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The construction process includes many choices about the best way to build something. But there might be even more tough decisions about how to help that “something” stay up. One of the most important is the installation of a barrier to protect the building envelope and reduce the risks of rotting, uncomfortable air, and high energy prices.
Barriers come in many forms and have different purposes. Construction experts should know the best barrier to apply to an external wall system based on building materials, environment, and other factors in each build.
Be careful – one product is often not interchangeable with the other!
Types of Envelope Protection: Air vs. Vapor & Weather Barriers
Understanding the various barrier types and the best investment for your new building means clarifying some misleading talk out there. “AVB" (air/vapor barrier) is a common term that’s actually incorrect. All of these products act as air barriers, but not all air barriers are vapor barriers. Let’s further break down the differences in air barriers vs. vapor barrier and weather barriers:
Air Barriers
Controlling air movement is important to sustaining energy efficiency within a structure.
Air barriers prevent the movement of air through the building envelope. However, they don’t stop the diffusion of vapor once it has made it into the building envelope. There are two main types of air barriers:
- As a secondary function, permeable barriers prevent the movement of vapor into a building.
- Impermeable air barriers block both air and moisture vapor.
Several of these products have been tested for NFPA 285-approved assemblies for exterior, non-load-bearing walls. NFPA 285-friendly barriers are designed to restrict the spread of fire across the building envelope. This extra fire protection typically comes at a higher price point than regular air barriers.
Vapor Barriers
Water molecules generally are more prevalent in hot air, but gravitate toward cold air. During that transition, condensation occurs and moisture can be transferred to the building – ultimately leading to rot and mold, along with cracked concrete.
In most U.S. climates, vapor barriers are applied on the warm side of the insulation to impede diffusion and prevent water molecule migration.
Another important distinction is vapor retarder vs. vapor barriers – they’re not the same product. While these two terms are used interchangeably, a vapor retarder simply reduces the rate at which vapor moves through the barrier (> 0.1 permeability, but ≤ 1 perm) . A vapor barrier has a rating of ≤ 0.1 perm.
Pro Tip: Vapor barriers do not interfere with air movement throughout the building. Therefore, vapor barriers are important in cold climates, as condensation can lead to damage. However, they’re also relevant in hot and humid climates, with different considerations for placement and permeability. The key is understanding your job site’s climate, the building's design, and how moisture interacts with the building envelope.
Weather Barriers
Think of a weather barrier as the building’s hooded rain jacket.
A more accurate expression of the purpose of these barriers is their alternative name – weather-resistant barriers (WRB). WRBs keep the external wall system dry and allow moisture vapor within the building to escape. This both protects the building’s outside and reduces the chances of water interfering with structural integrity.
There are two types of weather barriers that perform different functions:
- Permeable: Allows for drying potential in wall assembly
- Impermeable: A true air/vapor barrier, blocking airflow & moisture
Moisture Barrier
A fourth type of building envelope barrier!? Not really.
Construction crews throw out “moisture barrier” interchangeably with air and vapor barriers. Most often, they’re referring to a vapor barrier, or to moisture outside the building.
A “moisture barrier” still has the same job of protecting the building envelope.
Going Deeper: 3 Subtypes of Air Barriers
The catalog of weatherproofing products is as deep as the buildings are tall, and air barriers are no exception.
Fluid-applied vs. sheet air barriers are a big discussion point during the construction of a new building. These considerations influence not only the projected timeline for completion, but also the final budget of the build.
Liquid or Spray-Applied
Spray air barriers typically are easy to apply and conform to uneven substrates. However, consistency and accuracy are key in maintaining the spray thickness to ensure successful application. Weather also factors into the success of spray-applied barriers, as these need sufficient time to cure before exposure to rain or wind.
Liquid barriers can be more expensive, but they can be installed faster and are monolithic vs. having laps.
Sheet-Applied
Sheet-applied barriers have consistency baked in – they’re preformed to a variety of thicknesses.
However, these applications can be complex and labor-intensive.. Application of a sheet-applied barrier to uneven surfaces can be challenging. Also, a primer may be a necessary extra step
Primerless
A primerless barrier is generally made of silicone and can be put in place through a single-step application. These can be applied via spray or roll-on processes and involve a higher cost up-front.
[Note: GCP Applied Technologies offers self-adhesive applications that are easy to apply and increase energy efficiency for the structure.]
6 Considerations for Choosing Air vs. Vapor Barriers
Two main considerations influence the decision between weather, air, or vapor barriers:
- The type of construction
- The location of the insulation
However, construction envelope barrier types are aplenty, so your choice will depend on the answers to many questions about the project. Below are six of the most common:
- Climate – Placement and system complexity may depend on the level of cold or humidity in the environment. In a mixed climate, the best choice may be materials that act as both air and vapor barriers, but are carefully placed to handle seasonal conditions.
- Building design/materials – The success of barriers and even flashings can depend on the building materials’ permeability, adhesiveness, chemical compatibility, and thermal expansion.
- Moisture-management needs – How frequent is rainfall?
- Energy-efficiency goals – How much will air intrusion impact heating and cooling costs?
- Building regulations – Regulations often specify the minimum requirements for air and vapor barriers, with influence from the local climate and best practices.
- Durability – Buildings are sometimes constructed to allow water in behind the panel system, and the maintenance team opts to manage it from there. This is called a rain screen.
Final Thoughts: Air Barriers vs. Vapor Barriers
Air barriers are a near-universal need because of their ability to improve energy efficiency, air quality, and building durability. Air barriers are necessary to reduce the effect of wind and weather on the building envelope.
A vapor barrier’s value is more climate- and situation-specific, depending on factors like temperature differentials and humidity levels. Vapor barriers can reduce the risk of rot and mold due to condensation, and may even be a necessity, depending on the wall assembly’s design. If the insulation is outboard of the wall cavity, using an impermeable system will keep moisture out of the wall altogether. If the insulation is inboard or a combination of the two, a permeable system will preserve its drying potential.
Knowing these differences can help you streamline your building project and provide optimal energy efficiency for the completed structure.
Weather vs. Vapor Barriers | More to Learn
Air barriers vs. vapor barriers isn’t the only building product decision that impacts sustainability. If you’re looking for more information on this key topic, check out our blog library:
