Air Barriers and the Role of Self-Adhered Materials

Experts state that while heating and cooling account for 50 percent to 70 percent of the total energy used in the average home, air leaks can account for nearly half of those costs. Since heating and cooling accounts for the majority of utility costs in a typical home, stopping energy loss from air leakage offers the best opportunity for saving money and conserving resources. According to the U. S. Department of Energy, the recommended strategy in both new and old homes is to reduce air leakage as much as possible and to provide controlled ventilation as needed.

Air leakage is defined as the uncontrollable flow of air in and out of a building. It diminishes the insulating properties of a structure, and requires more energy to heat and cool a leaky building. There are two types of air leakage. Infiltration is the uncontrollable flow of outside air to the interior of the building. Exfiltration is the uncontrollable flow of inside air to the exterior of the building. In buildings, infiltration and exfiltration of air can have important implications, largely due to the unpredictable and unregulated nature of the air exchange and the possibility of admitting pollutants, allergens, and bacteria into buildings. In addition, the associated air pressure changes can impact the pressure equilibrium created by HVAC systems and work to move pollutants from certain places, say storage areas or garages, to the living space.

Exfiltration can cause moisture during the winter months when trying to heat a structure; this is seen predominantly in the northern climates. Infiltration can cause moisture during the warmer months as the hot, humid air is trying to get into the structure; this happens predominantly in the southern climates. As compared with diffusion, a type of passive transport in which molecules move from a higher to a lower concentration, the differentials in air pressure may act to move significantly more water vapor through building envelope leaks. Some experts say that hundreds of times more water vapor is moved by air pressure differentials than diffusion.¹

The types of air pressure that cause infiltration and exfiltration include wind, stack pressure, and HVAC fan pressure.

Wind. Wind pressurizes a building positively on windward side, and accelerates as it goes around the building, creating negative pressure on the sides, especially the corners. Negative pressure is created on the leeward side as wind moves past the building.

Stack pressure. Also called “buoyancy” or “chimney effect,” stack pressure is the movement of air in and out of a building caused by differences in exterior and interior air pressures resulting from temperature and moisture differences. The severity of stack pressure is dependent on height of the building and thermal difference.

HVAC fan pressure. Caused by HVAC system pressurization, fan pressure is usually positive and acceptable in warm climates, though in colder environments can cause envelope problems.

Air tightening is necessary to control air infiltration and exfiltration and is achieved most efficiently through the use of an air barrier system. As a combination of materials, flexible sealed joints and building envelope elements that create air-tight, effective air barrier systems have several things in common: They are continuous, durable, strong, impermeable, and stiff. Increasingly, air barrier systems are being required by code. There are several important reasons for this. Thermal performance is improved, and utility bills are lowered. Owners have better control of indoor conditions and better indoor air quality. Perhaps most importantly, moisture is prevented from entering the wall cavity as water vapor.

Air Barrier Systems—Keeping Up to Code

Energy codes are changing faster than ever so understanding code requirements is critical when building. Various codes have different requirements, but the intent to include an air barrier system is clear. To attain an ENERGY STAR certification on a new home, builders must prove that complete air barrier systems are in place throughout the house. ENERGY STAR requires that all openings in the building envelope be sealed, including joints, seams, and penetrations; openings between window and door assemblies and framing; walls and ceilings separating a garage from conditioned spaces; common walls between units; and attic access openings.

In addition, the current International Residential Codes (IRC) includes the International Energy Conservation Code requirement of an air barrier to prevent air leakage through the building envelope (IECC Section 402.4). Compared to 2009 when seven air changes per hour (ACH) were acceptable in any climate, in 2012, this number must be decreased to 3 or 5 air changes per hour, depending on the climate zone. Generally speaking, the lower the ACH, the lower the energy bill. A “blower door test,” which measures ACH, will also become mandatory to verify code compliance. A blower door is a diagnostic tool designed to measure the air tightness of buildings, and to help locate air leakage sites. Blower door testing is typically done as part of a whole home energy audit, and can usually be modeled to help determine how much energy is needed for heating or cooling purposes.

A blower door test measuring air changes per hour may soon become mandatory. Photo courtesy of Huber Engineered Woods