Ensuring moisture protection for manufactured stone

A filament nylon drainage mat is an effective means to provide an air gap and drainage cavity.
A filament nylon drainage mat is an effective means to provide an air gap and drainage cavity.

Facing these challenges, construction experts and specifiers have recommended additional or alternative means to promote drainage and drying within the wall. Some of these practices have included ventilated air gaps, which can promote drying, as well as impermeable membranes paired with air gaps, or rainscreen-type construction.

The findings are valuable not only for reducing premature wear, but also for increasing the durability of structures desiring a masonry look. The basic underpinnings of the new approach to adhered masonry can be summarized as two lists: minimal measures required by code, and supplemental measures for enhanced protection.

Minimal measures
Minimal measures include building papers, felts, and housewraps, along with WRBs.

Building papers, felts, and housewraps
A single layer of these water-permeable materials may not always be enough for adhered masonry veneer claddings. An application of two layers of building paper is better, but serves only as the minimum backup for walls faced with thin brick or manufactured masonry veneer. (For more, see the article, “Manufactured Stone Nightmares,” by Dennis McCoy in the December 2004 issue of the Journal of Light Construction [JCL.])

According to various studies—including the National Association of Home Builders’ (NAHB’s) 2008 report, “Improving Drainage and Drying Features in Certain Conditions: Rainscreen Designs for Absorptive Claddings”—using WRBs alone is not sufficient to prevent water infiltration and moisture-related damage behind adhered masonry veneers. One key to successful, durable construction is the location of the barrier (drainage plane), as well as its continuous nature across the entire enclosure and its integration with flashings, weep screeds, and other joint/penetration details.

Enhanced measures
The enhanced measures include ventilated airspace, capillary absorption, and drainage media.

Ventilated air space
The use of wall ventilation by means of an air space has been shown to improve drying within the wall, and reduce moisture-related damage. Straube’s study concluded the small resulting space by an air-gap membrane provides sufficient ventilation, allowing a faster rate of outward drying than traditional adhered veneers.

Walls using air-gap membranes or other materials to maintain the gap—such as furring strips, drainage mats, and profiled boards—tend to have lower sheathing moisture content. Walls with the proper air space generally do not experience moisture-related durability issues caused by relative humidity (RH).

According to the NAHB report, using a drainage spacer to provide a capillary break for ventilation and drying can improve drainage and drying performance.

To achieve the capillary break and maintain effective drainage, the researchers concluded the minimum depth of the cavity should be 3.2 mm (1/8 in.), maintained throughout the enclosure. A maximum depth of 19 mm (3/4 in.) has been recommended by some specifiers and manufacturers. The NAHB report described a drainage space between 5.9 to 10.3 mm (15/64 to 13/32 in.) in depth, maintained by a two-ply material with a filter fabric separator preventing materials from blocking drainage space. This report recognized mortar or debris may enter the air gap.

Offering a prescriptive approach, the 2005 National Building Code of Canada (NBC) has required a “drained and vented air space” of at least 10 mm (2/5 in.) with a cross-sectional area that is at least 80 percent open for coastal climates.

Having continuous air- and moisture-barrier protection around penetrations and window openings is critical to the long-term durability of the wall.
Having continuous air- and moisture-barrier protection around penetrations and window openings is critical to the long-term durability of the wall.

Capillary absorption
Measures to address capillary water movement—through both mortar and masonry units—are also vital to successful wall construction. According to construction consultant Steve Easley, in adhered manufactured stone masonry veneer (AMSMV), a main instrument for moisture movement is capillary action. Considered a reservoir cladding system—as it can store water—Easley says, AMSMV is similar to stucco in the way it behaves with water. The effect can also be described in terms of hydrostatic pressure.

The two-layer WRB system provides a drainage plane to protect against the capillary draw, explains Easley, but an even better option is a rainscreen system with a small airspace between the WRB and stone veneer—creating a drainage space which better prevents water intrusion. A ventilated air space allows bulk water to drain out, but also serves as the needed capillary break, meaning water penetrating behind the cladding is intercepted. (Steve Easley’s 2010 article, “Weatherproofing Synthetic Stone Veneers,” can be read online at www.greenbuildermag.com.) 

Drainage media 
For proper drainage behind adhered masonry veneers, designers and contractors have found it critical to keep the scratch coat and mortar out of the air gap, necessitating vigilance among installers. In many cases, designers and/or contractors have recommended using a drainage mat with a filter-fabric backing to prevent clogging the air space.

For example, the Residential Masonry Contractors Association (RMAC) made recommendations for manufactured stone veneer over wood-frame construction, which included a drainage mat with filter fabric backing. A similar recommendation was made by the Builders Association of Minnesota (BAM), with details including a drainage mat material (or furring strips) that separate the WRB applied to the sheathing away from a layer of building paper. The measure is intended to keep scratch coat or mortar out of the drainage space.

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