Water Infiltration through Exterior Walls - Momentum, Part 2 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 water is frequently blown at buildings by the wind. Wind is, of course, nothing more than moving air. When a moving mass of air reaches a building, it cannot pass through the enclosure. It also cannot build up in infinite quantities on the face of the building, so it diverts itself around the building. To a certain extent, the molecules that make up the moving mass never actually touch the building because they are buffered by a mass of air adjacent to the building.

Rain may get caught up by and carried along with the wind, enabling the water to approach the building in something other than a vertical direction. However, the rain has a tendency to keep moving in the same direction when it gets to the building, even as the air is diverting itself around the building. The tendency is a force we call momentum. Though the moving air might not reach the building, the moving water does, and it accumulates on the enclosure until it is removed by gravity.

If a particular drop of water reaches the building at a point where there is a hole in the facade, it will continue unabated into or even through the building enclosure, depending on the size and depth of the hole.

If the hole is large, such as a doorway, viewport, or ventilation opening, we are intuitively aware of the problem and we mitigate the problem of water entry by providing closures such as doors, windows, shutters, and louvers.

However, buildings tend to have other holes that more easily escape our attention such as weeps in veneer brick,  reveals in metal panels, joints between dissimilar materials, and even expansion joints. These holes cannot be blocked in the same way that a door or window is blocked, but the momentum of flying water can be blocked on the same principle that makes louvers work: designing labyrinths that interrupt every straight line and convert the primary force acting on the water from momentum to gravity.

Buildings also sometimes have unintentional holes resulting from a construction omission, natural disaster or aging, or vandalism. When these occur, the solution is to engage workmen to close up the hole with like material or a complementary material designed for that purpose, such as sealant.

As a final note, holes that are smaller than 5mm (3/16") do not need to be considered in terms of momentum. The reason is that it is nearly impossible for a water drop to pass through such a small opening without being attracted to or otherwise disrupted by the sides of the hole. However, capillarity tends to take over at the moment that momentum is arrested, introducing another means by which water can enter a building. But that is the topic of another essay.