An air barrier is any substance that prevents outside air from coming in, and inside air from moving out. In addition to causing drafts and lowering the comfort level of a building, air can carry moisture in the form of water vapor that can get inside of wall and roof assemblies and condense back into liquid form.
Air leaking into a building envelope can carry far more water vapor than could enter through diffusion. The driving force of air movement is differences in air pressure. Differences in pressure either cause air to move into a building from the outside (infiltration), or to move out of a building from the inside (exfiltration). Air pressure differences around building envelope assemblies are primarily caused by the stack effect, chimneys, wind, and forced hot-air heating systems.
The Stack Effect
The stack effect is caused when a building is heated. Warm air rises, creating a high-pressure area near the top floors of a building. The rising air in turn creates a low-pressure area in the basement and bottom floors of a building. The area midway between the two zones is called the neutral pressure zone. The high-pressure air above the neutral zone tries to escape from the building and move towards the lower- pressure area outside the building. Since the air in the basement and lower floors has lower pressure, the air from outside tries to enter the building through any available openings.
An active chimney acts as an exhaust fan and will suck air out of a building. This causes the neutral pressure zone to move up to the ceiling or roof area of a building, resulting in lower pressure throughout most of the house. The lower pressure inside can cause the infiltration of air from the outside.
Wind causes pressure differences around the wall assemblies of a building. Wind hitting a wall assembly can cause positive pressure on the side of impact, while creating a negative pressure on the opposite side of the building. Wind causing a positive pressure zone can lead to infiltration on the windward side of a building; the subsequent negative pressure zone can lead to exfiltration from the leeward side.
Forced Hot-Air Heating Systems
Forced hot-air heating systems generally have supply ducts that deliver heated air into a room and return ducts that supply air to the furnace to be heated. In many cases, the supply and return vents are not located in the same room. The location of the supply and return vents can cause pressure differences inside a building. Leaking ductwork is also a cause of pressure differences.
Richard T. Bynum writes in his book Insulation Handbook, “Infiltration can comprise almost 50% of all heat loss during the winter months.” Mr. Bynum then lists some of the areas in a house that are major sources of heat loss from air infiltration:
Other areas in a house that are sources of heat loss through infiltration are:
Where to Install the Air Barrier
The air barrier can be installed on either side of the wall assembly, where as the vapor retarder has to be installed on the side of the wall with the highest water vapor pressure (inside for a building in a heating climate). Because of the ease of installation, many builders choose to install the air barrier on the outside of the wall assembly. Many house wrap products are sold in nine-foot rolls, which makes it easier to apply an “unbroken” barrier around the wall assemblies with fewer seams to seal. House wrap manufacturers recommend that all seams and openings be sealed with a house wrap tape for their product to be effective as an air barrier. Without properly sealing the seams and openings, a house wrap looses its effectiveness as an air barrier. Unfortunately, house wraps are not properly sealed in many construction projects.
Foam is an Effective Air Barrier
Polyurethane foam insulation can be installed to form a continuous air barrier around the entire building envelope. Polyurethane foam is airtight and actually bonds to building materials; this creates an airtight seal that forms a continuous air barrier across the entire building assembly.
Some Air Barriers do not Stop Water Vapor Diffusion
Air barriers are designed to control the movement of air and bulk moisture like rainwater, but are not designed to stop the diffusion of water vapor. If water vapor does condense inside a building assembly, like a wall cavity, it will still be able to evaporate through the air barrier leaving the wall cavity dry. In most cases, a separate barrier is used to retard vapor so moisture doesn’t move into the wall cavity to begin with.
Polyurethane foam also performs as a vapor retarder in addition to an air barrier. The following chart is a comparison of different insulating material and their air, vapor and moisture retarding properties.
Foam is an Effective Air Sealant:
An air barrier is the material which prevents air movement through a building assembly. In order for this barrier to be effective, it must be continous without penetrations or openings. (There must be no openings in the barrier.) Air sealants are used to ensure a complete seal along seams, penetrations, and transition areas such as where the wall assembly meets the roof assembly. Foam is an excellent and necessary air sealant because:
Air leakage (infiltration, exfiltration) is the number one cause of poor building performance. Foam-insulated homes perform at levels much higher than conventional insulation without additional, complicated and labor-intensive air-sealing details.
Foam is airtight, it performs better in windy conditions, and resists R-value loss (drift).
Fiberglass batt insulation has virtually no air-sealing ability and has to rely on other components of a total thermal envelope system to maintain performance levels.
Air leakage at penetrations and around windows and doors creates an environment for condensation. This affects overall performance and can compromise indoor air quality (bugs, mold, and rot). Condensation can lead to premature structural failure in structural framing and sheathing materials.
Independent testing shows that the performance of foam-insulated buildings exceeds today’s energy standards by a factor of ten.
Bynum, Richard, 2001. Insulation Handbook, McGraw-Hill, New York, NY.
Lstiburek, Joseph and John Carmody, 1993. Moisture Control Handbook, Van Nostrand Reinhold, New York, NY.
Lstiburek, Joseph, 1998. Builders Guide: Cold Climates, Building Science Corporation, Westford, MA.