The size of the air gap has been questioned. What is the correct space for proper drainage and ventilation? When creating a drainage and ventilation void space in residential construction, a 6.35-mm (¼-in.) gap is the standard in the United States. A 9.5-mm (3/8-in.) gap is recommended in Canada.
A common question for building professionals is, “Does a drainable house wrap create a great enough air space to be considered a ventilated rainscreen wall system?” The short answer is no.
For an air space to be effective, the gap must be a minimum of 3 mm (0.11 in.). To this author’s knowledge, most drainable house wraps are not 3 mm (0.11 in.) thick. If used on their own with masonry veneer cladding, the scratch coat would clog the house wraps’ drainable surface areas, leaving them rather ineffective. The author has witnessed many restoration projects using just a drainable house wrap with stucco and manufactured stone—the moisture was trapped. WRBs often tear during installation and are not taped properly at the seams.
Drainable house wraps, using a two-layer approach, will be effective for drainage, but not so much for ventilation when compared to a 6-mm (0.23-in.) rainscreen drainage and ventilation mat. On the other hand, if ventilation is not a concern and the cladding is wood, vinyl, or cement siding, then a drainable house wrap is often considered good enough.
Another option would be to install a WRB and an engineered polymeric drainage and ventilation mat. Here, builders would install the house wrap and instead of a sacrificial second layer, they would install the rainscreen mat as the second layer. The capillary break created by the rainscreen will substantially outperform the drainage ability of a drainable house wrap. For example, a typical drainable house wrap with 1-mm (0.04-in) spacers has a hydraulic transmissivity of 2.25E-04 m2/s (0.11 gpm/ft) width, whereas a 6-mm (0.23-in.) engineered rainscreen has a hydraulic transmissivity of 8.67E-03 m2/s (4.22 gpm/ft). This is a substantial difference, but for the sake of practicality, if a wall system needs to drain 15 L (4 gal) of moisture in a few minutes, it has to deal with bigger issues.
The biggest advantage to building with an engineered rainscreen is the air transmissivity. For example, the air transmissivity of a 6-mm (0.23-in.) engineered rainscreen is 1.51E-02 m3/sec/metre width (9.8 ft) at an air gradient of 35; whereas, a drainable house wrap comes in at 4.85E-04 m3/sec/metre width (0.03 ft) at the same air gradient. As mentioned, the ventilation aspect of a successful drywall is greater than the drainage ability. Walls only drain when it rains. Walls with proper ventilation continuously dry 24 hours a day, seven days a week.
Conclusion
When it comes to WRBs, a house wrap is good, a drainable house wrap is better, and an engineered polymeric drainage and ventilation mat is best. These engineered mats create a cavity wall concept without a true cavity wall cost when building with typical, direct-applied claddings. They block mortar from clogging the drainage plane due to the filter fabric bonded to the entangled net filaments. In ventilated cavity wall applications, these mats also allow the designer to increase the amount of outboard rigid foam insulation being used while still maintaining a proper drainage and ventilation gap without extending the wall’s overall thickness. Engineered polymeric drainage and ventilation mats with a bonded filter fabric can be used with any absorptive cladding, which means they are a one SKU item for simple purchasing and inventory control, and can be used with any type of veneer.
Keith Lolley is the president of Advanced Building Products Inc. and also holds the position of chairman within the Building Enclosure Moisture Management Institute (BEMMI). Lolley has been involved in the moisture management end of the construction industry for the past 19 years. He can be reached at klolley@abp-1.com.
