Air leakage, or commonly known as infiltration, is the unintentional or accidental introduction of outside air into a building that affects both indoor air quality and building energy consumption. Buildings may conduct whole-building pressurization testing in accordance with ASTM E779 or ASTM E1827 to determine an air leakage rate, commonly calculated in cubic feet per minute per square feet (cfm/ft2). Actions taken to lower or limit the air leakage rate in the building thermal envelope is desired for energy efficiency and may even be required by code. For example, for the building envelope, ASHRAE Standard 90.1- 2022 uses an infiltration rate of 0.35 cfm/ft2 (under a pressure differential of 0.3 in of water or 75Pa), and the Passive House Standard has a value of 0.08 cfm/ft2.
Asset Score Report Recommendation: Seal building envelope reducing air infiltration
There are several areas for potential air leakage in a building, especially key transition or intersection areas including wall-to-roof, wall-to-foundation, and wall-to-fenestration junctions. Air leakage paths are typically driven by wind pressure, stack pressure (due to temperature differences) and mechanical building pressures. Due to the stack effect where warm air rises up in colder climates, the location between ceilings (unconditioned attics) and conditioned spaces, can become the largest area (square footage) of potential heat loss. Adding a continuous air barrier in such areas of a building’s envelope can greatly reduce building infiltration and exfiltration.
A continuous air barrier is defined as a combination of materials and assemblies that restrict or prevent the passage of air through the building thermal envelope. As outlined in the ASHRAE Advanced Energy Design Guide for K12 School Buildings, an air barrier system should have the following characteristics:
- It should be continuous, with all joints made airtight.
- Air barrier materials used in frame walls should have an air permeability not to exceed
- 0.004 cfm/ft2 under a pressure differential of 0.3 in. H2O (1.57 lb/ft2) when tested in accordance with ASTM E 2178.
- The system should be able to withstand positive and negative combined design wind, fan, and stack pressures on the envelope without damage or displacement and should transfer the load to the structure. It should not displace adjacent materials under full load.
- It should be durable or maintainable.
- The air barrier material of an envelope assembly should be joined in an airtight and flexible manner to the air barrier material of adjacent assemblies, allowing for the relative movement of these assemblies and components due to thermal and moisture variations, creep, and structural deflection.
- Connections should be made between the following:
- Foundation and walls
- Walls and windows or doors
- Different wall systems
- Wall and roof
- Wall and roof over unconditioned space
- Walls, floors, and roof across construction, control, and expansion joints
- Walls, floors, and roof to utility, pipe, and duct penetrations
- All penetrations of the air barrier system and paths of air infiltration/exfiltration should be made airtight.
Some materials serving as effective air barriers include drywall, sheathing, insulation, and house wrap.