Infiltration is the unintended and uncontrolled air leaking into a building through intentional and unintentional openings. Due to mass balance, air is also leaving a building through intentional and unintentional openings (exfilftration).
The amount of infiltration, and where it’s entering, is dependent on: indoor-outdoor temperature difference; wind (speed and direction); the operation of mechanical ventilation devices such as supply and exhaust fans; the operation of natural ventilation devices such as windows and relief vents; the airtightness of the building envelope (which in turn depends on the design, construction materials, and construction quality); distribution of openings and cracks around the building envelope; and other building characteristics such as height, footprint, and orientation in relation to wind direction.
While infiltration has been used to fulfill outdoor air requirements in buildings in the past, it is not generally accepted as a “best practice” anymore. The U.S. Department of Energy (DOE) Roadmap for Emerging Technologies (DOE 2014) indicates that in 2010 infiltration was responsible for 2.26 and 1.29 quads of space heating energy in the residential and commercial sectors, respectively, and 0.59 of space cooling energy in the residential sector. In aggregate, infiltration accounted for greater energy consumption than any other component of the building envelope, including fenestration; 2035 projections show similar trends. This report, with a 20-year target, states that:
“Computational tools are critically important for the design and construction of commercial buildings with energy-efficient envelope materials. As new technologies are developed, models and simulation tools must be updated to account for increased performance and durability.” -DOE Roadmap for Emerging Technologies 2014
The most accurate, but time-intensive way, to determine infiltration is to measure it following ASTM E741-11 Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution. This requires that a tracer gas be used and that the concentration of the tracer gas be uniform in the zone. This means that the building must be configured so that it can be treated as a single zone, which may be difficult to do in larger buildings or buildings with many compartmentalized spaces. Further, determination of an infiltration rate during an E741-11 test is limited to the weather and building configuration on the test day.
Modeling infiltration has many advantages: it can overcome the practical challenges of following E741-11 for large or complex buildings; it can be used to predict infiltration in new (not yet constructed) buildings; and it can be used to determine whether airtightening upgrades make sense in an existing building.
Empirical models for infiltration are available (e.g., effective leakage area) but they were developed for low-rise residential and the relationships developed specific to the buildings in the study (Coblenz and Achenbach 1963; Sherman and Grimsrud 1980). Building energy simulation programs like EnergyPlus include these as options for modeling infiltration (ZoneInfiltration:DesignFlowRate and ZoneInfiltration:EffectiveLeakageArea objects respectively). However, there is no guidance on how to use these EnergyPlus objects for larger, complex buildings such as commercial buildings.
Since 2012, NIST has been developing models, datasets, and tools for estimating the effects of infiltration on heating, ventilating, and air-conditioning (HVAC) related energy use. You can find all of that research on CHIMP, the Clearing House for Infiltration Modeling Purposes.