Vapor Retarder

With structural support in place and a barrier to protect the interior from the exterior, another moisture-related concern must be addressed within the wall cavity. The IBC and IRC recognize that water vapor or air-borne moisture is different from bulk water. Nonetheless, the potential for damage to a wood structure is just as significant from vapor condensing in an exterior wall and deteriorating construction materials through rot, rust, or mold. Therefore, the IBC and IRC require protection against condensation in the exterior wall assembly and go on to clearly require a specific solution to provide that protection: “Class I or II vapor retarders shall be provided on the interior side of frame walls in Climate Zones 5, 6, 7, 8, and Marine 4,” (IBC 1405.3, IRC R702.7). The climate zones referred to here can be identified by a map and listing in the International Energy Conservation Code (IECC). There are eight such climate zones identified, with zone 1 being the most southern and warmest ranging up to the most northern and coldest in zone 8. Although the determination for applicability of the vapor retarder is determined by the IECC Climate Zones, the requirement for its inclusion remains in the IBC/IRC.

The class of vapor retarder referred to is based on the same definitions found in both the IBC and the IRC (IBC 202, IRC R202). The IRC adds some specific language to deem acceptance of different materials as meeting the different class ratings (shown in parentheses after each class below per IRC R702.7.2).

• Vapor Retarder Class: A measure of a material or assembly’s ability to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method of ASTM E 96 as follows:
  • Class I: 0.1 perm or less. (Sheet polyethylene, unperforated aluminum foil)
  • Class II: 0.1 perm - ≤ 1.0 perm. (Kraft-faced fiberglass batts)
  • Class III: 1.0 perm - ≤ 10 perm. (Latex or enamel paint)

In the climate zones where the vapor retarder is required, it is called for on the “interior side of frame walls,” typically meaning on or behind the finished interior surface of the wall.

Air Barrier

A 2005 study sponsored by the National Institute of Standards and Technology (NIST) titled Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use (NISTIR 7238) identified the significance of controlling unwanted air infiltration in buildings. The study looked at the effects of air infiltration reductions in frame and other types of buildings in locations across different climate zones, including Bismarck, North Dakota; Minneapolis, Minnesota; St. Louis, Missouri; Phoenix, Arizona; and Miami, Florida. They summarized the results by saying: “The annual cost savings are largest in the heating dominated climates with potential gas savings of greater than 40 percent and electrical savings of greater than 25 percent.” Studies such as this have illustrated the significance of air infiltration on building energy usage. Therefore, since the direct impact is on energy use, it is the IECC, not the IBC or the IRC, that requires air barriers and air sealing in all residential and commercial buildings.

As with all significant items, the IECC offers some specific definitions that are in addition to those found in the IBC/IRC (IECC 202). • Infiltration: The uncontrolled inward air leakage into a building caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density or both. (Note that although not specifically defined by the code, the phenomenon of exfiltration is similar where an HVAC system can pressurize a building forcing the leakage of conditioned air outward from a building that is not appropriately sealed.)
• Air Barrier: Material(s) assembled and joined together to provide a barrier to air leakage through the building envelope. An air barrier may be a single material or a combination of materials.
• Continuous Air Barrier: A combination of materials and assemblies that restrict or prevent the passage of air through the building thermal envelope.

In a wood-framed wall with a multitude of individual materials and components, air infiltration can be difficult to control. The key to success lies first in the ability of a material to be considered a true air barrier. The IECC relies on ASTM E2178: Standard Test Method for Air Permeance of Building Materials as the basis for determination. This test establishes air permeance for a particular material, which the IECC will accept if such permeability is no greater than 0.004 cfm/ft2 (0.02 L/s • m2) under a pressure differential of 0.3 inches water gauge (w.g.) (75 Pa). In simpler terms, this threshold is based on the amount of air that penetrates through a sheet of gypsum wallboard. Hence, any individual material or combination of materials that can demonstrate this level of very limited permeance can qualify as an air-barrier material.

The second key to effectiveness of an air barrier is to look beyond the specific material and to its ability to be truly continuous. That means any joints, seams, penetrations, or other breaches of the barrier need to be addressed in some manner as part of a total system. The IECC points to specific tests in this case, namely ASTM E2357: Standard Test Method for Determining Air Leakage of Air Barrier Assemblies and ASTM E1677: Standard Specification for Air Barrier (AB) Material or System for Low-Rise Framed Building Walls. Using these tests, the IECC will consider the whole assembly to be compliant as a continuous air barrier if the average air leakage does not exceed 0.04 cfm/ft2 (0.2 L/s • m2) under a pressure differential of 0.3 inches of water gauge (w.g.) (75 Pa) is demonstrated. The difference between this testing for assembly at 4 hundredths of a cubic foot per minute and the testing just for a product at 4 thousandths of a cubic feet per minute might seem to be a big difference, but the net number is still very small. This current code-required level of air infiltration is dramatically less than found in typical wood-framed construction in the past. Its significance lies in the fact that all of the joints, seams, connections, etc. need to be included in the test to be sure that the total assembly does indeed perform as a complete, un-breached air barrier.

In the latest versions of the IECC, residential buildings are required to be tested for air sealing, while it is an option in commercial buildings to demonstrate compliance. While there are specific testing requirements and procedures, this process is commonly known as “blower door testing.” With all windows, doors, dampers, and other openings closed, the main entry door is fitted with fans that produce a code-defined difference in pressure between the inside and outside of the building. Once that pressure difference is attained, then the air leakage rate is measured and calculated using the appropriate testing equipment. If the building does not comply, then remediation measures must be undertaken until it does. Paying attention to the details of the air-barrier materials and assemblies in the first place, including penetrations and framing joints, will help assure the building complies when tested.

Just like the other aspects of building envelope that have been discussed above, a continuous air barrier is achieved through various products applied in a diversity of ways. House wrap materials have been popularly used because as a material they rate very well as an air barrier. Their weakness has been in securing it around penetrations and to the sheathing, as well as the sometimes inefficient ability to seal the joints and seams. In some cases, but not all, materials that qualify as a water-resistive barrier under the building codes may also qualify as an air barrier under the energy code. Such a material or system has the obvious benefit of meeting two different code requirements in one step, but there are certainly differences in the ways that is done. Stick-on or spray-on materials can fall into this category—as can properly detailed and installed sheet products—all with the same notable issues of field quality control, as noted earlier.

Photo used with permission of Huber Engineered Woods LLC

Some manufacturers require taped panel seams, which creates a rigid air barrier to reduce air leakage.

Taking things a step further, at least one manufacturer has made a structural sheathing product available that is manufactured with an integrated exterior barrier that qualifies as both a water-resistant barrier and an air barrier under the codes. Seams are addressed with specially formulated adhesive tape that covers and seals completely, when used in accordance with the manufacturer’s recommendations and instructions.