Roxul ComfortBoard IS - Manual (Page 10)

ROXUL
® The Better Insulation™ 8
Selection of Drainage Plane Material
Drainage planes are water repellent materials (building paper, house wrap, sheet membranes, etc.) that
are located behind the cladding and are designed and constructed to drain water that passes through the
cladding. They are interconnected with flashings, window and door openings, and other penetrations of
the building enclosure to provide drainage of water to the exterior of the building. The materials that
form the drainage plane overlap each other shingle fashion or are sealed so that water drains down and
out of the wall. The drainage plane is also referred to as the “weather resistive barrier” or WRB. A wall
design typically has a single primary drainage plane but may have multiple water-shedding layers as part
of a comprehensive water management strategy.
The most common drainage plane is “tar paper” or building paper. More recently, the terms “housewrap”
or “building wrap” have been introduced to describe building papers that are not asphalt impregnated
felts or coated papers such as polyethylene or polypropylene films. Drainage planes can also be created
by sealing or layering water resistant sheathings such as a coated structural sheathing. Finally, fully
adhered sheet membranes, or trowel and spray applied coatings can act as drainage planes.
Drainage planes can be vapor permeable or vapor impermeable depending on climate, location within
the building enclosure or required control function. Building papers and “housewraps” are typically vapor
permeable (more than 10 perms) whereas fully adhered sheet membranes and trowel applied coatings
are typically impermeable (less than 0.1 perms). There are a few recently developed spray and trowel
applied coatings that are semi-vapor permeable (1 to 10 perms) that are likely to see wider application in
the near future.
Recommendations for rain penetration control
The significance of rainwater management cannot be over-emphasized: along with the structural
support function it is usually this functional requirement that defines an enclosure design approach.
The climate and the site play a large role in defining the rain exposure that a building is exposed to. The
amount of annual rainfall is one factor in gauging the rain exposure for a wall assembly (see Figure 3
below) but this is modified by the coincidence of rainfall with wind events, the orientation of the building,
and height of the building. Most parts of the world experience a significant amount of wind-driven rain,
and those areas exposed to typhoons can have extreme exposure conditions. While this type of climate
demands good rain control strategies for enclosure walls, the rain deposited on walls can be significantly
reduced by good design and siting.