Drainage and Drying in Low- and Mid-Rise External Walls

Low profile drainable housewrap removes moisture vapor and bulk water from wall assemblies
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Sponsored by Benjamin Obdyke and Tamlyn
Karin Tetlow
Glossary of Moisture Management Terms

When it comes to how to select the right moisture management solution for each job, the first challenge is navigating through some potentially confusing terminology. Product names such as housewraps, building paper, and rainscreens are frequently used interchangeably, when in fact they can play distinctly different roles.

A water-resistive barrier (WRB) – Sometimes referred to as a weather resistant barrier, is a material that repels water and is installed behind the cladding as a secondary barrier and interconnected with flashings, window and door openings, or other penetrations of the building enclosure. There are many types of WRBs.

Housewrap – Plastic rolled sheeting designed to resist air and liquid water, while allowing some passage of water vapor. Strength and integrity is provided by a woven or non-woven fabric. Water and air resistance is achieved by fiber spun technology or the incorporation of a film layer. Vapor breathability is achieved through fiber pore size, micro-porous films, or mechanical micro-perforations.

Drainable housewrap – With an added feature of a built-in drainage gap, these products combine the benefits of flat housewrap with enhanced water-shedding capability. Drainable housewraps do provide drying, but not to the extent of a constructed rainscreen. Combining a rainscreen with drainable housewrap is redundant in most applications, but is recommended when maximum moisture management is required.

Building paper (asphalt-saturated kraft paper, ASK) – Rolled material consisting of single layer of mostly virgin cellulose fibers saturated with asphalt.

Felt (asphalt-saturated felt) – Rolled material with multiple layers of loosely laid recycled cellulose fibers saturated with asphalt. Tends to be heavier and more brittle than paper, but provides a varying vapor permeance depending on exposure to moisture.

Pre-weatherized sheathing – Sheathing product that is factory modified to provide the performance characteristics of a WRB.

Fluid-applied WRB – Spray, rolled, or trowel applied membrane that upon curing provides WRB performance characteristics.

Self-adhered WRB – Rollable polymeric sheet coated on one side with adhesive, which allows the product to stick to the wall.

Combination WRB/rainscreen products – Typically a housewrap pre-attached with a rainscreen. Unlike a drainable housewrap, these products provide a full capillary break and enhanced ventilation drying in addition to the water-resistive barrier properties of a housewrap.

Rainscreen – A constructed rainscreen provides drainage and drying. It controls rain entry by locating a 1/4– 3/4-in. pressure-moderated air space immediately behind the exterior cladding in addition to employing a water resistive barrier. A rainscreen system is recommended when drainage and ventilation are necessary for maximum wall-drying potential, such as when installing moisture-absorptive claddings in areas with frequent wind driven rain.


Impact of Building Codes

Among the factors driving the need for better moisture management solutions are the continued growth and standardization of requirements such as the International Residential Code (IRC) and the International Building Code (IBC), which requires a means of draining water that enters the wall assembly.

The IRC now mandates the use of water-resistive barriers, stating that “The exterior wall envelope shall be constructed in a manner that prevents the accumulation of water within the wall assembly by providing a water-resistant barrier behind the exterior veneer…and a means of draining to the exterior water that enters the assembly (R703.1.1 Water Resistance).”

Stucco and manufactured stone walls have an additional requirement regarding the draining of water that penetrates the exterior veneer: “Water-Resistive Barriers…shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D Paper. Exception: [one layer may be used] where the water-resistive barrier is separated by the stucco by an intervening, substantially non-water absorbing layer or designed drainage space. (R703.6.3)”

Future Direction of Codes

At least one U.S. state has added even more prescriptive measures to its codes. Oregon now requires a means of drainage in wall assemblies. Its code mandates that “…the [building] envelope shall consist of an exterior veneer, a water-resistive barrier (housewrap, building paper, etc.) and a minimum 1/8” (3mm) space between the WRB and the exterior veneer. The required space should be formed by the use of any non-corrodible furring strip, drainage mat, or drainage board.” An exception to this is that “a space is not required where the exterior veneer is installed over a water-resistive barrier complying with section R703.2 which is manufactured in a manner to enhance drainage and meets the 75% drainage efficiency requirement of ASTM E2273 or other recognized standards.” It should be noted that one drainable housewrap on the market meets 96 percent efficiency requirement of ASTM E2273.

Canada has always been ahead of the curve as it relates to moisture management. In coastal provinces of Canada, rainscreen systems are required: “…exterior walls exposed to precipitation shall be protected against precipitation ingress by an exterior cladding assembly consisting of a first plane of protection and a second plane of protection incorporating a capillary break (a drained and vented air space between the cladding and the backing assembly, over the full height of the wall…)”


Key to effective moisture management is proper installation of WRB products.

• The time between installation of the housewrap and the installation of exterior cladding should be kept to a minimum.

• Housewrap that is exposed to the elements for longer than its intended UV rating can deteriorate. Lengthy exposure can also result in tears and holes that can affect performance if not repaired.

• Wrapping should start at the foundation, extending at least an inch past the wall-to-foundation intersection. It should continue upwards like shingle installation, with the higher course overlapping the lower course. According to code, all horizontal seams should be overlapped a minimum of 2-in., and vertical seams a minimum of 6-in. with caution when installing housewrap.

• Most manufacturers recommend taping all seams both vertically and horizontally if the drainable housewrap is intended to act as an air barrier. However, the use of tape is not usually a code requirement and can add as much as 15 percent to installation costs. Additionally, the durability of tape is always a question. With the use of a drainage gap and proper overlap of the housewrap, water will naturally flow downward and drain out. Taping horizontal seams creates a negative overlay, which can trap moisture.

• Manufacturer-recommended tape should be used to cover tears and holes. Common galvanized roofing nails or plastic cap nails may be used for attachment of the housewrap. Staples can cause tears and should be used with caution.

HydroGap Drainable Housewrap Protects LEED® Certified Waterfront Home from Moisture Infiltration



Following the four Ds of managing moisture infiltration into buildings— deflection, drainage, drying and durability of materials—design professionals are turning to drainable housewraps as an efficient means for removing water and water vapor from wall assemblies as quickly as possible. In addition to the water and air resistance, vapor permeability and durability features of a standard flat housewrap, enhanced drainable housewraps have a drainage space that facilitates water drainage from behind the exterior cladding system. Typically achieved by adding 1mm-1.5mm spacers, drainable housewrap can be as much as 100 times more effective at removing bulk water from the wall as flat housewrap. The International Residential Building Code mandates the use of water resistive barriers in wall assemblies, but Oregon has added more prescriptive measures that can be met by using drainable housewrap. Other states could very well follow suit.


Benjamin Obdyke Benjamin Obdyke is a leading provider of high-quality roof and wall building solutions. Our roof and wall system products enhance the building to maximize performance, durability, and value of a building's most critical elements — its outer structure. www.benjaminobdyke.com
Tamlyn Tamlyn manufactures a wide variety of quality building materials featuring:
TamlynWrap™, a unique drainable housewrap system that improves water management in structure walls; XtremeTrim®, a sustainable extruded aluminum system of aesthetic profiles for fiber cement and other exterior claddings in a range of finishes and colors; plus many other conventional framing products such as lumber connectors, metal and other flashings, and masonry components. http://www.tamlynwrap.com/



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