An overview on how air flow works, how to understand it and control it.

Any type of sheathing material or continuous film or coating can function as an air barrier, as long as it is unbroken and airtight. Building wraps are often used for this purpose.

Restricting the flow of air against or through a building is an important step to ensure better energy efficiency and occupant comfort, especially in the coldest and warmest months of the year. For instance, in winter, the leakage of warm air to the exterior and thrust of cold winds against the exterior cladding of a building can decrease indoor temperatures. This requires the heating system to work harder to keep the building warm, which increases utility costs. A similar thing happens in the summer, when cool air leakage and warm air intrusion puts extra strain on the cooling system.

Air flow occurs only when there is a pressure difference between the exterior of a building and the interior, moving from a region of high pressure to one of low pressure. The air flows faster as pressure differences increase. This article will give a concise overview of air flow and how it works, so it is easier to understand how to control it. We will begin with a closer look at air pressure differentials.

Figure 1: Shown above are the three types of air pressure differentials: wind pressure caused by external forces, stack pressure created by warm air rising and mechanical pressure created by a building’s mechanical systems.

Air Pressure Differentials

As demonstrated in Figure 1, there are three different types of air pressure differentials: wind pressure caused by external forces, stack pressure created by warm air rising and mechanical pressure created by a building’s mechanical systems.

Wind Pressure Effect

Wind pressure creates a high positive pressure on the upwind side of the building and a low negative pressure on the downwind side of the building. Wind also intensifies the impact of rain on building surfaces. To combat wind pressure and prevent wind-driven rain from penetrating the building envelope, it is essential to combine exterior air barriers with water-resistive barriers.

Stack Pressure Effect

Stack pressure occurs when atmospheric pressure differences exist between the top and bottom of a building because of differences in temperature. The stack effect causes infiltration at the bottom and exfiltration at the top of buildings during the heating season. In warm southern climates, the stack effect is lessened due to the short heating season. Figure 2 shows a good example of the results of stack pressure effect.

Mechanical Effect

The mechanical effect is caused by the HVAC system pressurizing the building. Many designers create systems with a slight positive pressure in the building to reduce the chances of air infiltration through the mechanical effect. At the very least, they try to create a neutral pressure to avoid constant air infiltration.

The next factor to consider is how air flows and what course it takes. There are three air flow paths: direct air flow, diffuse air flow and channel air flow.

Direct air flow is somewhat of a linear path through an assembly. For example, a gap under a sliding glass door or a gap that goes straight through the assembly is considered direct air flow.

Diffuse air flow happens when air moves through what seems to be a homogeneous material, but is actually porous. Concrete block with mortar joints can support diffuse air flow two ways: through the block and through cracks that form in the mortar joints.

Channel air flow is an indirect path between openings in the building envelope. These openings are often a space hidden from view, where a wall and the roof deck interface. It is important to block these spaces.

To control these different types of air flow, ensure that the building envelope is airtight. Airtight building envelopes are key in a building’s overall performance, as they help control heat and sound energy, and airborne moisture flow and contaminants. They even help impede the spread of fire if cavities are properly blocked. In short, airtight building envelopes create more sustainable, energy-efficient buildings. The best way to make an airtight building envelope is by installing an air barrier system.

Figure 2: Stack pressure occurs when atmospheric pressure differences exist between the top and bottom of a building because of differences in temperature. This can cause cracks in the cladding and the entrance of air at the bottom and exit of air at the top of buildings during the heating season.

