The critical role of sheathing membranes in exterior wall assemblies
The process of selecting the right sheathing membrane for your project is often rigorous, particularly when there are various project requirements in place. What is the role of sheathing membranes to begin with and, when it comes to building code requirements, what are the key things to keep in mind? This two-part blog series will uncover answers to these often sought-out questions. The first part will focus on vapor permeable sheet membranes, describing the functional roles of these important building materials, and part two will then expand upon this introduction—describing North American building code requirements for sheathing membranes.
To start off, sheathing membranes serve as a key environmental separator in exterior wall assemblies as they control water (and sometimes water vapor) and air from freely exchanging between the inside and outside. There is more than one terminology used for sheathing membranes, they are also known as: water-resistive membranes, weather-resistive barriers, building papers, building wraps, house wraps, wall underlayments, and various other names. Typically, but not always, a sheathing membrane entails a thin sheet or membrane that is placed outside of structural sheathing board in exterior wall assemblies. As reflected in the assortment of names, sheathing membranes can be made with different materials and serve numerous purposes.
In particular, vapor permeable sheathing membranes are not completely waterproof—meaning that moisture can pass through them by vapor diffusion and capillary flow. Additionally, the level of water protection required depends upon expected moisture loads (i.e., climate) and cladding type. The material type and manufacturer, on the other hand, influence the level of water resistance. Vapor permeability is an important material property for sheathing membranes. Vapor permeability impacts the drying capacity of an enclosure and is a vital characteristic for moisture-sensitive assemblies. That said, incidental moisture is almost always present, and extended periods without adequate drying may cause significant damage (i.e., mold, mildew, rot, corrosion etc.). Vapor permeable membranes permit drying by allowing entrained water vapor to migrate to the outside air.
The original role of sheathing membranes was to resist air infiltration and drafts in wood-framed buildings in the early 20th century. However, sheathing membranes’ better well-known role as a water control layer developed sometime around the 1920’s and 30’s. Although known for this role, sheathing membranes are actually a secondary line of defense (a.k.a. drainage plane or secondary plane of protection) against water intrusion. While most rainwater is prevented from entering the enclosure by the cladding system, some can still bypass the system through cracks or other discontinuities.
Air tightness in a sheathing membrane is often thought of as a secondary function in today’s modern buildings. However, the sheathing membranes’ role in providing airtightness continues to increase as policy and industry move toward more energy-efficient buildings. While airtightness in buildings is provided by many different elements (e.g., windows, doors, sealants, etc.), many modern North American building codes require a designated primary layer of airtightness. Sheathing membranes are a good choice for this role in many wall assemblies and climate zones, as they are often impermeable to air and can be easily connected to other critical components.
Three commonly used vapor permeable sheathing membranes
There are three commonly used vapor permeable sheathing membranes: asphalt-saturated felt, asphalt-impregnated kraft paper, and synthetic or plastic membranes. A number of other materials can act in a similar role as sheathing membranes:
- Liquid applied membranes: These materials can be used as both the air barrier (AB) and water-resistant barrier (WRB). Liquid applied membranes can either be spray or trowel applied and are more common on larger building projects.
- Insulated sheathing: Typically consists of vapor impermeable foam boards taped at the edges. Insulated sheathing may act as both the WRB and air barrier or may be a composite material of insulation faced with a ‘sheathing’ membrane. These materials combine thermal control with air and water control.
- Composite Materials: Composite materials integrate the WRB and/or AB into one product. Insulated sheathing, as noted above, may be considered a composite material. Other composite materials include sheathing boards integrated with a water-resistant facer.
There are numerous products and materials that may serve similar roles and the market is continuously evolving; however, it is pertinent to understand the most commonly used sheathing membranes. These materials often define sheathing membrane requirements and are referenced in today’s building codes.
