Exterior Water Sources
In order for water to penetrate a wall, all three of the following conditions must be true at the same time and location:
- Water on the surface of the wall.
- An opening through which water may pass.
- One or more of the five forces (see below)2.
Rain, striking a wall at an angle, may have enough horizontal momentum to drive it through an opening (see Figure 2a). The water will penetrate the wall if the opening is continuous and properly oriented. The solution in Figure 2b, shows how a labyrinth shaped opening interrupts the flow of water. Other solutions include covering the gap, or orienting the opening downward similar to a louver or baffle4.
The adhesive and cohesive properties of water allow it to cling to a surface and even oppose gravity. In Figure 3a, water is flowing along the underside of the block until it reaches the edge where it drips onto the backside of the wall. To fix this problem, a groove is cut into the bottom edge of the top panel (see Figure 3b) and the surface tension is interrupted5.
Capillary Action (Capillarity)
Capillary action is the migration of water (or any liquid) through a tube (see Figure 4a). The amount of absorption is determined by the molecular attraction between water and the absorbing substance. Absorption is also greatly affected by the diameter of the capillary tubes; typically, a more narrow tube will draw more water. Adhesion is the attractive force between the liquid and other objects, which tends to move the water along the capillary. Cohesion, the bond of water molecules to one another, will resist adhesion and limit the degree of absorption6.
The material will absorb water until it is fully saturated, and if left alone, it will remain saturated. At that point, if gravity or air pressure forces act on the material, the water may evacuate the capillaries. Figure 4b, illustrates how widening the capillary will limit and shorten the distance water will penetrate7.
Air Pressure Differences
The wall diagram in Figure 5a shows a difference in air pressure between the outside and inside of the wall. If the exterior and interior pressures are unequal, then air must flow from the high-pressure area to the low-pressure area in order to achieve equilibrium. The subsequent air currents can raise and carry water inward8. As mentioned above, these currents can also draw water out of a saturated material such as masonry.
By allowing weak air currents to flow in and out, the pressure within can match the outdoor pressure moment by moment (see Figure 5b). Thus when there is equilibrium at the joint, suction is eliminated, and water will not penetrate the opening. Please refer to the section “The Rainscreen Principle” for further explanation on how this solution is implemented9.
Interior Water Sources
The primary source of water and moisture originating from the inside of a building is humidity. Several conditions can agitate the problem by creating higher levels of interior moisture and pressure. One such condition, is “The Stack Effect” where warm, moist air inside of a building rises and collects at the upper levels (see Figure 6). This can increase the barometric pressure inside the building, and if that pressure is greater than the outdoor pressure, then air may exfiltrate into the wall assembly. This movement of air can often carry moisture through the wall as well. Once moisture migrates through the wall structure or insulation materials, for example, it may condense into liquid form10.
Results of Water Damage
Water which infiltrates a wall structure will do much more than create an eye sore, in fact often times the damage done by water is hidden within the wall. Water will degrade many building materials, some worse than others. Water build-up on wall insulation may also reduce the quality of the insulation11.
Perhaps the most alarming and recent development has been the outbreak of toxic mold growth within walls and the dangers to the public health12.
“Health effects from exposures to molds in indoor environments can result from allergy, infection, mucous membrane and sensory irritation and toxicity alone, or in combination. Mold growth in buildings (in contrast to mold contamination from the outside) always occurs because of unaddressed moisture problems.”13