By Donald R. Mettauer
Chief Estimator
Metropolitan Restoration and Waterproofing Corporation
70 Von Hillern Street
Boston, MA 02125

There has been much talk about Air Barriers lately and much confusion regarding them; what they are, how they are achieved and even, why bother? The answers to these questions are often not as simple as one might think, and most often, quite surprising.

The United States Department of Energy has concluded that as much as 40% of the energy needed to cool or heat a building is lost due to air leakage into and out of buildings. Structural damage due to moisture condensation in building walls has been documented. And there are increasing problems with mold growth in buildings, causing severe health risks.

These facts led to the State of Massachusetts incorporating requirements for Air Barrier systems into the Massachusetts State Energy Code for Commercial and High-Rise Residential New Construction (780 CMR 13) as of January 1, 2001. Paragraph 1304.3.1 Air Barriers states: "The building envelope shall be designed and constructed with a continuous air barrier to control air leakage into, or out of the conditioned space. An air barrier shall also be provided for interior partitions between conditioned space and space designed to maintain temperature or humidity levels which differ from those in the conditioned space."

To understand what constitutes an effective Air Barrier system, we need to understand how air and moisture are related to each other and how they move through a building. There are two sources of moisture in a building. One is in the form of liquid, such as roof leaks, plumbing failures, ground water, etc. The other is in the form of vapor: water molecules suspended in air. It is this latter form that we are concerned with when we design an air barrier system.

Water vapor is present in the air of any normal building, generated by people, plants, mechanical humidification and the like. This moisture moves through the building and the building materials, from one side of a material to the other, driven by the differences in the pressure caused by the air vapor itself. This difference in pressure will actually drive the vapor molecules between and through the molecules of the building materials. This process is called diffusion.

The movement of this vapor through the building envelope is controlled by what is referred to as a "vapor barrier" or, more correctly, "vapor diffusion retarder"; essentially a material placed in the building envelope that will limit the vapor diffusion so that moisture does not accumulate within the walls. These materials are typically applied as a membrane or coating, although other types of materials such as rigid insulation, or reinforced plastics are sometimes used. Probably the most commonly used material in cold climates is polyethylene sheeting. The location of the vapor retarder within the wall system is critical and should be placed so that the retarder material does not get so cool that the water vapor condenses into liquid water (the dew point), accumulating in the walls and causing damage to the building. Thus we can see the importance of an effective vapor diffusion retarder to control the movement of moisture through the building envelope.

However, the fact is, that almost 200 times more moisture will move through the building envelope due to air leakage than from diffusion, wasting energy for heating, cooling, and humidity control, causing structural damage, and possibly creating a "sick building".

The forces that drive air through a building are: Wind Effect, Stack Effect, Ventilation Effect, Interior Mechanical Pressurization and Air Leakage.

• Wind Effect is caused by the positive pressure of the wind pushing against the building on the windward side, while negative pressure is created at the roof by the wind passing over it, and negative pressure is also created on the leeward side of the building as the wind rushes by, thus literally driving air into the building. (See Illustration 1.)
  
  
• Stack Effect is the action of the heated air rising in a building, much like in a chimney, causing strong positive (outward) pressure in the upper stories and negative (inward) pressures at the base of the building. (See illustration 2.)
  
  
• Ventilation Effect is negative air pressure inside a building caused by the emission of air by ventilation fans without an adequate supply of replacement air, creating a vacuum like effect and thus drawing outside air into the building. (See Illustration 3.)
  
  
• Interior Mechanical Pressurization occurs in most buildings because HVAC systems are typically designed to create positive pressure inside a building, pushing outward against possible drafts and to reduce pollutants being drawn into the building from outside. This positive pressure drives the inside air out through the building.

• Air Leakage is the result of humid air moving through gaps and holes in the building envelope. Typical problem areas are around windows, electrical outlets and wall penetrations, but masonry materials are also likely to contribute to leaking. Concrete blocks and bricks are, by their very nature, quite porous, and mortar, because it shrinks as it dries, will contain small cracks that allow air to pass through.

All five of these factors can stress a building's H.V.A.C. system causing it to perform poorly, thereby wasting energy and increasing maintenance costs. However, other problems can also occur.

Efflorescence is a condition caused by the moisture in the air dissolving salts and other minerals contained in concrete and masonry as the moisture is driven through the material, thereby breaking down its chemical composition and leaving a white stain of mineral deposits on the exterior.

The first step, and of utmost importance, for creating an Air Barrier system is good architectural detailing. More often than not, conditions in specific areas of a building will be unique to themselves and will have to be addressed individually. Because of this a system will be comprised of several components or materials acting in conjunction with one another to form a continuous barrier. For example, in a given area such as at a window, there might be a self-adhering membrane on the outside face of the exterior wall sheathing (between the sheathing and rigid insulation board), window head and sill flashings plus foam sealant around the perimeter of the unit, all acting as the Air Barrier for that location. The exact location of these and how they interface with one another will depend on design factors such as the construction of the window unit and the wall system and the finish materials to be used. (See Illustration 4.)

The Air Barrier system must be continuous around the envelope of the building, blocking the migration of air while still being flexible enough to withstand the movement inherent in a building. It must be able to stand up to wind loads, wide variation of temperatures, shrinkage/expansion of the materials to which it is connected and be durable over time.

Areas of particular concern are:

1. The building envelope cavities. (See Illustration 5.)

3. Across roofs.

4. Intersections between walls and roofs. (See Illustration 6.)

6. Wall, roof and floor control and expansion joints.

7. Pipe and duct penetrations.

8. Access openings.

9. Stairwell and elevator lobby doors.

10. Vestibules that separate conditioned space from unconditioned space.

11. Louvers for machine rooms and mechanical systems.

12. Recessed light fixtures when installed in the building envelope.

Materials designed specifically for Air Barrier systems and multi-use materials that may be incorporated into a system are being developed at an extraordinary rate these days. New products are released practically on a monthly basis and it is a job in itself to keep up with new developments in the technology. Liquid membranes, sheet membranes, foams and rigid sheathings are available today that provide a broad spectrum to draw from in designing an effective Air Barrier system.

Of course even the best designed Air Barrier system utilizing the most advanced materials will not be effective if not installed properly. It is imperative that installers be trained in the installation of all materials utilized in a system and knowledgeable of all applicable manufacturer's specifications, as well as being familiar with the detail drawings. Supervisors need to fully understand not only the materials and details but have a working knowledge of Air Barrier systems theory, how to maintain quality control in the field and be able to document the work as it progresses. There may also be times when product manufacturer's representatives may be required on site to answer questions regarding the unforeseen problems that often arise in the course of a project, and so, Project Management will need to have this resource available.

So, an effective Air Barrier system is a result of an interconnected system of informed, detailed design, thorough specification and use of materials combined with expertise of installation.

The result of a well designed and properly installed Air Barrier system, over and above meeting the State Building Code requirements, is a building whose HVAC system performs efficiently, resulting in energy cost savings as well as maintenance cost savings, reduction of air pollutants, elimination of health hazards such as mold growth, and let us not forget, a more comfortable building to be in. These benefits, gained by incorporating Air Barrier systems in our buildings, far outweigh and, in fact, offset the initial cost of installing these systems.

It makes sense. And it is now the law.