Infiltration Equations & Conversions

A key factor in building heat gain and loss may be the infiltration rate, or the rate at which outdoor air is exchanged with conditioned interior air through the envelope. This spreadsheet tool outlines a set of simplified equations aimed at converting typical, real world infiltration measurements into metrics that can be input into EnergyPlus.

In using methods outlined in the document Infiltration Modeling Guidelines for Commercial Building Energy Analysis by the Pacific Northwest National Laboratory, we were able to convert common metrics of I75 and ACH50, into ones that could be conveniently input into an Energy Plus Model (Idesign and ACHnat).

NOTE: At this time, this calculation tool does not take into account infiltration from stack pressure, only horizontal wind pressure.

Key Definitions

  • ACH50-The number of complete air changes that occur within an hour when the building is pressurized at 50 Pascals.  This metric is usually used in residential infiltration measurement.
  • ACHnat-The number of natural air changes that occur with an hour when the building is naturally pressurized.
  • I75- The infiltration flow rate of air in cubic feet per minute per square foot of exterior exposed surface area when the building is pressurized at 75 Pascals. This metric is more commonly used in commercial infiltration measurement.
  • Idesign- The infiltration flow rate of air in cubic feet per minute per square foot of exterior exposed surface area when the building is naturally pressurized.

    • Spreadsheets

  • Spreadsheets 1 and 2 can be used to convert I75 into Idesign. Spreadsheet "1. I75 to Idesign Text," explains the method and equations for the conversion. "2. I75 to Idesign Calculations," is an interactive spreadsheet that takes your project's input and provides an output that can be used in EnergyPlus.
  • Spreadsheets 3 and 4 can be used to convert ACH50 into ACHnat. As in spreadsheets 1-2, "3. ACH50 to ACHnat Text," explains the method and equations for the conversion. "4. ACH50 to ACHnat Calculations," is an interactive spreadsheet that takes your project's input and provides an output that can be used in EnergyPlus.
  • Spreadsheets 5 and 6 are for comparing ACH50 into Idesign metrics. As in spreadsheets 1-4, "5. Compare ACH and I Text," explains the method and equations for the conversion. "6. Compare ACH to I Calculation," is an interactive spreadsheet that takes your project's input and provides an output of comparisons between the different metrics.
  • Spreadsheet 7 is a provides a reverse calculation. "7. Reverse Calcs" allows you to convert from an EnergyPlus input into I75.
  • Spreadsheet 8 is a reference tab. "8. Appendix" contains useful reference charts for spreadsheets 1-7.