Definition

Air infiltration is most commonly used to refer to uncontrolled air leakage from a whole building. However, it can also refer to leakage through individual components or building assemblies such as doors, windows and skylights. In these cases, whole building air infiltration can be calculated by aggregating all the component air infiltrations, and then factoring in the additional infiltration through the joints and interfaces between them.

[INSERT IMAGE: 3D view of air leakage paths.]

Air infiltration takes place primarily through inadequate and imperfect sealing between building elements, the frames of openings and their host construction as well as between the different components of individual door and window assemblies. It can also result from ill-fitting and unsealed doors frames, window frames, outlets, walls, floors and roofs, as well as through exhaust appliance such as roof vents, bathroom fans, range hoods and dryer vents. Reduced air infiltration combined with proper ventilation can not only increase energy efficiency but also improve the quality of indoor air.

The term infiltration means air entering a building or space whilst exfiltration means air leaving a building or space. Most buildings maintain a relatively constant internal pressure, which means that instantaneous infiltration pretty well equals exfiltration, so infiltration is usually used as catch-all for both. An exception to this is some fully air-conditioned or mechanically ventilated buildings where an intentional choice is made to have the system maintain a constant positive internal pressure by pumping in more air than is extracted. This can be used to significantly reduce air infiltration and turn it to mostly exfiltration.

Factors Affecting Infiltration

Air infiltration is an important factor in a building’s heating and cooling costs. In winter, if a building is not well sealed, cold air seeps in from outside and replaces warm internal air that escapes through gaps and cracks, increasing overall heating loads. In the summer, warm outside air replaces cool inside air, this time increasing overall cooling loads.

Winds that blow on a building create positive pressure on the windward facades and negative pressure on the leeward facades. This causes infiltration in the areas of positive pressure and exfiltration in the areas of negative pressure. These wind effects are greatly influenced by the terrain surrounding the building as well as nearby trees, large areas of vegetation and other physical obstructions.

External temperature can also drive air infiltration, especially in multi-storey buildings. This occurs when there is a large variation in temperature between the inside and outside air, which occurs most often in very cold winters. In this case, warm inside air rises up and creates a positive pressure at the top of the building and a negative pressure at the bottom. This draws in cold outside air from near the ground as the warm inside air leaks out through gaps and cracks around the ceiling and roof.

Units

Air infiltration is measured as a volume flow rate as opposed to a mass flow rate. Thus the units are always volumetric, typically the volume of outside air replacing inside air over some specified period of time.

Air Flow Rate

Air infiltration is usually given as an absolute flow rate, being the total measured amount of air flowing into or out of a building over a set period of time. Any of the following units can be used.

  • SI: Litres per second (LPS) or Cubic meters per second (m³/s).
  • Imperial: Cubic feet per minute (CFM).

The important thing to note about absolute air flow rates is that values are linearly related to the overall size of a building and the number of external doors and windows it contains. Values for different buildings are not directly comparable without some form of relative volume compensation.

Air Exchange Rate

Another commonly used unit is an air exchange rate, often referred to as air changes per hour (ACH), being the number of interior volume air changes that occur per hour over the whole building. Thus a value of 2ACH would means that the entire volume of inside air within a building is replaced by outside air twice within the space of an hour.

Unlike absolute air flow rates, air exchange rates are directly comparable between buildings of different sizes. It is therefore possible to compare the air infiltration of a hospital with that of a house as the unit already compensates for relative volume.

Air exchange rates can be calculated directly from absolute flow rates if the interior volume of the building is known or can be calculated with reasonable accuracy. This is done using one of the following equations, selected based on the units of measured flow rate. The key to these equations is to ensure you give the volume (V) in the same units as the volume used to measure the air flow rate.

$$ \begin{align} ACH &= (LPS * 3600) / V(l) \\ &= ((m³/s) * 3600) / V(m³) \\ &= (CFM * 60) / V(ft³) \end{align} $$

Thus, if your flow a rate is given in litres per second (LPS), make sure V is given in litres. Similarly, with cubic meters per second (m³/s), make sure V is given in cubic meters (m³). Or, if you have cubic feet per minute (CFM), make sure V is given in cubic feet (ft³).

Measurement

Blower Door tests

Figure 2: A blower door fitted for testing.

The air infiltration rate of a building is typically determined using a blower door test. One of the doors of the building is replaced by the blower door (a door with a large fan and air flow meter installed within it) with all other external doors and windows shut tight, but all internal doors left open. The blower door fan is then used to pressurise the building, usually to 50Pa, and then the air flow through the fan is measured using the flow meter whilst keeping the pressure constant. This measurement yields the average absolute air flow rate.

Blower door tests only measure air infiltration under exaggerated conditions. They do not take into account varying wind effects, changes in atmospheric pressure or any activities by the occupants that might affect infiltration rates over time.

Tracer Gas Tests

Figure 3: Example tracer gas detection equipment, courtesy of Cincinnati Test Systems.

Another form of air infiltration test involves the use of tracer gasses. In these tests, an emitter gives off a constant amount of tracer gas, usually perfluorocarbon gas or sulfur hexafluoride which are both colourless, odourless and harmless, and a detector in another part of the building or space measures and records the concentrations of tracer gas it detects.

Variations in tracer gas concentration are directly related to air infiltration rates. Thus, by recording over an extended period of time and matching values against recorded changes in internal and external temperatures, atmospheric pressures and prevailing wind conditions, it is possible to develop a much more comprehensive understanding of variations in infiltration.

Example Values

The following tables give typical values ranges in both ACH and CFM.

AIR-TIGHTNESS RATE (ACH)
Good 1 to 5
Moderate 5 to 10
Leaky 10+
Table 1: Air Changes per Hour (ACH) at 50pa.
AIR-TIGHTNESS RATE (CFM)
Good 1500 or less
Moderate 1500 to 4000
Leaky 4000+
Table 2: Cubic Feet per Minute (CFM) at 50pa.

Useful References


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