Roxul ComfortBoard IS - Manual (Page 14)

ROXUL
® The Better Insulation™ 12
Basic requirements of Air Barrier Systems
Typically, several different materials, joints and assemblies are combined to provide an uninterrupted
plane of primary airflow control. Regardless of how air control is achieved, the following five
requirements must be met by the air barrier system (ABS):
1. Continuity. This is the most important and most difficult requirement. Enclosures are 3-D
systems! ABS continuity must be ensured through doors, windows, penetrations, around
corners, at floor lines, soffits, etc.
2. Strength. If the ABS is, as designed, much less air permeable than the remainder of the
enclosure assembly, then it must also be designed to transfer the full design wind load (e.g., the
1-in-30 year gust) to the structural system. Fastenings can often be critical, especially for flexible
non-adhered membrane systems.
3. Durability. The ABS must continue to perform for its service life. Therefore, the ease of repair
and replacement, the imposed stresses and material resistance to movement, fatigue,
temperature, etc. are all considerations.
4. Stiffness. The stiffness of the ABS (including fastening methods) must reduce or eliminate
deflections to control air movement into the enclosure by pumping (movement of the air barrier
pulls and pushes air into and out of enclosure cavities). The ABS must also be stiff enough that
deformations do not change the air permeance (e.g., by stretching holes around fasteners)
and/or distribute loads through unintentional load paths.
5. Impermeability. Naturally, the ABS must be impermeable to air. Typical recommended air
permeability values are less than about 1.3 x 10-6 m
3
/m
2
/Pa. However air barrier materials are
commonly defined as materials which pass less than Q< 0.02 lps/m
2
@75 Pa. Although this is an
easy property to measure it is not as important as might be thought. In practice, the ability to
achieve other requirements (especially continuity) are more important to performance, and the
air “permeance” of joints, cracks, and penetrations outweighs the air permeance of the solid
materials that make up most of the area of the ABS. Hence, a component should have an air
leakage rate of less than Q< 0.2 lps/m
2 @75 Pa, and the whole building system should leak less
than Q< 2.0 lps/m
2
@75 Pa.
Joints, penetrations, and transitions are the critical link in achieving airtightness. At penetrations and
transitions, details must show how an uninterrupted, strong and airtight plane continues from the wall
element to other components of the enclosure such as windows, roof and foundation assemblies, while
accommodating dimensional construction tolerances and in-service movements.