Air Barriers Keep Good Air In, Bad Air Out
Understanding and controlling air flow in the building envelope will maximize energy efficiency.
Stanley D. Gatland II, CertainTeed Corp.
Unrestricted flow of air against or through a building can have an enormous impact on the building’s temperature and energy efficiency. In cold months, warm air leakage to the exterior, and the thrust of cold winds against the exterior surface of a building, can cause interior temperatures to drop, requiring extra work from the heating system and additional utility bills to keep the interior warm. The same is true with cool air leakage and warm air intrusion in summer months. Like heat flow, air flow has a strong impact on the building envelope.
In this third installment of Valley Forge, PA-based CertainTeed’s nine-article series on commercial building science, we’ll discuss how to control air leakage, air-pressure differential effects, air flow paths, air barrier systems, fenestration products, and compartmentalization. To learn how to correct the problems caused by air flow in buildings, we must first take a look at what it is and how it works.
The force behind air flow
Air flow occurs only when there is a difference between the exterior and interior of a building. Air will flow from a region of high pressure to one of low pressure-the bigger the difference, the faster the flow. Air-pressure differentials are thus the driving force behind air flow. There are three 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 has a significant effect on buildings, as it creates a high positive pressure on the upwind side of the building and a low negative pressure on the downwind side of the building-the taller the building, the higher the pressure. Wind also has a strong influence on the impact of rain on building surfaces. It is essential to combine exterior air barriers with water resistive barriers to prevent rainwater from penetrating the building envelope.
Stack-Pressure Effect: Stack pressure occurs when atmospheric pressure differences exist between the top and bottom of a building due to temperature differences. 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.
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 potential for air infiltration. At the veryleast, 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 types of air flow paths: direct flow, diffuse flow, and channel flow.
Direct Air Flow: Direct air flow can be thought of as a linear path through an assembly. So, for example, a gap under a sliding glass door or a gap that goes straight through the assembly for whatever reason is considered direct air flow.
Diffuse Air Flow: Diffuse air flow happens when air can move through what seems to be a homogeneous material, but is in reality, 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: The third type of air-flow path, 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 are connected, for example. Such spaces must be blocked.
To restrict these different types of air flow, it is important to employ an efficient air-barrier system. There are a variety of choices in this area.
Types of air barriers
Designing an airtight building envelope is crucial to a building’s performance. Airtight building envelopes help control heat and sound energy, as well as airborne moisture flow and airborne contaminants. They even help to control the spread of fire if cavities are properly blocked. In short, airtight building envelopes create more energy-efficient, healthy buildings, which are more durable and require less maintenance. The best way to make an airtight building envelope is by incorporating an air-barrier system.
Any type of sheathing material or continuous film or coating can function as an air barrier, provided it is unbroken and airtight. Some of the most common materials used as air barriers are:
- nylon film-also used as a smart vapor retarder
- polyethylene film-used mostly in colder climates
- building wrap
- roofing membrane-effective only if the seams and overlaps are sealed
- self-adhering asphalt roofing membrane
- built-up modified asphalt roof
- extruded polystyrene board
- aluminum foil-faced polyurethane.
A building material must meet a range of requirements before it can be approved as an air barrier. The most important requirement for air barriers is air impermeability, or not allowing any air to pass through them. They must also be continuous, as well as strong and durable, to stand the test of time and a variety of weather conditions. Air barriers installed on the exterior of buildings must be able to withstand UV light in addition to precipitation, freezing, and thawing.
For a list of building code-specific requirements for air barriers, refer to ASHRAE 90.1, which has code-specific requirements for the material alone, the material in an assembly, and for the whole building. Also, the Air Barrier Association of America (www.airbarrier.org) provides detailed information on the concept, design, and specification of air-barrier systems in building enclosures.
Air barriers are separated into four different categories: mechanically fastened materials, rigid sheathings, self-adhered or peel-and-stick membranes, and fluid or trowel-applied coatings.
Mechanically Fastened Materials
The most common mechanically fastened air barriers are exterior building wraps, often used in residential construction, but sometimes in commercial construction as well. For use in interiors, there are also polyethylene and nylon films, such as CertainTeed MemBrain. Air barriers must be continuous to be effective. All penetrations and terminations must be sealed completely. Installers must repair any rips and tears, and seams must be overlapped and sealed. Wherever there’s a supporting frame or substrate, the barrier must be properly attached. The goal is no air leaks whatsoever.
