Infiltration Rate Converter | University of Idaho Integrated Design Lab

Convert measured infiltration metrics (I₇₅, ACH₅₀) into the naturally–pressurized values EnergyPlus expects (I_design, ACH_nat), following the Pacific Northwest National Laboratory Infiltration Modeling Guidelines for Commercial Building Energy Analysis. Enter values in the input panel; outputs update live in the gold panel.

Blower-door / commercial measurement → EnergyPlus Flow/ExteriorArea

I_design = (α_bldg + 1) · I₇₅ · ( 0.5 · C_s · ρ · U_H² / 75 )ⁿ

Input units

I₇₅ — infiltration rate at 75 Pa (measured or typical, see Reference) m³/s·m²

Terrain factor α_bldg (building site — Chart 1)

Advanced parameters (PNNL defaults)

U_H — wind speed at building top m/s

C_s — avg. wind pressure coefficient (Swami & Chandra)

ρ — air density kg/m³

n — flow exponent

I_design — EnergyPlus Flow/ExteriorArea

– m³/s·m²

– cfm/ft²

EnergyPlus: in ZoneInfiltration:DesignFlowRate, set Design Flow Rate Calculation Method = Flow/ExteriorArea and enter this value in Flow per Exterior Surface Area (m³/s·m²). Then set the modifier coefficients to the DOE-2 set: A = 0, B = 0, C = 0.224, D = 0 — not the BLAST defaults. I_design is calibrated to the 4.47 m/s reference wind (0.224 × 4.47 ≈ 1), so EnergyPlus reproduces it at design wind and modulates correctly hour by hour. Complete the PNNL recommendation with the schedule fraction: 0.25 while the HVAC system runs, 1.0 when it is off.

Residential blower-door measurement → EnergyPlus AirChanges/Hour

ACH_nat = ACH₅₀ / LBL Factor

ACH₅₀ (air changes/hr at 50 Pa)

LBL climate zone (DOE map — Chart 2)

Number of stories

Shielding

LBL Factor (auto from table — override if known)

ACH_nat — natural air changes per hour

– ACH_nat

LBL Factor used: –

EnergyPlus: in ZoneInfiltration:DesignFlowRate, set Design Flow Rate Calculation Method = AirChanges/Hour and enter ACH_nat in Air Changes per Hour. Use constant coefficients here: A = 1, B = 0, C = 0, D = 0. The LBL factor already de-rates ACH₅₀ to an average natural rate, so applying the DOE-2 wind term on top would double-count and is not appropriate.

Bridge volumetric ↔ area metrics through building geometry (CFM)

ACH_nat → CFM = ACH·V/60 → I = CFM/A → I₇₅ = I_design / [(α+1)(0.5C_sρU_H²/75)ⁿ]

Starting metric

Input units

Infiltration — ACH₅₀

Above-grade exposed surface area ft²

Building volume ft³

LBL Factor

Terrain factor α_bldg

I_design — EnergyPlus Flow/ExteriorArea

– m³/s·m²

ACH_nat — EnergyPlus AirChanges/Hour

– ACH_nat

Equivalent I₇₅

– cfm/ft²

EnergyPlus: use the I_design output with Flow/ExteriorArea and DOE-2 coefficients (A=0, B=0, C=0.224, D=0); use the ACH_nat output with AirChanges/Hour and constant coefficients (A=1, B=0, C=0, D=0). Pick one path — the two describe the same leakage and should not both be added to a zone.

EnergyPlus model value → field-comparable I₇₅

I₇₅ = I_design / (α_bldg + 1) / ( 0.5 · C_s · ρ · U_H² / 75 )ⁿ

Output units

I_design (from EnergyPlus model) m³/s·m²

Terrain factor α_bldg

Advanced parameters (PNNL defaults)

U_H m/s

C_s

ρ kg/m³

I₇₅ — infiltration rate at 75 Pa

– m³/s·m²

–

Weather-correlated coefficients → two EnergyPlus infiltration objects

The NIST infiltration correlations (Ng et al. 2018) replace the single DOE-2 wind term with building-type and climate-specific A/B/D coefficients fitted to multizone (CONTAM) airflow simulations — capturing stack effect and HVAC operation. Pick a prototype building, an Idaho climate zone, and an airtightness level; the tool emits ready-to-paste ZoneInfiltration:DesignFlowRate objects with verified coefficients.

