Air Leakage & Vapor Control

Address air leakage and vapor control as durability and IAQ drivers. In most homes, air leakage moves far more water vapor than diffusion, so airtightness is the first lever.

Builds on: P6 (four control layers), P7 (bulk water)  ·  Leads to: P9 (heat flow), P10 (assemblies), P11 (HVAC)  ·  Cross-series: A8 Energy Performance

P/A bridge: Air sealing is a low-cost, high-verification-value measure - but only affordable if ventilation is included in the package.

Core Concepts

• Air leakage moves moisture. Can treat air control as a moisture control strategy.
• Vapor strategy depends on climate and drying potential. Can avoid moisture traps by aligning vapor control with drying paths.
• Testing is essential; don't guess. Can set airtightness targets and verify with blower door/inspection.

What Good Looks Like

• A continuous air barrier you can point to, draw, and inspect (roof, walls, foundation tie-ins).
• Fewer unplanned pressure pathways: chases, attics, garages, crawlspaces, ducts intentionally managed.
• Controlled ventilation sized and delivered intentionally (not "ventilation by leakage").
• Condensation risk managed by keeping surfaces warm enough or able to dry reliably.

Drying caution: Do not block all drying paths. Drying direction depends on cladding, insulation ratio, vapor retarder class, and solar drive - verify drying potential for the actual assembly, not a generic rule.

Air Sealing QA Sequence

1. Plan the air barrier line on drawings before framing
2. Seal ceiling plane (top plates, penetrations, chases)
3. Seal rim/band joist and all wall penetrations
4. Rough-in blower door test before insulation/drywall

Targets depend on adopted code, ventilation strategy, and program goals. Mechanical ventilation is non-negotiable once tightness reaches levels where leakage ventilation becomes unreliable.

Where Things Go Wrong