Rigid sheathing, such as gypsum board, can be used as an air barrier. Extruded polystyrene and faced polyurethane foam also fall into this category. 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 so there is not any moisture behind the assembly.
Another category of air-barrier material is self-adhered or peel-and-stick materials. These are heat- or pressure-applied membranes or films, which are often water-vapor impermeable. These must be exceptionally well installed, because, should water get behind them, the moisture will be trapped with no way to escape and can cause concealed damage over time. So, all penetrations must be thoroughly sealed. These films may not adhere properly unless the substrate is cleaned or primed, and if applied in cold weather, they may require a primer to adhere properly.
Air barriers may be asphalt- or polymer-based, including those that are trowel-applied or spray-applied. A good installation includes sealing all penetrations, including around brick ties. Cleaning or priming of the substrate may be required. The job also must be applied with care to avoid overspray and solvent vapor inhalation.
Each application requires a specific type of air barrier. The next step is determining the best way to use air barriers and other methods to control air flow in each portion of the building.
How to use air barriers
Roofing is also considered an air barrier, and this can include asphalt- or polymer-based roofs, typically membrane roofs and built-up roofs. 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.
Glass presents special challenges in creating air barriers. Many buildings have large expanses of glass: curtain walls, windows, and doors. Most of these products are steel or aluminum framed. Proper airtight installation is critical to the integrity of the building envelope and critical to the energy efficiency and comfort of the building occupants.
Airtight fenestration products are a must, and ASTM 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 sq. ft. 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 rates less than or equal to 0.4 cu. ft./min./sq. ft. of window area. Glazed doors should not exceed 1 cu.ft./min./sq.ft.
Installation is critical. All joints between the window and the rough opening must be thoroughly sealed. Flashing and sealing must be airtight and watertight. Windows should remain operable and be well maintained.
When your goal is to control air flow, you want to compartmentalize the building as much as possible. The purpose of compartmentalization is to isolate connecting spaces and minimize the impact of the stack effect. Disconnect building spaces between the foundation and occupied spaces above, and between the roof and occupied spaces below. Floors, rooms, and connecting corridors should be disconnected.
Isolate Continuous Vertical Paths
Continuous vertical paths, such as stairwells and utility shafts, need to be isolated. Airtight doors are an asset here and, once again, remember the importance of sealing all penetrations. If you have access panels to electrical boxes and telephone equipment, 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 typically located on the roof or a high point in the building, so it follows that airtight elevator doors are essential. Incidentally, sloppy elevator installations can pull airborne contaminants, along with stack-driven air, through 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 you’re 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, you should isolate the potential sources as the building is constructed. Garages are a major source of pollution, so prevent automobile emissions from entering the rest of the building. Isolate chemical storage areas and mechanical rooms. Other pollution sources include commercial kitchens, photocopy rooms, and lavatories.
Isolate Entry Lobbies
Entry lobbies should be isolated from the rest of the building because this is the place where doors open and close frequently-perhaps constantly. To minimize exterior air from entering, isolate lobbies with vestibules, use revolving doors whenever possible, and use automatic closures on conventional doors. If the lobby area has recessed lighting, it’s 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
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 sound control.
Install Sealed, Air-Distribution Systems
HVAC air-distribution systems should be well insulated and and made as airtight as possible. This is another key place where air sealing results in reducing room pressurization. When there are necessary functional penetrations, such as fresh-air intakes and exhaust hoods they should be equipped with airtight dampers to maximize air control through those types of systems.
Closing with continuity
The most important piece of advice is to remember that continuity counts. Effective air barriers require special attention at all penetrations. Areas of discontinuance in the building are where problems begin. These include roof decks and parapets, windows and doors, wall and floor intersections, expansion joints, wherever there are brick-ties, and at all faade supports.
Attention to detail is key to maximizing air control and minimizing related problems. Taking all of these guidelines into consideration should help create a more efficient, healthy building.
Stanley D. Gatland II is the manager of building science technology for CertainTeed Corp.’s Valley Forge, PA, Insulation Group. He is responsible for generating and providing technical information to architects, engineers, builders, trade contractors, building-envelope consultants, building scientists, and building- code officials on the system performance of new and existing building-envelope materials, as well as building science educational training. Stan has expertise in the areas of building science and architectural acoustics. He is a graduate of the Univ. of Massachusetts, Amherst, with BS and MS degrees in mechanical engineering. He is a member of ASHRAE, ASTM, ASME, and BETEC.