Prototype building type

Idaho climate zone (NIST representative city)

Envelope airtightness

Idaho note. CZ 6 uses Helena, MT — itself a dry-cold 6B city, so it is a close match for eastern Idaho. CZ 5 (Chicago) and CZ 7 (Duluth) are humid-climate reference cities; since infiltration is driven by wind and ΔT rather than humidity, they are reasonable for Idaho's 5B and zone 7, but the values carry their reference-city weather.

I_design @ 4 Pa — Flow per Exterior Surface Area

–

Reference data

Chart 1 — Terrain factor α_bldg (and boundary-layer thickness δ)

Category	Description	α	δ (m)
1	Large city centres (≥50% of buildings >21 m over ≥2000 m upwind)	0.33	460
2	Urban, suburban, wooded areas with closely-spaced obstructions	0.22	370
3	Open terrain with scattered obstacles generally <10 m high	0.14	270
4	Flat, unobstructed areas exposed to wind over open water (≤500 m inland)	0.10	210

Chart 2 — LBL Factor (ACH₅₀ → ACH_nat)

Columns are number of stories. Climate zone from the DOE Energy Star Home Sealing map.

Zone	Shielding	1	1.5	2	3
1	Well-shielded	18.6	16.7	14.9	13.0
	Normal	15.5	14.0	12.4	10.9
	Exposed	14.0	12.6	11.2	9.8
2	Well-shielded	22.2	20.0	17.8	15.5
	Normal	18.5	16.7	14.8	13.0
	Exposed	16.7	15.0	13.3	11.7
3	Well-shielded	25.8	23.2	20.6	18.1
	Normal	21.5	19.4	17.2	15.1
	Exposed	19.4	17.4	15.5	13.5
4	Well-shielded	29.4	26.5	23.5	20.6
	Normal	24.5	22.1	19.6	17.2
	Exposed	22.1	19.8	17.6	15.4

Typical infiltration values

Residential national building-stock average ≈ 1.8 I₇₅. For commercial/institutional ranges and additional residential ACH₅₀ by construction tightness, see Persily (1998) and the DOE Energy Star Home Sealing Specification (the original workbook's Charts 3–4).

Scope note. This conversion accounts for horizontal wind pressure only — it does not include stack (buoyancy) pressure. All calculations are performed internally in SI; outputs are reported in the EnergyPlus convention (m³/s·m² or AirChanges/Hour) with standard-unit equivalents shown alongside.

Method status. PNNL-18898 (the method used here) has not been revised since 2009 and remains appropriate for schematic-stage conversion from a single measured airtightness value. For detailed modeling, NIST Technical Note 1829 (2014) and its successor TN 2072 (2019) refine this approach using multizone (CONTAM) airflow simulations of the DOE prototype buildings, adding stack-pressure and HVAC system-operation effects this wind-only conversion omits. They use the same ZoneInfiltration:DesignFlowRate object and equation form, but with building-type- and location-specific coefficient sets rather than a single DOE-2 coefficient.

EnergyPlus modifier coefficients

EnergyPlus does not use the design value directly. Each timestep it computes: Infiltration = I_design · F_sched · [ A + B·|ΔT| + C·U + D·U² ], where U is the zone-height wind speed. Choose the coefficient set that matches the value you entered:

Set	A	B	C	Use for
DOE-2 (wind only) — PNNL recommended	0	0	0.224	I_design (Flow/ExteriorArea)
Constant	1	0	0	ACH_nat (AirChanges/Hour)
BLAST (E+ default — avoid here)	0.606	0.0364	0.1177	—

PNNL-18898's full recommendation also sets the schedule fraction F_sched to 0.25 when the HVAC system is on and 1.0 when it is off (mechanical pressurization suppresses infiltration during occupied hours). Apply it with a schedule on the infiltration object, or with two objects switched by the fan schedule.

Definitions

ACH₅₀: Complete air changes per hour with the building pressurized to 50 Pa — common in residential measurement.
ACH_nat: Air changes per hour under natural pressurization — an EnergyPlus infiltration input.
I₇₅: Infiltration flow rate per unit exposed surface area (cfm/ft² or m³/s·m²) at 75 Pa — common in commercial measurement.
I_design: Infiltration flow rate per unit exposed surface area under natural pressurization — the EnergyPlus Flow/ExteriorArea input.

Reference data

Chart 1 — Terrain factor αbldg (and boundary-layer thickness δ)

Chart 2 — LBL Factor (ACH50 → ACHnat)

Typical infiltration values

EnergyPlus modifier coefficients

Definitions

Chart 1 — Terrain factor α_bldg (and boundary-layer thickness δ)

Chart 2 — LBL Factor (ACH₅₀ → ACH_nat)