Air Barrier System
The function of the air barrier system is to stop INSIDE air from EXFILTRATING through the envelope:
- Air movement = moisture movement
- Air movement = energy loss
...and to stop OUTSIDE air from INFILTRATING through the envelope:
- Carries in pollutants from within the building assembly
Approximately 100 times more moisture is carried into exterior house cavities by air leakage then by any other mechanism.
This means that the air barrier is 100 times more important than the vapour barrier.
Air Barrier Criteria
- Can be installed anywhere in building envelope assembly
- IMPERMEABLE to air flow
- CONTINUOUS, by sealing all seams, edges, gaps, holes and tears
- Not necessarily impermeable to moisture flow
- Rigid and strong enough to withstand air-pressure differences
- DURABLE during construction and for the expected life of the building
Air Barrier vs. Vapour Retarder
| AIR BARRIER CRITERIA | VAPOUR RETARDER CRITERIA |
|---|---|
|
|
| Air Barrier Permeance | Vapour Retarder Permeance |
| Measurement of a material's resistance to air flow when subjected to a pressure difference Both NBC and OBC state that air barrier material must have a permeance no greater than 0.02 L/(s*m^2) at 75 Pa |
Measurement of a material's resistance to vapour diffusion Lower perm rating = more effective vapour retarder Both NBC and OBC state that vapour barrier material must have a vapour permanence no greater than 60 ng/(Pa*s*m^2) |