Why Should Air Barriers Be Incorporated into Post Frame Construction?
Energy efficiency of post-frame buildings has become a huge topic of discussion. Rather than trying to impress you, gentle reader, with my limited degree of knowledge, here are the words of an expert. The general public does not get to read Frame Building News. Here is an excerpt from the June 2018 edition written by Joseph Zulovich.
“The issue of energy conservation in buildings, including post frame buildings, will likely maintain importance and even increase in importance over time. Incorporation of a compliant air-barrier system is one way to be compliant with air infiltration code requirements.
Our understanding of the crucial importance and value of air barriers in a building system’s energy performance for all types of building construction is increasing. Those working in the residential home sector have been discussing air infiltration and air tightness for a number of years, and blower-door tests are commonly used to evaluate air infiltration in completed homes. At one session of the 2018 ASHRAE winter meeting, the importance of air barriers and their impact on forced-air heating and cooling systems in commercial multistory buildings were discussed. Air leakage or infiltration through the exterior building envelope causes inefficient operation of heating and cooling systems. In severe cases, excess air leakage or infiltration decreases the thermal comfort of the occupants. Those working on post-frame buildings can and should capture the knowledge about air barriers being gained in other building sectors.
The cost of installing a code-approved air barrier may be prohibitive if one considers only the energy efficiency for a given project. However, post-frame construction uses diaphragms as an integral part of the structural building system. Two code approved air barriers are plywood or OSB with a thickness of not less than 3/8 inch. Structural diaphragms are often constructed using plywood or OSB. Installing continuous structural diaphragms with sealed joints and seams can also provide for a code-approved air barrier. Although the expenses for the materials and labor associated with installing a sealed diaphragm do add to a building’s cost, the sealed diaphragm provides a code-approved air-barrier system without adversely affecting the structural integrity of the building. Another way to provide for a code-approved air barrier is to use sheet steel or aluminum. For some post-frame building projects, the interior cladding is either steel or aluminum sheeting. When the seams of these sheets are caulk-sealed during installation, the interior cladding creates a continuous air barrier. The edges of the interior metal cladding should be sealed to prevent indoor air washing from moving moisture-laden warm air from the inside space into the exterior wall cavities. Outside metal siding should not be sealed to create an air barrier on the outside surface of the exterior wall. Any moisture migration through the wall will collect on the inside surface of the siding if the siding is the best vapor retarder in the exterior wall envelope. Cavity ventilation or a drainage plane should be available to allow any moisture that migrates through the wall to dry. If wind washing is a concern, a building wrap should be installed before the siding is installed.
Overall, the IECC addresses air barriers only as a control for exfiltration and infiltration through an exterior building assembly. It does not address the other four types of airflow within an exterior building assembly. Wind washing and indoor air washing do not really contribute to air exchange– based energy impacts because air does not completely pass through the entire building envelope. However, as discussed, these types of airflow can adversely affect the longevity of the building assembly because of moisture problems. The potential for airflow through and within exterior building assemblies is minimized during the design phase.
Attic ventilation chutes and attic insulation stops at eaves help keep air movement away from problem areas. If wind driven air can get underneath ceiling insulation through an eave opening of a building, the ceiling insulation essentially gets wind washed, which negates its effectiveness. During cold weather, wind-washed ceiling insulation results in cold ceiling surfaces, where condensation can form on the inside surface of the ceiling and cause an increase in heat loss. During warm or hot weather, wind-washed ceiling insulation results in warm ceiling surfaces. If the interior space is not air conditioned, this wind washing may not be much of a problem. However, if the interior space is air conditioned, the outside surface of the ceiling is likely to be cold, and condensation can form on the outside surface of the ceiling material. Ceiling material that absorbs moisture will likely get moldy on the inside because of the moisture condensing on the outside surface of the ceiling. Attic insulation stops minimize the potential for wind washing of ceiling insulation. Attic ventilation chutes transfer air into the attic space and protect the insulation from wind currents.
As energy efficiency in buildings becomes more crucial, air barriers in post-frame construction will continue to gain importance. The design of post-frame construction often includes structural diaphragms. These structural diaphragms can provide part of an air-barrier system when they are sealed and connected with other parts of the air-barrier system.“
Joseph Zulovich, PhD PE, is extension agricultural engineer at the University of Missouri, Columbia, Missouri. He can be reached at zulovichj@missouri.edu.
