BPI IDL cheat sheet

Pressure is the driving force for leakage: airflow through an opening scales with the pressure difference across it, so test results are only comparable at the stated reference pressure.

CFM50 is the blower-door airflow needed to hold the house at 50 Pa; CFM25 is the duct-leakage reference pressure used for most duct tests.

ACH50 = CFM50 × 60 ÷ conditioned volume in cubic feet; ACHnat is estimated by dividing ACH50 by a climate-dependent n-factor, typically 10-20.

CFM50 per square foot of conditioned floor area is an alternative tightness metric; lower values indicate a tighter envelope.

ELA (effective leakage area) converts measured leakage to an equivalent orifice area, commonly referenced at 4 Pa for envelope work.

Leakage curves fit the power law Q = C × ΔP^n; n near 0.5 indicates orifice flow, n near 1.0 indicates laminar/crack flow.

Duct leakage is expressed as CFM25 total, CFM25 to outdoors, or as a percentage of system airflow at the same pressure.

Envelope leakage raises infiltration load, drives moisture transport, and can destabilize pressure balance and combustion appliance venting.

Blower door setup: seal the door frame with the panel, install the gauge, and confirm the reference (outside) pressure tap is unobstructed.

Baseline (bias) pressures are recorded before the fan runs and subtracted from test readings to remove stack and wind effects.

Multi-point testing records airflow at several pressures (e.g., 15-60 Pa) and fits the power law to derive C and n for extrapolation.

Single-point CFM50 reading is valid only when the induced pressure is at least about 50 Pa and wind is low enough to keep readings stable.

Duct pressurization: seal registers, connect the fan to the air handler or a register, and pressurize the duct system to 25 Pa.

Total duct leakage is measured with the house (envelope) open to outdoors; leakage to outdoors is measured with the house sealed and pressurized.

Duct leakage to outdoors = total duct leakage minus leakage to the conditioned space; both are reported at 25 Pa.

Gauge calibration, hose routing, and ring selection must be checked, since a wrong ring or kinked hose corrupts every reading.

Zone pressure testing (door closure, zonal pressure) locates connected zones and hidden leakage paths between rooms, attics, and garages.

A pressure pan over a register or outlet shows whether that penetration is connected to the outside; readings above roughly 1-2 Pa suggest a leak path.

Thermal imaging paired with a blower door reveals air leakage and missing insulation, since depressurization makes leaks cold in winter and warm in summer.

Smoke pencils and theatrical fog trace leakage direction and confirm the path when pressure readings are ambiguous.

Duct leakage in unconditioned spaces (attics, crawlspaces, garages) is the highest priority because it is effectively outdoor leakage.

Return-side leakage in a depressurized house can draw attic or garage air into the supply, degrading indoor air quality and pressure balance.

Pressure imbalance between supply and return, or between rooms, causes closed-door pressure differences and comfort or backdrafting problems.

Combustion safety: depressurization can backdraft natural-draft appliances; test with the blower door running and check for spillage.

Record test conditions: indoor and outdoor temperature, wind, baseline pressures, gauge model, ring, and fan configuration.

Report both measured values and derived metrics (CFM50, ACH50, CFM25, leakage percentage) with the reference pressure stated.

Standard references include RESNET/ANSI and ASTM E779 for envelope testing and ASTM E1554 for duct leakage; cite the version used.

The test report should document pre- and post-retrofit values so improvement can be verified against the same protocol.

Safety: never leave the blower door running unattended; secure the panel and fan, and protect openings and occupants.

Before testing, confirm combustion appliances are off or monitored, and check for spillage after depressurization.

Document anomalies (open windows, obstructed registers, wind gusts) that could invalidate results, and note corrective actions.

Retain raw data and calculations for traceability, since third-party verification and program compliance require an auditable record.