Zonal Pressure Diagnostics (Series Leakage Split for Attics and Crawlspaces)

Two leakage paths in series -- house to attic, attic to outside -- divide the pressure between them exactly as two resistors divide a voltage, so the intermediate zone's pressure is a direct readout of which path has the resistance. A zone sitting near house pressure has almost all its resistance between itself and outdoors, meaning the plane between it and the house is wide open and the zone is effectively part of the house. A zone sitting near outdoor pressure has a tight plane on the house side and is effectively outdoors. That single reading redirects the work, which is the whole value of it. A high pressure ratio at the attic says seal the ceiling plane -- top plates, wire and pipe penetrations, the attic hatch, recessed lights -- and blowing more insulation on top of an unsealed ceiling is close to wasted money, because insulation does not stop air. A low ratio says the ceiling is already tight and the money is elsewhere in the building. The leakage path ratio reported here puts a number on it rather than leaving it as a direction, because the square-root relation is not intuitive: a pressure ratio of a half means the two paths are equally leaky, and the ratio climbs steeply on either side of that. Two readings are usually taken together, because the priority between planes is what a scope of work needs. The add-a-hole method extends this: opening a known hole to outside and re-reading isolates each path separately, which turns the ratio into an actual leakage area rather than a comparison. That refinement is not computed here. This interprets ENTERED readings from a blower-door test with the house held at a reference pressure: it does not perform the test, correct for the sign or reference of each gauge (a reading taken against the wrong reference inverts the conclusion), estimate leakage areas in absolute terms, or account for a zone with more than two significant paths, such as an attic connected to a garage. The BPI or RESNET procedure in force and the technician's own judgment govern.

pressure ratio = zone pressure with reference to outdoors / house pressure with reference to outdoors, both taken during a blower door depressurization; the ratio of the two series leakage paths follows as sqrt(ratio / (1 - ratio)) on the standard n = 0.5 flow exponent.

The zonal pressure diagnostic as building-performance field practice states it, with the series-leakage interpretation on an assumed flow exponent of 0.5. It is a RATIO diagnostic: it says which of two planes is leakier, not how many cfm either passes.

One measured ratio and one square-root relation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: house_pressure_pa, zone_a_pressure_pa, zone_b_pressure_pa, zone_a_label, zone_b_label, zone_a_ratio, zone_a_path_ratio, zone_b_ratio, zone_b_path_ratio

• Flow exponent 0.5 real envelopes run roughly 0.5 to 0.75the power-law leakage model
• Series leakage, two planes a zone with a third path is not this modela multi-point diagnostic
• A ratio, not a flow it does not report cfm through either planean add-a-hole measurement