Roxul ComfortBoard IS - Manual (Page 8)

ROXUL
® The Better Insulation™ 6
Rain Penetration Control
There are three recognized design strategies to control rain penetration within and through the
enclosure: Storage, Drained Screen, or Perfect Barriers.
In a Storage (or Mass) approach, it is assumed that water penetrates the outer surface of the wall and
then is eventually removed by drying to the inside or outside. The maximum quantity of rain that can be
controlled is limited by the storage capacity available relative to drying conditions. Some examples of
mass systems include adobe walls, thatched roofs, solid multi-wythe brick masonry, and single-wythe
block masonry that is still employed for some modern buildings.
Drained enclosures assume some rainwater will penetrate the outer surface (hence the cladding “screens”
the rain) and therefore the assembly must be designed to remove this water by providing drainage
(comprised of a capillary breaking drainage plane, a drainage gap, flashing, and weep hole/drain). Many
cladding systems, such as brick veneer and stucco, leak, as do the joints between other cladding types,
such as shakes, terra-cotta, small metal panels, or natural stone. For these cladding types drainage is a
practical and successful system of rain penetration control.
Perfect Barrier systems stop all water penetration at a single plane. Such perfect control required the
advent of modern materials. Because it is difficult to build and maintain a perfect barrier with many
materials, it is common to recommend the use of drained walls. However, some systems, usually factory
built, provide wall elements that are practical perfect barriers. For example, architectural precast
concrete can be considered watertight, as can glazing, and roof membranes. The joints between perfect
barrier elements should almost always be drained joints in the form of two-stage sealant joints or similar.
Drained Screen Approach
The drained screen approach is considered to be best practice for rain control for residential buildings
using insulating sheathing.
The term “rain screen” is applied to some drained systems, but the term is imprecise, as it means
different things to different people. Drained walls may also be vented (a single, or single line of openings
in the cladding connected to the exterior), ventilated (at least two openings through the cladding usually
distributed between the top and bottom of the cavity), or even pressure-moderated (the air pressure in
vented & ventilated walls tends to follow the exterior wind pressure, thereby “moderating the pressure”).
Rain screen is applied loosely to all three different types of drained walls.