Drainable is Sustainable



Why the best waterproofing system includes a drainage plane system.

Stone veneer removal on home with entrapped moisture damage to rainscreen building envelope

Most exterior surfaces of a vertical wall system do allow some moisture to penetrate more deeply into the exterior building envelope. Why do building professionals allow this? Don’t we have adequate waterproofing systems? Of course we know, and of course we do, but that isn’t the “hole/whole” story.

The hole/whole story – the more complete story – is that some exterior building surfaces of the exterior building envelope are responsible for more than just moisture management. These responsibilities include aesthetics, structural support, signage support, mechanical support and protection (veneers acting as shields). In addition to all these responsibilities, they also have to manage moisture.

So, the hole/whole story is a little more complicated. As part of the process of moving forward, let’s take a look at an example of a typical exterior wall (exterior building envelope). Obviously, many types of wall systems exist, but let’s use this one (see Figure 2.) to simplify the discussion as much as possible.

Figure 2

Rainscreen components
The rainscreen is the most exterior surface of an exterior building envelope. It is the first surface that has the potential to change the exterior environment to satisfy a desired interior environment: wet to dry, hot to cool, cold to warm. It can be constructed of various materials, including brick, stone, stucco, steel, wood, glass, plastics and fabrics.

The next section, the rainscreen drainage plane, is the space from the backside of the rainscreen to the front surface of the moisture-resistant material. It can be either a wall design feature (such as a space) or a product.

The third section, the weather-resistant surface, can be a waterproofing product (usually a sprayed or rolled on chemical compound) or a weather-resistant material (two layers of construction paper or a building wrap). It also can be both.

The final section, the structural wall, can be composed of a variety of materials, including wood or steel studs, CMUs or poured concrete.

In most cases, a requirement for insulation exists. It comes in many forms and can be placed in various locations in this detail. It is an extremely important and complicated character in the hole/whole story, and must be included in the moisture-management equation. (See Figure 3.) All forms of insulation involve unique moisture-management issues beyond the scope of this article, However, they must be seriously examined when constructing a moisture-management plan for the rainscreen building envelope.

Figure 3

Figure 10

Figure 11

Figure 11

Figure 12


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