Air & Vapor Barriers

Vapor Barrier vs. Air Barrier

In building science, both vapor barriers and air barriers are critical components for a building's envelope, but they serve different purposes. The key distinction lies in what they are designed to control.

An air barrier is a material or system of materials designed to stop the uncontrolled flow of air into and out of a building. Air movement is a primary cause of heat loss, as it can carry a significant amount of heat with it. Controlling this movement prevents drafts and improves a building's energy efficiency. Air barriers are not necessarily waterproof, but they are continuous and sealed to prevent air from leaking through gaps and cracks. Examples of air barriers include house wraps (like Tyvek), rigid foam insulation, and sealed drywall.

A vapor barrier (or vapor retarder) is a material used to restrict the movement of moisture in its gaseous state—water vapor—through building materials. This is important because as warm, moist air moves through a wall, it can reach a colder surface, causing the water vapor to condense back into liquid water. This condensation can lead to mold growth, rot, and degradation of building materials. The primary purpose of a vapor barrier is to prevent this condensation from occurring within the wall assembly. Materials like polyethylene plastic sheeting, foil-faced insulation, or certain paints can act as vapor barriers.

Understanding Water at the Molecular Level

The difference between how water behaves as a liquid and a vapor is fundamental to understanding these barriers. At the molecular level, water molecules (H2​O) are always in motion. The way they interact and "clump" together determines their phase.

In liquid water, molecules are relatively close to each other and are held together by strong attractions called hydrogen bonds. A drop of water contains an astonishing number of molecules—more than 1.5 sextillion. These bonds cause the molecules to stick together in large, constantly shifting clusters. These large clumps are too big to easily pass through small pores in materials like house wrap. This is why many building wraps can stop liquid water (rain) while still allowing water vapor to pass through.

In water vapor (gas), the molecules have much more kinetic energy and move too fast for hydrogen bonds to hold them together in large clusters. Instead, they exist as individual, very small molecules or in tiny, very temporary groups. Because of their small size, these individual molecules can easily pass through the microscopic gaps and pores in materials that would block much larger liquid water clumps.

This molecular difference is what allows some "smart" or "vapor-permeable" barriers to function. These materials are designed with a pore structure that is small enough to stop the large clusters of liquid water molecules (preventing rain from getting in) but large enough to allow the much smaller individual water vapor molecules to pass through and escape from inside the wall (allowing the wall to "breathe" and dry out). This is a major concept in building design, as it allows a wall to be both water-resistant and able to manage internal moisture.

Air and vapor barriers are installed in different locations within a building's envelope, and their placement is highly dependent on the local climate zone. Incorrect placement can trap moisture and lead to major building damage.

Air Barrier Placement

The primary goal of an air barrier is to stop air from moving in and out of the building. To be effective, the air barrier must be continuous around the entire thermal envelope of the structure, including the walls, floors, ceilings, and roof. It can be installed on the interior, exterior, or even in the middle of a wall assembly.

• Exterior Air Barriers: Often used in all climates, this is the most common approach for new construction. The air barrier, such as house wrap or rigid foam insulation, is applied on the outside of the sheathing, just under the exterior siding or brick veneer. This placement is effective at preventing outside air from moving into the wall cavity, a process known as "wind-washing."

• Interior Air Barriers: These are typically installed on the inside of the wall, behind the drywall. They are most effective in very cold climates where the goal is to prevent warm, moist indoor air from escaping and condensing in the wall cavity. However, they can sometimes make it harder for the wall to dry out if moisture does get in.

Many modern building designs use both an interior and exterior air barrier to get the benefits of both approaches.

Vapor Barrier Placement

The correct placement of a vapor barrier depends entirely on the climate. The goal is to place the barrier on the warm side of the wall assembly to stop moisture vapor from the warmer, more humid side of the wall from moving to the colder side, where it could condense.

• Cold Climates: In cold climates, the interior of the building is warm and humid during the winter. Therefore, the vapor barrier is installed on the interior side of the wall, between the drywall and the insulation. This prevents warm, moist indoor air from reaching the cold outer sheathing and condensing.

• Hot/Humid Climates: In these climates, the outdoor air is hot and humid during the summer. To prevent this external moisture from entering the wall cavity and condensing on the cooler, air-conditioned interior surface, the vapor barrier is installed on the exterior side of the wall.

• Mixed Climates: In climates with both hot, humid summers and cold winters, vapor barrier placement can be more complex. A "smart" vapor retarder—a material that can change its permeability depending on the humidity—is often used. It allows some vapor to pass through, so the wall can dry in both directions. In these cases, it's also common to use a vapor-permeable air barrier on the exterior.

Basements and crawl spaces require special attention. A vapor barrier, typically a thick polyethylene sheet, is installed on the ground or under the concrete slab to stop moisture from the soil from rising into the structure.

The Importance of a Continuous Air Barrier

For an air barrier to be truly effective, it must be continuous and sealed across the entire thermal envelope of a building. The thermal envelope is the boundary that separates the conditioned, interior space from the unconditioned, exterior environment. Think of it as the building's "skin." If there are any gaps, holes, or discontinuities in this skin, the air barrier fails at its primary job: stopping the movement of air.

Even a small gap can compromise the entire system. Imagine trying to keep water out of a boat with a single hole in the bottom. No matter how strong the rest of the hull is, that one hole will allow water to pour in. The same principle applies to air barriers. A small unsealed seam, a hole around a plumbing pipe, or a gap where the wall meets the floor is an open invitation for air to leak in or out.

This uncontrolled airflow, known as convection (the movement of heat through the circulation of a fluid like air), can dramatically reduce a building's energy efficiency. Warm air escaping in the winter and cool air escaping in the summer forces heating and cooling systems to work harder, increasing energy consumption and utility bills.

Key Areas for Continuity

A building scientist focuses on ensuring the air barrier is sealed in every critical area, including:

• Wall to Foundation: The joint where the wall framing meets the foundation or slab must be sealed.

• Wall to Roof: The transition between the wall and the roof or attic space is another common leakage point.

• Around Windows and Doors: These are notorious for air leaks. Proper sealing with tapes and sealants around the frames is essential.

• Penetrations: Any hole made for plumbing pipes, electrical wiring, or HVAC ducts must be meticulously sealed to maintain the integrity of the air barrier.

Modern building codes and best practices emphasize "airtight" construction, which means building a thermal envelope with very low air leakage. This is measured using a blower door test, where a powerful fan is used to depressurize the house. A gauge then measures how much air is leaking in, quantifying the effectiveness of the air barrier. A well-sealed home is more comfortable, healthier (by reducing the entry of pollutants and moisture), and significantly more energy-efficient.