Types of Air Barriers

Any type of sheathing material or continuous film or coating can function as an air barrier, provided it is unbroken and airtight. Here is a list of some of the most common materials used as air barriers:

• Nylon film
• Polyethylene film
• Building wrap
• Roofing membrane
• Self-adhering asphalt roofing membrane
• Built-up modified asphalt roof
• Plywood
• Extruded polystyrene board
• Aluminum foil-faced polyurethane

A building material must meet several requirements before it can be approved as an air barrier. The most important requirement for air barriers is air impermeability, meaning that they should not allow any air to pass through them. Air barriers must also be durable and continuous to be effective, with all penetrations and terminations sealed completely. Installers must repair any rips and tears, and seams must be overlapped and sealed. Plus, wherever there’s a supporting frame or substrate, the barrier must be properly attached. The goal is to have absolutely no air leaks. Air barriers installed on the exterior of buildings must be able to tolerate ultraviolet light, precipitation, freezing and thawing.

ASHRAE Standard 90.1 provides a list of building code-specific requirements for the air barrier material alone, the material in an assembly and for the whole building. The Air Barrier Association of America (, which provides detailed information on the concept, design and specification of air barrier systems in building enclosures, is another good resource.

Air barrier materials are separated into four different categories: mechanically fastened materials, rigid sheathings, self-adhered or peel-and-stick membranes, and fluid coatings able to function as air barriers.

Mechanically Fastened Materials

Exterior building wraps, often used in residential construction and sometimes in commercial construction, are the most common mechanically fastened air barriers. There are also polyethylene and nylon films, such as CertainTeed MemBrain, for use as interior air barriers.

Rigid Sheathing

Rigid sheathing, including gypsum, extruded polystyrene and faced polyurethane foam boards, can be used as an exterior air barrier. These materials should be thoroughly sealed and the seams or butt joints must be airtight, covered with durable sealants, specialized tapes or membranes. All penetrations must be sealed. Rigid sheathings must be properly integrated with the water-resistive layer to prevent moisture accumulation behind the assembly.

Self-Adhered/Peel-and-Stick Membranes

Self-adhered, or peel-and-stick, materials are heat- or pressure-applied membranes or films which are often also impermeable to water vapor. These must be installed with extra care to prevent the trapping of water behind them, which can cause concealed damage and mold growth over time. So, seal all penetrations thoroughly. These films may not adhere properly unless the substrate is cleaned or primed; if they’re applied in cold weather, they may require a primer for them to adhere properly.

It’s important to isolate and seal off return air plenums from occupied spaces when designing an air barrier system.

Fluid Coatings

Air barriers may also be asphalt- or polymer-based fluid coatings, usually trowel-applied or spray-applied. A good installation includes sealing all penetrations, such as those around brick ties. Cleaning or priming of the substrate may be required. And, the job must be applied with care to avoid overspray and inhaling solvent vapor.

Each application requires a specific type of air barrier. The next step is determining the best way to use air barriers and other techniques to control air flow in each portion of the building.

How To Properly Use Air Barriers Roofing Applications

Roofing is also considered an air barrier if it employs asphalt- or polymer-based membranes in membrane and built-up roof applications. Membrane roofs require ballast if the material is not adhered to the substrate. All penetrations through the roof must be flashed and sealed carefully, with special attention to detail at the critical roof and wall interface.

Fenestration Products

Fenestration refers to any opening in a building envelope, including windows, doors, curtain walls and skylights. Most of these products are steel- or aluminum-framed and feature glass. Proper airtight installation is critical to the integrity of the building envelope and to the energy efficiency and comfort of the building occupants.

Airtight fenestration products are a must and ASTM Standard E 283 is a guide to selecting high-quality fenestration products. A unit’s air leakage is expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the air leakage rating, the less air will pass through the seams and joints in the assembly. Building codes require window assemblies to have ratings of less than or equal to 0.4 cubic feet per minute per square foot of window area. Glazed doors should not exceed 1 cubic foot per minute per square foot.

Proper installation is critical. All joints between the window and the rough opening must be thoroughly sealed. Flashing and sealing must be airtight and watertight, and windows should remain operable and well maintained.


