Air vs. Vapor Barriers

Air Barriers Versus Vapor Barriers

Building a tight, secure house is more than simply building to code. It’s a source of pride among construction professionals and a signature element of a quality build. You want the finished structure to be warm and dry in winter, cool and dry in summer, just right in the shifting seasons in between and able to withstand all of those challenges long term.

So, when you’re assembling building materials and components, the ultimate goal is to create—and keep— a neat envelope of protection shielding the home from weather, the elements and issues that might result if the envelope weren’t in place to do its job.

Key to that protection are air barriers and vapor barriers—two categories of building materials that can prove confusing as concepts because they often overlap, work together or—in some cases—do double duty. However, each has a very specific task to address:

  • Air barriers block unwanted air from traveling through open spaces like cavities, gaps or cracks.
  • Vapor barriers block the movement or unwanted transfer of moisture through solid surfaces or materials like wood, drywall or masonry, for example.


Something to keep in mind, however, is the U.S. Department of Energy’s often-cited observation that “air movement accounts for more than 98% of all water vapor movement in building cavities.” This is why stopping air is the first priority and why—when you can’t stop the infiltration of air, temperature shifts or resulting moisture—vapor barriers also become a critical tool of choice. It’s a matter of choosing the right barrier to counter the challenges of the job.

Air, Heat and Moisture Problems in Construction

When you build, your intent is to create a stable environment that is easily controlled, adjusted and maintained. Accounting for the natural behaviors of air, heat and moisture within a design is crucial to achieving that goal.

  • Air— Air moves quickly, constantly seeking an outlet that will release it from a high-pressure area to a lower pressure one. It can be as simple as air slipping through a gap in a door seal, for example, or an electrical outlet. However, air—still or moving—also carries with it the more complex elements of temperature and humidity.
  • Heat—The second law of thermodynamics governs heat transfer and the fact that warmth moves from hot objects to cooler ones until equilibrium is achieved—that is, both objects reach the same temperature. Highly conductive materials like steel, for example, allow this to happen more easily while materials with low conductivity—insulation, for example—slow or even stop the process.
  • Moisture—Moisture can form due to dewpoint and condensation. Depending on its temperature, air can hold varying amounts of water. Generally, the warmer the air, the more moisture it can hold. If warm, moist air is able to move to a cooler destination, it will release the moisture it can no longer hold as condensation. The temperature at which this release begins to occur is the dewpoint, often a variable that shifts with environmental conditions. What can make this especially challenging is that once moisture forms, vapor drive will cause it to spread from areas with high concentrations of moisture and saturate drier areas.


Weather, seasons, climate, geography and even altitude can directly impact the intensity of effects like air movement, heat transfer, and condensation and vapor drive. That’s why understanding the range of options available in building materials and matching a home plan’s design and barrier systems to the surrounding environmental conditions are so important.


Air Barriers

The purpose of an air barrier is to prevent air—along with the heat and moisture that it carries—from infiltrating or escaping from a building. While air barriers can consist of a single material, more often, they incorporate a whole assembly of materials that together create a protective envelope that keeps desired conditions within the envelope and undesired conditions outside the envelope—an air barrier system.

An air barrier system can be external or internal, and most homes have multiples thanks to accepted construction principles, but basic requirements are the same:

  • It should be impermeable to air flow.
  • It should be continuous, without gaps, cracks, open seams or joints, or other compromising points of ventilation.
  • It should be structurally sound and able to withstand forces that may act upon it—wind, for example, or weight loads.
  • It should last the expected lifetime of the building.


Many of the most commonly used building materials serve as effective parts of an air barrier system, securing every aspect of a home—from the roof system to the floor system as well as the above-grade and below-grade wall systems. Air barrier components can include everything from concrete, glass, plywood and OSB to drywall or gypsum board, rigid or spray foam insulation, polyethylene sheeting and rubber membranes as well as the gaskets, tapes, sealants and adhesives needed to make a barrier truly continuous.

The term continuous is important because it bars any interruptions or breaks—flaws that degrade air barrier performance, result in moisture damage, provide access to pests, and increase heating and cooling costs for a home. While flat, single-material surfaces are the most basic element of an air barrier, areas typically requiring extra materials to interface or overlap transitions, for example, include joints between different building materials, angled joints or corners, and plumbing or electrical penetrations.


Vapor Barriers

The purpose of a vapor barrier is to prevent moisture from directly contacting or moving through solid surfaces. They’re often installed in areas prone to condensation where an air barrier alone is not enough. The idea is to protect building materials from moisture that is likely to form. However, the use of a vapor barrier and its placement need to work with—rather than against—the home’s design and environmental conditions.

While a home may have a number of layered air barrier systems in a given area, vapor barriers work solo—only one impermeable layer protecting a particular space—and correct placement is crucial. Since condensation occurs when hot air cools, vapor barriers typically sit on the warm side of a wall—also referred to as the warm side of the insulation or frame wall. The idea is to stop the movement of moisture vapor before it can reach dewpoint, condense and diffuse into adjacent materials like insulation, for example, where it can result in mold and water damage.

Some of the areas of a home most in need of a vapor barrier are basements, foundations and crawlspaces as well as floors, walls and ceilings. Because these features are perimeter interfaces delineating inside and outside, climate is often used as a general guide for vapor barrier placement.

  • In warm climates, the vapor barrier is typically installed on the exterior side of the frame wall and insulation layer. With cool, air-conditioned interiors, vapor drive moves inward.
  • In cold climates, the vapor barrier is typically installed on the interior side of the frame wall and insulation layer. With heated interiors, vapor drive moves outward.


However, many variables come into play in modern homes designed for all seasons and weather extremes—including modern building advances in techniques and technologies.

The International Residential Code issues requirements according to climate zones broken into varying classifications. In addition, state and local building codes often give further guidance on not only whether a vapor barrier should be installed but also where it should be placed and what the required specs are for appropriate vapor barrier materials.

What can prove confusing is the seeming interchangeability of the terms vapor barrier and vapor diffusion retarder. Materials that qualify as vapor diffusion retarders are grouped into three classes that measure permeability to moisture in perms. Class I represents the vapor barrier category:

  • Class I—Actual vapor barriers have a permeability factor of 0.1 perm or less, featuring materials like glass, metal, unperforated aluminum foil, polyethylene sheets and rubber membranes.
  • Class II—With a range of 0.1 perm to 1.0 perm, the second class of vapor retarders includes materials like polystyrene, 30-pound asphalt-coated paper, plywood and bitumen-coated kraft paper.
  • Class III—This final class of vapor retarders has a broad range, from 1.0 perm to 10 perms, and includes many of the materials often used within an air barrier system—materials like gypsum board, insulation, lumber, concrete block, brick, 15-pound coated asphalt paper, latex or enamel paint, and house wrap.


Note that environmental conditions like humidity, for example, can affect some retarders, shifting them from one class to another. Especially interesting to note is the development of smart vapor diffusion retarders designed to limit condensation and vapor drive in cold months when heating is necessary as well as allow drying when air-conditioning and weather flip conditions.


Construction in Idaho’s Climate

Idaho may initially look like a simple cold-climate state on a zone map. However, its geography offers considerable variability in conditions. Most of the state has a dry, even high desert-like climate that extends down into northern Nevada, but northern climes are mountainous with a good bit of winter rain and snow from the Northern Rockies.

Matching building materials designed for optimal control of the movement of air, heat and moisture with environmental conditions to create a home that will withstand the tests of seasons, climate and time is actually a complex process. If you’re ready to plan, design or build a house that will look and feel like home, Franklin Building Supply is the perfect place to start. We have all the materials and expertise you need.