for more information on air barriers vs. vapor barrier.
*Bastien, Diane & Winther-Gaasvig, Martin. (2018). Influence of driving rain and vapor diffusion on the hygrothermal performance of A hygroscopic and permeable building envelope. Energy. 164. 10.1016/j.energy.2018.07.195.