16 January 2010

When to Use Pressure Equalized Rainscreens

Pressure equalized rainscreen systems are typically neither inexpensive nor (as with any other technology) infallible. Therefore, there is value in applying the technology only where you can expect reasonable return on the investment. Here are several considerations.

Susceptibility. Wood and steel studs are very susceptible to decay and corrosion due to water damage. If there is any failure of the exterior envelope, significant loss to the structural integrity of the building can occur before it is discovered. Therefore, it is prudent to use the best technologies available to skin a building with a wood or steel stud structural or exterior wall backup system. Masonry backups, on the other hand, are relatively immune to loss of structural integrity due to water infiltration, so it's only necessary to be sure that water is excluded from the interior of the building. In most cases, this can be achieved without pressure equalization. I would also put structural steel backup in with masonry because, although it is susceptible to corrosion, it is more likely that the water damage will clearly manifest itself in other ways long before significant structural loss has occurred. Therefore, if you have wood or steel studs, consider pressure equalization.

Water. As obvious as it is, it must be pointed out that if there is no water, there will be no water infiltration. Similarly, if there is little water, as in a desert, little protection is required because the drying periods are so significant. So, at what point is there enough water that additional protection measures should be taken? G. Adaire Chown has recommended that a reasonable criteria is 50 cm (20 inches) of rain per year*; if the building receives more rain that that, pressure equalization should be used.

Enclosing Material. Many building materials such as brick mortar joints are naturally porous, and others such as cedar siding contain surfactants. The former provide pores for capillary ingress of water and the later is a wetting agent that increases the effectiveness of capillary ingress. If either of these are present, pressure equalization is in order.

Wind. Although there are multiple sources for pressure differentials, wind is the prime generator of those that cause water infiltration. Buildings subject to heavy wind-driven rain will profit from pressure equalization more than buildings subjected to light winds. Some designers consider wind gusts greater than 40 km/h (90mph) as their criteria for considering pressure equalization.