Air Leakage & Vapor Control
Air leakage is a major energy, comfort, and durability driver—and it carries moisture. Vapor control only makes sense when you understand air pathways, cold surfaces, and drying direction.
Potential agenda
- Intro (2 min): “Air does most of the moisture transport.”
- Desired outcome (6–8 min): What “good air control” looks like in the real world
- Common failures (6–8 min): Leakage sites + why vapor layers get misused
- Core concepts (6–8 min): Airtightness, pressure drivers, and vapor logic
- Takeaways (2–3 min): A small set of high-payoff moves
- Discussion (30–45 min): Prompts embedded below
Desired outcome
- A continuous air barrier you can identify and inspect.
- Controlled ventilation rather than accidental ventilation through leaks.
- Comfort stability: fewer drafts, fewer cold spots, fewer pressure-driven surprises.
- Vapor strategy that supports drying (instead of creating traps).
Discussion prompt
- Air barrier identity: In your go-to wall and roof assemblies, what is the air barrier—specifically?
Common failures
Common air/vapor failure patterns These show up as comfort problems, moisture problems, and “mystery” odors
- Top-of-building leaks: stack effect drives warm, moist air upward into cold zones.
- Band joist / rim leaks: big leakage area, hard-to-see consequences.
- Chases and penetrations: dozens of “small” holes become a big hole.
- Wrong vapor layer placement: creates a cold condensing surface with poor drying.
Examples (worked into failures)
Example — Attic frost / roof deck moisture in winter
Warm interior air leaks upward; the roof deck is cold; moisture accumulates quietly until damage appears.
Example — Drafty home despite “good insulation”
Insulation slows heat flow but doesn’t stop air movement; occupants experience drafts and uneven temps.
Discussion prompt
- Leakage hotspots: What are your top 3 “always-leaks” locations?
Core concepts
Air barrier is a system, not a material
Pick an air barrier strategy that survives trades, sequencing, and penetrations.
Pressure drives airflow
Stack effect, wind, and duct pressures matter. Airtightness without pressure awareness can still disappoint.
Vapor control is about the cold surface
Identify the likely condensing surface, then choose materials so it can stay warm enough or dry reliably.
Measure and verify
Blower-door and smoke/IR investigations turn “opinions” into actionable fixes.
Takeaways
- Air leakage is a first-order driver of comfort, energy, and moisture risk.
- Stop guessing: identify the air barrier and test/inspect for continuity.
- Vapor layers should support your drying plan, not fight it.
- Next (P09): we go deeper into heat flow, thermal bridges, and condensation risk.
Further reading
Optional links will be added here (short, high-credibility readings that match the session focus).