08 October 2009

Water Infiltration through Exterior Walls - Capillarity, Part 3 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
Free drops of water will by nature take on a spherical shape - the geometric shape that encloses the most volume with the least surface area - because of the cohesive properties of water. However, many things can interrupt the cohesive forces, making water both a versatile and destructive material. Those forces that can overcome the cohesion of water include gravity, which tends to flatten the sphere, and attraction and adhesion to other materials, which can completely overcome the water's cohesion and "stretch" it along a surface.

Water tends to have a natural adhesion to many building materials, but various pollutants and cleaning agents may break down the cohesion and viscosity of water more than normal to increase its adhesion. We call those combination of forces the wetting ability.

This would not be a problem except that those same building materials often come with pores and cracks - little tubes composed of the wettable surface. The tubes increase the available surface area for a given volume of water, increasing the relative effectiveness of the adhesive forces. If the tubes are the right size, the adhesion can completely overcome the water's cohesion, turning the pore or crack into an effective transport medium to drag the water from one place to another, even over great distances. If the pores or cracks are even further right sized, these forces can even overcome gravity, allowing the water to be drug uphill.

One thing about capillarity is that it tends to take time to transport the water. If the walls are thick enough, the rainstorm will end before the water has made it the whole way to the interior, and the transport will reverse during the following dry period. Therefore, capillarity could more or less be ignored in traditional thick construction.

With our modern high performance thin construction, building designers need to take capillarity into account, just as they do gravity and momentum, if they will design durable structures. The first line of defense is to acknowledge that capillarity will happen, and to use materials with enough integrity to withstand freeze-thaw cycles with very little deterioration over a long period of time. Even porous materials such as brick come in varieties that meet this requirement.

The second line of defense is to provide capillary breaks so that the water cannot be transported the whole way into the interior. The capillary break can be a 10 mm (3/8") or larger gap, or a layer of water-impermeable material. In many modern wall constructions, there may be multiple capillary breaks intentionally or not. The multiple breaks aid in the durability of the building by providing a back-up in case the first break is bridged or compromised.

The right size for water transport tends be be in the range of 5 mm (3/16") to 0.01mm (the thickness of a human hair). Within this range, pores and cracks less than 0.5 mm (the thickness of a business or credit card) can even transport water uphill.  Pores and cracks larger than this range will tend not to support capillarity, but will allow water intrusion by momentum. Cracks smaller than this range can still take on water, but they do not support capillarity in the sense that the adhesion bonds tend to be so strong they will not release the water. Then, if they are subject to freeze-thaw cycles and the expansion of the water as it turns into ice is sufficient to overcome the cohesive forces of the building material, the entrained water will widen the cracks as it freezes, enabling future capillary transport.

Although there is a right size for capillarity, pores larger and smaller can still be effective and even rapid conduits for water transport, if other forces such as gravity or differential pressure are added to the capillary forces. The next essay will look at how pressure differential impacts water intrusion.

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