The purpose of compartmentalization is to isolate connecting spaces and minimize the impact of the stack effect. It is important to disconnect building spaces between the foundation and occupied spaces above and between the roof and occupied spaces below. Installers should disconnect floors, rooms and connecting corridors. It is important to compartmentalize as much as possible when trying to control air flow.

Isolate Continuous Vertical Paths

Continuous vertical paths, such as stairwells and utility shafts, need to be isolated. Airtight doors are helpful here, and again, it is key to seal all penetrations. If there are access panels to electrical boxes and telephone equipment on the project, install airtight covers.

Isolate Elevator Lobbies

Elevators move lots of air, so it’s important to isolate elevator lobbies from elevator shafts. They often run from the ground to the roof, and the mechanical rooms are usually located on the roof or a high point in the building, so it makes sense for airtight elevator doors to be essential. Incidentally, sloppy elevator installations can pull airborne contaminants, along with stack-driven air, throughout the building. Parking garages are one such source of contaminants. A good design recommendation is to separate the elevator lobby from adjacent spaces with an airtight doorway.

Isolate Hidden Plenums

Hidden plenums should always be isolated. If plenums are concealed behind a suspended ceiling, remember that these ceilings are not considered airtight. So, disregard suspended ceilings when designing air barriers, and remember to isolate and seal off return air plenums from occupied spaces.

Isolate Pollution Sources

Since the stack effect and the mechanical effect pressures can transfer contaminants throughout buildings, it’s a good idea to isolate these potential pollution sources as the building is constructed. Isolate chemical storage areas and mechanical rooms, as well as garages. Other pollution sources include commercial kitchens, photocopy rooms and lavatories.

CertainTeed’s MemBrain is a breathable vapor retarder product that also serves as an interior air barrier. It changes its permeability seasonally to prevent the flow of humid exterior air into homes in the summer and aid the evacuation of moist interior air from homes in the winter.

Isolate Entry Lobbies

Entry lobbies should be isolated from the rest of the building because they have big doors that open and close frequently-perhaps constantly. Isolate lobbies with vestibules, use revolving doors whenever possible and use automatic closures on conventional doors to minimize exterior air from entering. If the lobby area has recessed lighting, it’s important to air seal it, lest you encourage unwanted air and moisture to get into plenums and other spaces.

Use Airtight I.C. Rated Recessed Lighting

If a recessed light fixture is intended for direct contact with insulation, it will require an Insulated Contact rating. An IC-rated fixture must, by definition, “be approved for zero clearance insulation cover by an Occupational Safety and Health Administration Nationally Recognized Testing Laboratory,” such as Underwriters Laboratory. It is important to use IC-rated, airtight recessed lighting. The interface between the ceiling and the light should be sealed using an airtight gasket or adhesive sealant.

Use Fire-Rated Sealants

A thorough sealing job is necessary around plumbing penetrations between floors. Sealants and caulks should be fire-rated for the application and often the sealants must be certified for code approval. Check the requirements to be safe.

Seal Vertical and Horizontal Paths

In order to minimize room pressurization, make sure that vertical and horizontal paths are sealed. Again, the idea is to compartmentalize as much as possible. Another benefit of air sealing here is you’ll also be contributing to sound control.

Install Sealed Air Distribution Systems

HVAC air distribution systems should be both well insulated and airtight. This is another key place where air sealing results in reducing room pressurization. And, when there are necessary functional penetrations, such as fresh air intakes and exhaust hoods, they should have airtight dampers to maximize air control throughout.

Closing with Continuity

With air barrier systems, always remember that continuity counts. We cannot stress this enough. Effective air barriers require special attention at every penetration. Areas of discontinuance in the building are the source of many air leakage-related problems. These include roof decks and parapets, windows and doors, wall and floor intersections, at expansion joints, brick-ties, and at all façade supports.

These are just a few of the areas to concentrate on, stressing the fact that attention to detail across the board is the key to maximizing air control and minimizing related problems. Studying and practicing all of these guidelines should help create a more efficient, healthy and sustainable building.