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Schematic-design web tool

Infiltration Rate Converter

Convert real-world envelope airtightness measurements into the inputs EnergyPlus actually uses — right in the browser, no spreadsheet download. The successor to the legacy I75 / ACH50 conversion workbooks.

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The full tool is embedded below. For more room, use Launch the tool above to open it in its own tab.

If the embedded tool does not appear, open it directly: infiltration-converter.html.

What it does

A key driver of building heat gain and loss is the infiltration rate — the uncontrolled exchange of conditioned indoor air with outdoor air through the envelope. Field metrics such as I75 and ACH50 are measured at test pressures and cannot be entered into an energy model as-is. This tool bridges that gap with five modes:

I75 → IdesignCommercial leakage at 75 Pa to the EnergyPlus design flow per exterior area, with terrain adjustment.
ACH50 → ACHnatBlower-door air changes to a natural air-change rate via the LBL n-factor (climate, shielding, stories).
Compare ACH ↔ ICross-check the two pathways through building geometry so the numbers agree.
ReverseWork back from an EnergyPlus Idesign value to the equivalent field I75.
NIST coefficientsBuilding-type and climate-specific A/B/D coefficients that emit ready-to-paste EnergyPlus objects capturing stack effect and HVAC operation.

Key definitions

ACH50

Complete air changes per hour with the building pressurized to 50 Pascals — the blower-door metric common in residential airtightness testing.

ACHnat

The natural (untested-pressure) air-change rate. Entered in EnergyPlus with the Air Changes/Hour method.

I75

Infiltration flow per square foot of exterior exposed surface area at 75 Pascals — the metric more commonly used in commercial work.

Idesign

The EnergyPlus design infiltration flow per exterior surface area at design conditions, entered with the Flow/ExteriorArea method. EnergyPlus then modulates it every timestep by a coefficient equation in wind speed and indoor–outdoor temperature difference.

Method & data sources

The I75 / ACH conversions follow PNNL's Infiltration Modeling Guidelines for Commercial Building Energy Analysis (PNNL-18898) — the DOE-2 linear wind-velocity coefficient (C = 0.224, referenced to a 10 mph design wind), paired with PNNL's full recommendation of an infiltration schedule fraction of 0.25 while the HVAC system runs and 1.0 when it is off.

The NIST coefficients mode uses the weather-correlated EnergyPlus inputs of Ng et al. (2018), with coefficient values drawn from the published NIST dataset (DOI 10.18434/mds2-2598). For each of eleven prototype building types, climate zone, and airtightness level, it returns the A/B/D coefficients — fitted to multizone (CONTAM) airflow simulations — as two ready-to-paste ZoneInfiltration:DesignFlowRate objects (system-on and system-off). Idaho's zones map to the NIST representative cities: 5B → Chicago, 6B → Helena, zone 7 → Duluth.

On stack pressure. The I75 and ACH conversions account for horizontal wind pressure only. The new NIST coefficients mode additionally captures stack (buoyancy) effect through its temperature-difference term, so it reflects both drivers of infiltration.

University of Idaho Integrated Design Lab · Elevating occupant comfort, health, and affordability. Intended for early-stage schematic design analysis, prior to detailed energy simulation.

LEGACY_Infiltration Equations & Conversions
IC_Tech_Memo