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.

Glazing in Fire Doors

Wired glass is traditionally used in fire doors, not because it survives the fire tests, but because until recently, there has been no viable alternatives that do survive the tests.

Now, however, we have options. Therefore, current codes have eliminated the use of wired glass in most cases. Unfortunately, it’s not immediately obvious when reading the code how severely its use has been restricted.

IBC 715.4.6.1 and 715.5.3 clearly permit wired glazing in doors as long as it meets the size limitations, at least from a fire point of view.

Now let’s turn to chapter 24 to consider impact resistance. According to 2406.3.1, if the glass opening is larger in 3 inches, the glass needs to be safety glazing [2406.3.1]. Since most wired glass does not comply with CPSC 16 CFR 1201, it cannot be used in doors that use the maximum openings allowed in chapter 7.

The options available to us are (in approximate ascending cost):

1. Safety-Rated Wired glass such as SuperLite I-W and Film-Faced Ceramic glazing such as FireLite NT have a film on one face of the wired or ceramic glazing material to provide safety characteristics, but are susceptible to scratching.
2. Laminated Ceramic such as FireLite Plus has the safty film laminated between two layers of ceramic glazing material.
3. Laminated glass with intumescent interlayers such as PyroStop has intumescing safety interlayers between two or more sheets of anealed glass.
4. Gel-filled, dual glazed units such as SuperLite II are similar in construction to an insulating unit, but filled with an intumescing gel rather than air.

Note that gel-filled units also provides temperature rise control, and can be used to create unlimited-area, transparent, fire-rated walls. (Other glazing assemblies are limited to a maximum of 25 percent of the wall area.)