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Design Infiltration Tool Guide

What this tool does – [Tool]

This tool calculates the stack-induced, the wind-induced, and combined infiltration rates, as well as the resulting sensible heat loss or gain, forgiven indoor/outdoor temperatures and wind speed. The calculations use the same, powerful infiltration model, AIM-2, that was used to calculate the weather and shielding factors for the ASHRAE 62.2-2013 ventilation standard. The primary use cases include:

  • Determining the design infiltration rate that can be used in a heating or cooling load calculation for system sizing.
  • Investigating how infiltration rate depends on building leakage, building height, flues, leakage distribution, sheltering, terrain, and weather conditions.
  • See how the stack-induced and wind-induced infiltration rates combine in a sub-additive way because of their mutual interference. By sub-additive we mean that the combination is less than the sum of the two.

Calculated values

  • Stack-induced infiltration for given indoor and outdoor temperatures.
  • Wind-induced infiltration for given wind speed.
  • Design (combined) infiltration for given indoor/outdoor temperatures and wind speed.
  • Design infiltration sensible heat loss (heating) or heat gain (cooling) energy.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs).
  • This tool, based on the AIM-2 infiltration method, is not intended for buildings of more than three stories above grade, or for multi-unit buildings.
  • An important use for this tool is the determination of the design infiltration heating and cooling load for the purposes of sizing heating and cooling systems. To use it for this purpose, you must enter the design indoor and outdoor temperatures and wind speed at the location of the building. Please see the References section below for help finding the appropriate weather data.
  • Use the default percentages for leakage distribution unless you have reason to believe you have more accurate values. The default percentages are dependent on the building height and whether or not the building has a vented crawlspace.
  • Make sure you read the pop-up help for “Terrain category” and “Building shelter class” before making your selection from the drop-downs. Click/touch on the input label to read the pop-up help.
  • Regarding the “Building has open flue/chimney” section inputs:
    • If the building has flues and/or chimneys that are closed, such as fireplaces with tight-fitting dampers, do not include them in this section.
    • If there are multiple open flues, add their areas and average their heights.
  • If the “Design indoor temperature” is higher than the “Design outdoor temperature”, “Design infiltration heat loss” appears as the last result of the tool. On the other hand, if the “Design indoor temperature” is lower than the “Design outdoor temperature”, “Design infiltration heat gain” appears as the last result of the tool.
  • The stack-induced and wind-induced infiltration rates combine in a sub-additive way because of their mutual interference. By sub-additive we mean that the combination is less than the sum of the two.
  • When either the stack-induced or wind-induced infiltration rate is more than four times the other, you will find that the combined infiltration rate is actually less than the larger of the two. This is a consequence of the way the two modes of infiltration interfere with each other.


The Alberta Infiltration Model (AIM-2) applies to low-rise (up to three stories) detached, single-family, residential structures. The development of the empirical model began in the 1980s and was more finely developed over the next decade by David Wilson, Iain Walker, Larry Palmiter, Tami Bond, Max Sherman, and others.

For the mathematical model used for this RED Calc Free tool, “over 3400 hours of measured ventilation rates from the test houses at the Alberta Home Heating Research Facility were used to validate the predictions of infiltration rates and to compare the AIM-2 predictions to those of other ventilation models. The AIM-2 model had bias and scatter errors of less than 15% for wind-dominated ventilation, and less than 7% for buoyancy (“stack-effect”) dominated cases.”


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