Roxul ComfortBoard IS - Manual (Page 13)

INSULATING SHEATHING FOR RESIDENTIAL CONSTRUCTION APPLICATION GUIDE 11
Air Control
There are three primary classes of reasons why the control of air flow is important to building
performance:
1. Moisture control – water vapor in the air can be deposited within the enclosure by condensation
and cause serious health, durability, and performance problems.
2. Energy savings – air leaking out of a house must be replaced with outdoor air, which requires
energy to condition it. Approximately 30% to 50% of space conditioning energy consumption in
many well-insulated houses is due to air leakage through the building enclosure. Air movement
within the enclosure, either through low-density insulation or in spaces around insulation can
reduce the effectiveness of thermal insulation and thus increase energy transfer across the
enclosure.
3. Comfort and health – cold drafts and the excessively dry wintertime air that results from
excessive air leakage directly affect human comfort, wind-cooled portions of the interior of the
enclosure promote condensation which supports biological growth which in turn affects indoor
air quality, airborne sound transmission control requires good airflow control, and odors and
gases from outside and adjoining buildings often annoy or cause health problems.
There are other circumstances that require the control of air flow; for example, to control smoke and fire
spread through air spaces and building voids and shafts in multi-family residential buildings, but these
are situations that deal with extreme events, not typical service.
The primary plane of air flow control in a wall is generally called the air barrier. Because such a plane is in
practice comprised of elements and joints, the term air barrier system (ABS) is preferred. In framed, low-
rise residential buildings, the primary air barrier system is often located on the interior of the exterior wall,
comprised of either an inner layer of drywall (sealed around the perimeter and at all penetrations) or
sealed polyethylene. However, an exterior air barrier system is preferred because fewer penetrations
need to be accommodated and it is more easily inspected. An exterior ABS can be constructed using
outer layers of sheathing (such as gypsum, waferboard, and fiberboard) with tapes or sealed housewrap.
Unsealed housewrap or building paper provides additional resistance to out-of-plane air flow through the
enclosure assembly. In many modern building assemblies, exterior sheathing is designed and detailed to
be part of an outboard air barrier system and additional layers providing resistance to air flow are
provided to the interior. Note that the plane of airtightness labeled by the designer (and all building
sections should indicate what is intended to be the air barrier) or builder as the air barrier system may not
in fact act as the ABS if that plane is not made continuous through all construction details.