Air and Vapor Barriers
Understanding Air and Vapor Barriers
A properly installed air barrier system represents about 1% of the building initial costs but reduces the structure’s heating and cooling costs by as much as 36%. (Source: The National Institute of Science and Technology)When encountering an air and/or vapor barrier detail on your contract, take time to review how you can protect your productivity and margins by opting for the best system for your project. White Cap offers all components, including adhesives, flashings, transition membranes, sealants, and more, for a successful job.
To understand the best air barrier system for your building assembly, let’s dive into some basic information and options on this important construction detail.
An air barrier is placed on exterior exposures to create a seamless building envelope. The system eliminates any uncontrolled air leakage in and out of structures. Restricting air flow improves control of temperature, moisture, and humidity in commercial and residential structures. Air barriers also minimize moisture condensation in wall systems that can cause steel reinforcement corrosion, mold, and loss of insulation value.
What is a Vapor Barrier?
Vapor barriers, or vapor retarders, limit the amount of water vapor transmission through a material as a result of different vapor pressures. Thre are no vapor assemblies, only vapor barrier materials.
What is a Water Resistive Barrier?
As stated by the Air Barrier Association of America, the function of a water resistive barrier is to keep liquid water from entering the building enclosure. Combined with flashing and other materials, the water resistive barrier ensures that there is a shingled effect to direct liquid water to the exterior sheathing.
What is the difference?
According to Carlisle Coatings & Waterproofing, “the function of a vapor barrier is to retard the migration of water vapor, which is not typically intended to retard the migration of air. This is the function of air barriers. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material. Remember that air leakage – and not vapor diffusion – is the real concern because air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.
A single material can function as all three, an air, vapor and water resistive barrier. There are also materials that function only as a water resistive barrier that are not air barriers. Understanding the functions of the materials used is critical, so that the building performs in accordance to the project requirements.”
Kinds of air barriers
During the pre-job review, be sure to understand requirements with respect to the air barrier’s permeance specification. Determining whether you need to apply either a permeable or impermeable air barrier depends primarily on climate and the structure’s wall design.
Permeable or Impermeable?
Air barrier products are rated for their permeance, which is a laboratory measurement that indicates the rate of water vapor transmission through them. Monitoring the permeance requirement is key, as products are specified by permeance and not by millage (the material’s thickness). The less permeable a material, the greater the material’s resistance to water vapor transmission.
Engineers determine whether to use a vapor permeable or vapor impermeable air barrier—and how to use it on two key factors: the climate where the wall is located, and the wall design – focusing on the insulation layer location with respect to the building exterior.
Thus, there’s an important difference between air barriers and vapor barriers. When an air barrier material is rated with a permeance of greater than 10 perms, it is classified as permeable. At this rating, air barriers are designed to avoid trapping moisture within the wall assembly by simply restricting air movement.
When an air barrier is required to block moisture vapor, it is referred to as a vapor barrier. These materials are more restrictive and must have a permeance of less than 0.01 perms. At this rating, these materials are classified as impermeable.
If you opt for an air barrier that doubles as a vapor retarder and water resistive barrier, be sure to review the product specifications for compliance.
You have several options when selecting both permeable and impermeable air barriers.
1. Commercial Air Barriers
Commercial air barriers can be self-adhering sheet membranes, fluid-applied membranes, and building wraps. So, in your selection process, there are some key factors to consider. These factors include the surface texture of the material onto which the air barrier will be affixed, the building geometry, project conditions, weather conditions, project timeline, and past install experience.
There are other special considerations. If the barrier will be permanently exposed to some UV light, opt for products that provide resistance. If the air barrier system is to be installed inside an enclosed space, check to see if fire resistance is a concern. Some manufacturers offer an air barrier option with built-in mold resistance which helps to resist exponential proliferation of mold, mildew, and fungus.
- Liquid/Fluid Applied Air Barriers
The key is to apply the manufacturer’s recommend thickness on all parts of the exterior wall. Most fluid-applied membrane products are “thick-coat” products with a specified cured thickness of 40 mils. (For a practical reference, your credit card is about 30 mils in thickness.)
To stay on target, you should check the applied material’s thickness about every 500 square feet using a wet mil gauge. These devices are about the size of a business card and have a series of notches cut into their sides much like the teeth on a comb. To ensure consistency, place the end of the comb into the material to gauge thickness during the application.
- Self-Adhered, Sheet Applied Membranes
Self-adhered, sheet-applied membranes can also reduce the risk of applicator error. The sheets come with a manufacturer guarantee of a minimum thickness insuring proper permeance performance.
But there are some other aspects to consider. Self-adhered sheet applied membrane installation can be labor-intensive if the structure’s shape is complex – and effective application can be difficult on uneven substrates.
- Building Wraps
It’s important to pay close attention to their permeance rating. It’s critically important that joints, seams, and penetrations be carefully detailed to complete the building envelope.
2. Residential Air Barriers
Manufacturers have designed air barriers for the special conditions that exist in residential construction. These products are designed for frame buildings and are generally easy to install. Residential air barriers also offer homeowners a secure barrier against moisture, resulting in increased building longevity. You can easily install self-adhered weather resistive barriers with common hand tools. These products feature a peel-n-stick system, allowing you to cover structures efficiently with minimal labor costs. Many of these products also function as a water-resistant barrier and rain barrier. They can achieve 95% drainage efficiency and provide excellent moisture and water protection.
- Building Wraps
Residential building wraps are cost-effective for both the contractor and the owner. For you, the light-weight materials are easy to handle allowing your crews to quickly envelop the structure following framing. Opt for a building wrap that includes a non-woven fabric structure that is easy to work with. These qualities can help you speed up construction without affecting quality.
For the homeowner, building wraps restrict air flow and water intrusion. These qualities help keep homes cool in the summer, warm in the winter, and dry all year round.
Treating Penetrations
To create an effective air barrier, attention to the small details will distinguish a high-quality job. The greatest challenge is in properly closing and treating penetrations and openings.
Using the right type of flashing that matches the air barrier material is an important step. Look to White Cap for a wide array of products that aid you in both productivity and building performance when sealing these penetrations. These flashings include formed elements, sheets of all types of materials, and wet materials that match every substrate transition and exposure.
It’s also important to select high-quality adhesives, primers, and sealants that are matched to the air barrier’s physical qualities. Your White Cap expert can provide guidance for an efficient air barrier system.
