Passive House Airtightness (ACH) Calculator

The Passive House (Passivhaus) standard requires airtightness of ≤0.6 ACH50, meaning the entire building volume exchanges less than 60% of its air per hour at 50 Pascals pressure difference. This is roughly 10-20 times tighter than typical construction. Achieving this requires meticulous attention to the air barrier, sealing every penetration, joint, and seam in the building envelope. Blower door testing at both mid-construction and completion verifies compliance. Failing to meet 0.6 ACH50 means the building cannot be certified as Passive House, though slightly higher rates (0.6-1.0 ACH50) may still qualify for EnerPHit (retrofit) certification.

Natural ACH is estimated by dividing ACH50 by a conversion factor (N-value) that depends on climate, building height, and shielding. For Passive House, the standard conversion uses ACHnat = ACH50 / 20. So 0.6 ACH50 gives approximately 0.03 ACHnat — meaning only 3% of indoor air is replaced per hour naturally. This extremely low infiltration rate is why Passive Houses need mechanical ventilation with heat recovery (MVHR). The general formula uses N-values ranging from 17 (well-shielded, tall buildings) to 27 (exposed, low-rise). Colder climates typically use higher N-values since stack effect is stronger.

Exceeding 0.6 ACH50 has several consequences: (1) Certification failure — Passive House certification requires ≤0.6 ACH50 with no exceptions. (2) Energy penalty — every 0.1 ACH50 above target adds roughly 0.3-0.5 kWh/m²/year to heating demand. A building at 1.0 ACH50 instead of 0.6 ACH50 may see 15-25% higher heating energy. (3) Comfort issues — drafts, cold spots near penetrations, and potential moisture damage from exfiltration. (4) MVHR imbalance — leaky envelopes disrupt the balanced ventilation, reducing heat recovery efficiency. Remediation involves finding leaks with a blower door and smoke pencil, then sealing with appropriate tapes, gaskets, or sealants.

Wood frame: Use an exterior smart vapor retarder or interior airtight drywall (Airtight Drywall Approach, ADA). Seal all bottom plates with acoustical sealant and gasket drywall. Concrete/ICF: The concrete itself is airtight; focus on sealing wall-to-roof connections, window rough openings, and MEP penetrations with fluid-applied membranes. Steel frame: More challenging due to many penetrations; require continuous air barrier membranes on the exterior sheathing. All types benefit from: (1) an airtightness layer clearly marked on drawings, (2) a dedicated air barrier inspector, (3) blower door testing at drywall stage before ceilings are enclosed, (4) using pre‑gasketed Passive House certified windows and doors. Typical cost for achieving 0.6 ACH50 is 1-3% of construction cost, mostly in labor for careful detailing.