What is the major problem with walls? It is not insects! Our industry spends more when companies improperly market products as a solution, like a bug screen with mortar collection. A 2-mil fabric covering a portion of a weep hole will not prevent bugs. Anything sealed? Anything caulked or fastened around the weep hole to assure the seal is proper? No, the product is loosely installed and does not even cover the entire opening. Ask ourselves, should our industry pay more?
I doubt that bugs are causing an issue in masonry as much as moisture-laden cavities have. In fact, the number one litigated issue in wall design is excess moisture. No, the little critters with eight legs are not what people complain about. In fact, complaints for bugs are solved with another trade – EXTERMINATORS!
Lets review a few things that are important to a cavity wall. First, ventilation is the most important aspect of a wall. If a wall does not vent the incidental moisture caused by vapor drive, then condensation and temperature change will allow small amounts of moisture to accumulate behind the brick, stucco or manufactured stone. We are all very familiar with this issue. Wall science has become the most important technical design consideration in the industry. We don’t see bug experts consulted when an architect is specifying a school project. We contact a BUILDING ENVELOPE consultant to assure our dew point is in the proper location and verify our ventilation and drainage is proper after we connect our flashing properly to our air barrier. Those highly technical design considerations are why our walls work well today. And by the way, that bug screen that is manufactured on the mortar collection device has a gap at the bottom in half the pieces from the die cutter. Really, even if it was flush do you think an ant can’t crawl under the fabric?
What is not occurring: bugs in the cavity.
Important Design
Ventilation of a cavity is essential. The free-flow of air through a weep hole into the cavity is required so that the air movement can take moisture-laden air out of the cavity. The perfect cavity would be perfectly clear. Inhibiting a perfectly clear cavity is the mortar from stone, brick and stucco. Rainscreen products help provide a 95% open cavity with a high percentage of uninhibited open area for free air movement. Why inhibit the weep hole if we don’t need to? Fabrics limit airflow, and the pressure needed for air to pass through increases with the density of the fabric. Open head joints enhance airflow.
Cavity walls are not designed for high water flow; they are designed to enhance air movement. Decade’s old studies have shown that air begins to move efficiently over a large area in a thickness approximating 0.25-inch. Less than about 0.25-inch and minor changes in cavity dimensions create air movement impediments and limit efficiency. The BIA has required cavities greater than an inch in width for over a decade to assure the ease in ventilation. None of this is about liquid water drainage. Cavities are not designed for significant water drainage since in no case is there a good reason for bulk water to enter the cavity. Designing to a drainage level measured in gallons per minute is immaterial since no cavity should ever see water measured in gallons.
Conclusion: Bugs or Ventilation?
Hype sells products. Bugs scare people. Ventilation solves cavity wall woes. All three of those statements are true, but only one is important to wall design. Walls need to ventilate, and providing a high amount of ventilation will keep masonry walls performing well.