Understanding engineered rainscreens

As shown in Figure 5, a typical pressure-moderated wall system consists of:

  • backer wall;
  • through-wall flashing;
  • air/moisture/vapour barrier;
  • outboard rigid foam insulation;
  • clear vented air space with ventilation devices at the top and bottom of walls; and
  • tough exterior cladding.

Since new energy codes are causing designers to increase the overall cavity depth, these wider cavities will take longer to neutralize because the wider the air space, the more air needed to enter for neutralization. By incorporating an all-wall drainage mat, the air space can be reduced without compromising its intended functionality.

For example, a 406-mm (16-in.) wide wall system with 51-mm (2-in.) rigid outboard insulation could have its 70-mm (2 ¾-in.) air space reduced to 44.5 mm (1 ¾ in.) to neutralize the air pressure quicker (Figure 6). More importantly, it also allows the designer to increase the R-value of the same 406-mm wide wall system. If increased insulation is not desired, the overall wall system can be greatly reduced and still drain and ventilate as intended.

A common misconception is using an engineered rainscreen drainage and ventilation mat adds cost to a building. (The same was said when air barriers came on the market years ago.) From a commercial standpoint, designing with an engineered drainage and ventilation mat narrows the air space, which reduces the size of many wall components and, in turn, costs.

Figure 7 uses through-wall flashings as an example. A typical cavity wall application has a mortar deflection usually 254 mm (10 in.) in height. To avoid mortar bridging atop these mortar deflections, it is recommended the through-wall flashing extend a minimum of 102 mm (4 in.) above the mortar deflection. When building with an engineered rainscreen wall, the all-wall drainage and ventilation mat eliminates mortar bridging issues and allows the flashing to only extend 204 mm (8 in.) up the wall, knowing the air/vapour barrier will be installed and shingled over the flashing.

As Figure 7 illustrates, the flashing costs are significantly reduced. For a typical 1524-m (5000-ft) long wall, the potential flashing savings are illustrated in Figure 8. Thanks to the proper drainage and ventilation associated with all-wall drainage mats, excess moisture will not be present within the wall system. This means significant advantages, such as:

  • increased energy efficiency;
  • reduced exterior surface staining and efflorescence;
  • protection against the deterioration of interior finishes;
  • reduced chances of toxic mould;
  • promotion of good indoor air quality (IAQ);
  • decreased overall maintenance needs;
  • reduced corrosion of building materials; and
  • increased lifespan of the building.
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