Roxul ComfortBoard IS - Manual (Page 11)

INSULATING SHEATHING FOR RESIDENTIAL CONSTRUCTION APPLICATION GUIDE 9
Figure 3: Annual Rainfall Map
(From Building Science Corporation. Based on
information from the U.S. Department of
Agriculture and Environment Canada)
Drained Screen Wall Recommendations
Screened wall systems are inherently more forgiving than either mass or perfect barrier systems.
Properly designed and built screened wall systems will provide economical and durable rain penetration
control. Failures in screened systems tend to occur because drainage was not provided (either through a
design or construction failure).
The most reliable and widely applicable approach is to follow the mantra:
“Deflection, Drainage/Exclusion/Storage, and Drying”.
Proper siting of the building and the use of sloped hip roofs and generous overhangs deflect driving rain,
even for tall buildings. Water on the surface of the wall is shed from and deflected around openings by
surface features, drip edges, and protruding flashing. Water is removed from the base of the wall by
sloping the grade, and siding is kept at least 8” (200 mm) above grade to protect it from splashes.
Rainwater will penetrate the cladding at joints, laps and penetrations. This water should be removed by
drainage through a drainage space and redirected to the exterior by the use of waterproof flashing with
all lap joints sealed.
Water will remain within the drainage cavity, will be absorbed into the cladding, and may even penetrate
into the structural sheathing or stud space. This water should be removed by drying to the exterior and
the interior by allowing diffusion drying and ventilating the space behind the cladding.