3.4 CONTROL OF RAIN PENETRATION
While the relation between rain penetration and heat, air and water vapor transfer through the
building envelope is indirect, it is still an important envelope performance issue. In fact, when most
people in the field of building envelope design and construction discuss leakage, they are referring
to rain penetration and not air leakage. Rain penetration is important to the discussion in these
guidelines because, like all envelope design concerns and subsystems, the means for controlling
rain penetration must be integrated into the building envelope so that all the various subsystems
can function effectively.
The control of rain penetration is primarily an issue of keeping water away from building materials
whose performance will suffer if wetted, and preventing water leakage to the building interior.
Water leakage can lead to dimensional changes in envelope materials, rust and corrosion, decay of
disintegration of materials, and dislodging of envelope components due to freezing (Brand). In
addition to leakage into the building envelope, the flow of rainwater across the building surface must
be controlled to prevent dirt-marking and staining of facade materials and etching of glass.