Rain penetration and rain penetration control is a very important aspect of building science and building engineering. As the majority of low-rise residential buildings are constructed using traditional platform frame construction, moisture that penetrates the cladding to the interior wall cavity can cause significant damage to the performance, finishes and structure of a building. Rain penetration can prove detrimental to concrete and steel (especially light-gauge steel) structures as well.

The damage in the widely publicized British Columbia (the "Condo Crisis") and Wilmington, North Carolina problems can be linked to rain penetration combined with the long rainy season and short drying season of the warm coastal climate. The vast majority of the condos that were damaged were designed using a "face-sealed" wall system. The face-seal relies on the integrity of the outer surface of the cladding to prevent the entry of water into the wall system. Water that reaches the sheathing or enters the stud-space can initiate decay and corrosion of structural members. More wide-spread rain problems in North America due to rain penetration include damage to finishes, mold problems, and cladding deterioration. 

Rain penetration occurs through a number of different mechanisms:

  • The most obvious method of rain penetration is the movement of moisture through cracks and openings in the cladding due to the kinetic energy of the falling rain (i.e., the open window scenario). This is not often an important mechanism since the joints at material interfaces are typically designed to prevent the entry of moisture into the wall cavity in this manner. Even failed caulking will often resist most kinetic energy. Rain penetration due to kinetic energy requires a clear path through which the water can travel into the interior of the wall.
  • The high surface tension of water is due to the polar nature of the molecule. This polar nature also increases water's ability to wet surfaces and gives rise to the phenomenon of capillarity (or "wicking"). Capillarity allows water to penetrate very small openings in the cladding without an external driving force. Capillarity can only occur if the crack is sufficiently small (less than about 3 mm) and the surface hydrophylic. As the size of the crack decreases, the capillary force increases and rainwater can thus be driven into small cracks with huge pressures. Capillarity is an important mechanism of rain penetration for stucco and brickwork.
  • Water can also penetrate cracks and openings in the cladding under the influence of gravity. This is the most common cause of serious problems, although it is the easiest to understand and avoid. Lack of flashing or proper seals around openings (especially windows, doors, balcony penetrations, etc) can collect surface rain water and direct it into the enclosure assembly. This is the mechanism that causes serious problems quickly.
  • Air pressure differences across the cladding or the entire enclosure assembly can drive rain water through openings and cracks. This mechanism is highly over rated in importance -- the primary reason walls leak when it is windy is that falling rain is directed onto the walls under these conditions, not that the air pressure is driving the leakage. During 99 percent of the time, most enclosures experience relatively small air pressure differences, nothing like the pressures imposed in some standard tests or laboratory experiments. Claddings such as brick and stucco leak profusely with absolutely no air pressure applied. The provision of a relatively small drainage in a joint or behind cladding will drain any water that is pushed across. Air pressure can be important for hydrophobic materials with no large gaps and openings (curtainwalls, window frames).

A rain control primer can be downloaded Click here. The publications section contains much more information on rain control, especially as presentations.