Add-a-Hole Zone Leakage (Flow Through an Attic, Garage or Crawl Space)
How much blower-door air actually passes through an attic, garage or crawl space.
Example
You enter
- House to outside (Pa) 50
- Hole opened between house
- Zone to outside before (Pa) 5
- Zone to outside after (Pa) 50
- Blower door flow before (CFM) 3595
- Blower door flow after (CFM) 4503
You get
- Blower door flow change 908 CFM
- House-to-zone plane alone at house pressure 280 CFM
- Zone-to-outside plane alone at house pressure 1,170 CFM
- Path flow (CFM) 262
- Path share (%) 7.3
Checked against published worked examples from: The Energy Conservatory.
Details, formula, and sources
ScopeA zone pressure alone gives only the ratio of its two planes; opening a hatch, door or vent in one plane and re-reading the zone and the blower door gives the flows. A garage reading 5 Pa to outside with the house at 50 Pa, whose door to the house takes the blower door from 3,595 to 4,503 CFM, passes 262 CFM, 7.3% of the house leakage. Both planes use the 0.65 field exponent. Readings are entered. The BPI or RESNET procedure in force governs.
Formula or tableTwo planes in series carry the same flow: C_hz (P - z)^n = C_zo z^n, with P the house pressure and z the zone pressure, both to outside, n = 0.65. A hole in the house-side plane leaves C_zo unchanged, so C_zo = dQ / (z_after^n - z_before^n); a hole in the outdoor-side plane leaves C_hz unchanged, so C_hz = dQ / ((P - z_after)^n - (P - z_before)^n). dQ is the rise in blower-door flow at the same P. Path flow before the hole = C_hz (P - z_before)^n; each plane alone at house pressure = C P^n.
Source and editionThe add-a-hole and open-a-door methods of zone pressure diagnostics (Michael Blasnik and Jim Fitzgerald), as charted by Cox and Olson in The Energy Conservatory's Zone Pressure Diagnostics (ZPD) Trainer Software User's Guide, 2014, cited by name.
Free accessFree at energyconservatory.com/wp-content/uploads/2014/07/zpd_trainer_manual.pdf. Its Open a Door #1 demonstration (p. 8: garage 45 Pa to the house, 3,595 then 4,503 CFM50) prints 280 CFM50 garage to house, 1,170 garage to outside, 262 total path leakage and 7.3% of total leakage; this tile returns 280.5, 1,169.9, 261.9 and 7.29%.
What governsEstimate. AHJ and licensed professional govern.
API field nameshouse_pressure_pa, hole_side, zone_before_pa, zone_after_pa, cfm_before, cfm_after, flow_change_cfm, house_to_zone_cfm50, zone_to_outside_cfm50, path_flow_cfm, path_share_pct
- Flow exponent 0.65 on both planes real envelopes run roughly 0.5 to 0.75the power-law leakage model
- Unopened plane unchanged its coefficient is the same before and after the holethe add-a-hole method
- Series leakage, two planes a zone with a third path is not this modela multi-point diagnostic
- Small-change caution flagged when the zone moves less than a tenth of the house pressurescope of this tile