Is a drainable wrap enough? Weather-resistive barriers and gaps

All photos courtesy Advanced Building Products

By Keith Lolley
The construction industry has seen a tremendous influx of new building materials over the past 10 years. Many of these products serve a real value, but with new offerings come new questions. Sometimes, innovation and evolution need to slow down and allow for proper training and education to catch up.

All building professionals will agree moisture is one of the leading factors in wall failures in both commercial and residential construction. There is still no cure for moisture intrusion, but there are ways to reduce the harmful resulting effects.

The big question, though is, ‘how can builders build a drier wall?’ The first step is to understand the proposed construction site’s climate. After all, a building in Vancouver is far different from one in Whitehorse. When a site is chosen, professionals must consider the annual rainfall and snowfall, whether the area is in a high wind zone, and which type of cladding to use.

The Building Enclosure Moisture Management Institute (BEMMI), along with others, recommend that walls being built in areas receiving more than 508 mm (20 in.) of rainfall annually—Vancouver and Atlantic Canada fall into this category—have a drainage component, or capillary break.

The most commonly used cladding systems in construction today are brick, stucco, fibre cement, manufactured stone, metal, vinyl, wood (clapboards and cedar shingles), and glass. These claddings can be divided into two categories—absorptive and non-absorptive. Roughly 81 per cent of all cladding systems used today are absorptive. Therefore, these wall systems need to be constructed in order to allow moisture to drain and not linger behind the cladding.

In older construction, this was not difficult to achieve. This author was recently on a restoration job from the 1940s. When the cladding was removed there were no signs of rot. One could see right through the sheathing as there were gaps in the joints. More air flowed through the structure than one could imagine, but that kept the wall dry for all those years.

Wall systems
Currently, there are three main wall systems:

  • direct-applied;
  • vented; and
  • ventilated.

Direct-applied walls are typically constructed in this order, from interior to exterior: sheathing, weather-resistive barrier (WRB), window flashings, and then cladding. When moisture gets past the cladding it often remains trapped between the back of the cladding and the WRB’s exterior side. What happens when the weather-resistive barrier is torn or not taped properly? The moisture takes the path of least resistance, often getting behind the WRB and into the sheathing. At this point, a moisture issue is inevitable.

An entangled matrix design can give moisture and air the ability to move in multiple directions for maximum ventilation and drainage.

A vented wall, also known as a cavity wall, is constructed in the following order—sheathing, flashings, WRBs, mortar deflection, and various absorptive claddings.

Commercially, one is seeing less and less concrete masonry unit (CMU) backup walls and more steel stud construction with various forms of sheathing. New energy codes call for increased outboard rigid foam insulation to maintain higher R-values and reduce energy costs. Air or vapour barriers, depending on the building’s geographical location, greatly reduce the amount of moisture and air travelling through the building. Remember the 1940s restoration project that showed no sign of rot and how the structure had so many gaps in the sheathing for air to pass through easily? Air no longer has the ability to move through the wall with new construction technologies. To compensate, the wall system requires a gap, or capillary break.

Vented cavity wall designs, created for commercial assemblies, are designed to ‘drain the rain.’ This author thinks of such systems as ‘reactionary walls.’ They ‘know’ moisture is coming in, so they are designed to handle the problem by means of through wall flashings, mortar deflections, and weeps. These components allow moisture entering through the veneer a path to escape. Organizations such as the International Masonry Institute (IMI) recommend having a 50-mm (2-in.) air space. However, now there is an increase in outboard rigid foam insulation in these wall systems, so one of two things happen. The wall system is either becoming wider as design professionals retain the width of the gap, or the gap is being replaced with rigid foam insulation. The wider the wall, the more expensive it becomes. Further, the elimination of an air space causes the wall to become more susceptible to moisture.

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