Asphalt-impregnated or saturated felt paper (a.k.a. tar-paper), in use since the 19th century, is the oldest of the common sheathing membrane types. Recycled organic material (historically cloth rags and currently recycled paper products such as cardboard and other cellulose fibers) are impregnated with varying degrees of asphalt to form a sheet type membrane. Typically, the more asphalt, the higher the water resistance, and consequentially the heavier the sheet. Designations for felt paper are based on density and historically refer to the weight of 100 square feet of the material, with #15 felt paper or 15-pound felt weighing 15 pounds and #30 felt paper weighing 30 pounds. Modern designations (#15 or Type I and #30 or Type II) no longer indicate exact weight but still reflect higher densities, robustness, and water penetration resistance and are governed by ASTM Standards D226 and D4869 (the latter being used for roofing felt paper).
Asphalt-Treated Kraft Paper or Grade D Building Paper
Asphalt-impregnated kraft paper is made by infusing virgin kraft paper (cellulose or organic fibres) with asphalt to make it water and rot resistant. Due to the manufacturing process, asphalt kraft paper is lighter and more malleable than felt paper, but may be more susceptible to damage from extended periods of moisture exposure. Asphalt kraft paper is commonly used under stucco cladding systems and is therefore most common in western North America. Kraft paper’s designation stems from the US federal specification UU-B-790, in which grades are based on water resistance. This specification, created in the 1960’s, sets requirements for tensile strength (20 lbs/inch) and water vapor permeance (minimum of 5 US perms), which seem to have been adopted in some of today’s sheathing membrane specifications. Water resistance is tested with the ASTM D779 standard (a.k.a. the “boat test”) and is measured in minutes. The time to failure provides the rating of the paper; 10 minutes (Grade D) is generally the minimum requirement.
Polymer Sheathing Membranes
Originally marketed as energy-saving products during the energy crisis of the 1970’s, plastic or polymeric sheathing membranes are typically thin, lightweight fabrics made of polyolefins, such as polyethylene or polypropylene. Synthetic sheathing membranes are the only common membrane that can be designated as the primary air barrier layer. These “air infiltration barriers” double as water-resistant membranes after undergoing water-resistance testing and obtaining equivalency recommendations from building code organizations (e.g., CCMC in Canada or ICC-ES in the US).
Polymeric sheathing membranes can either be perforated (small pin holes through a uniform sheet) or non-perforated. Vapor permeability is provided by the pin holes or through the special properties of weaved fibers (non-perforated type). Vapor permeability can range from 5 US perms to over 500 US perms. Due to the high permeability of some of these membranes, water resistance testing is sometimes problematic with the standardized “boat test”. A water ponding test and a water column test were created as an alternative assessment for water resistance. As may be expected, perforated membranes tend to provide less water resistance than non-perforated types.
Mechanically fastened membranes and self-adhered membranes
A relatively recent development has been the advent of self-adhering sheathing membranes, which have typically been mechanically fastened to buildings. While mechanically fastened membranes can work well if properly designed and constructed, the intended functions of water and air control may be compromised by the many penetrations and loose-fitting nature of the product. Proper installation requires maintaining overlaps, taping at seams, and specifying fasteners that cover the perforations. In addition, the membrane must be able to withstand the fastener point stresses over the life-time of the building.
Self-adhered membranes, on the other hand, are fully self-supporting and relay the bond between the substrate and the membrane adhesive. Self-adhered membranes may also reduce complexity of installation as they can be easily detailed and installed around complex geometric shapes.
The choice of membrane type, whether mechanically fastened, self-adhered, perforated, non-perforated, depends on project specific requirements and building code requirements.
About Graham Finch, Principal, Building Science Research Specialist:
Graham Finch, Principal and Senior Building Science Specialist, is a building science engineer who specializes in research and investigation work. His work experience includes a wide range of projects including building enclosure condition assessments, forensic investigations, research studies, energy assessments, building monitoring programs, field review, and testing services for new and existing buildings across North America. He has worked with numerous building product manufacturers on product research and development, performance monitoring, and field testing.
Graham has authored and contributed to many publications, including industry guideline documents related to durable and energy-efficient building enclosures. He is regularly invited by building industry organizations and clients to speak on practical and technical issues related to a broad range of building science topics, and actively presents technical papers and presentations at local and international conferences. Examples of his work can be found on the RDH Building Science Laboratories website.