PLOT’s approach to Water-Resistive and Air Barriers
Choosing the right WRB (Water-Resistant Barrier) is a critical part of your home's performance and the health of its occupants. It’s not as straightforward as it may seem, and many products make various performance claims. Three major components control air flow, heat flow, and moisture flow:
Water-Resistant Barrier (WRB): Deflects liquid water flow on the exterior of the building, typically combined with flashing systems.
Air Barrier (AB): Controls airflow from the exterior into the conditioned space.
Vapor Barrier (VB): Controls moisture (usually in gas form) transmission through an assembly. Not all buildings need a vapor barrier in the wall or roof assembly; this depends on climate zone, based on International Building Code 2021 and International Residential Code 2021 definitions, and the materials used. This post will not focus on Vapor Barriers.
The Air Barrier (AB) should not be confused with the Vapor Barrier (VB), whose purpose is to reduce water vapor movement through an assembly and is installed on the warm side of insulation. Common materials for vapor barriers include plastic or foil-faced insulations. If improperly installed, they can cause significant moisture-related damage in wall and roof assemblies.
"Housewrap" commonly refers to the WRB layer of a wall assembly, but it is also a brand name for a popular product: DuPont’s® Tyvek Housewrap. Additionally, not all WRBs are inherently Air Barriers. The WRB is a critical component for managing exterior water, while the Air Barrier reduces air leakage. These two functions can often be combined in the WRB/AB system, but successful implementation requires careful detailing, skilled craftsmanship, and appropriate products.
Exterior cladding systems are not waterproof, so the primary purpose of an exterior-applied WRB is to prevent liquid moisture from penetrating the interior of a wall assembly, while still allowing moisture vapors to diffuse outward. This is essential to avoid trapping moisture within the wall. Traditionally, asphalt-impregnated felt paper was used for this purpose, and the International Residential Code still allows it as the WRB. However, asphalt felt has limitations, such as brittleness and susceptibility to damage during installation, leading to the development of more robust materials. Most modern WRBs are made from woven-plastics or polymer-based materials. Therefore, we advise against using asphalt felt and instead recommend using an “approved substitute” per the International Residential Code.
Example of Sheet Applied WRB/AB with pressure treated furring strips to create a drainage plane, often referred as a rainscreen.
Without a proper exterior water-control layer, building sheathing is at risk of rot and deterioration over time. Additionally, if liquid water enters the wall cavity, it can degrade thermal insulation, and in the worst-case scenario, lead to mold and serious health issues. In climates with mixed temperatures and moderate rainfall, like the Pacific Northwest, an additional control layer called a rainscreen is often recommended. This system creates a space between the WRB layer and the exterior cladding, allowing water to drain freely through the wall assembly and enabling the WRB to dry properly. Typically, furring strips create this drainage gap and provide an attachment point for the cladding, but some WRB products have integrated dimples or corrugations that eliminate the need for furring strips. When detailed and installed correctly, this approach minimizes the risk of trapped water behind the cladding, preventing damage.
The air barrier component is critical for thermal comfort of the interior. Many green building and energy codes quantify this with Air Changes performed by a blower door test. Lower the number the more air tight the building envelope is. As buildings get more air tight, more thought has to be placed around fresh air intake and whole house ventilation to remove interior toxins and moisture vapors that can find it way into a wall assembly and become trapped. Another major benefit of quality air barriers is control from outside pollutants such as fire smoke and plant pollan,
Continuity is key for air barriers. Foundations, thru-wall penetrations, wall to roof transitions, and in sheet applied - the seams, are the typical weak spots for air barriers continuity. These areas need special attention to detailing and installation.
Typical weak spot in detailing and construction for WRB and AB Continuity
There are several factors that can decide on a product and system. At PLOT, we ask ourselves and our clients these questions before specifying and detailing a WRB/AB system:
Building performance goals, such as thermal comfort, energy costs, etc.
Passive House, or any other green building certification?
Need to meet higher baseline performance to meet accepted Energy Codes credits?
Contractor comfortability with install? Some products require special certifications for installers.
Availability of products in your region/location?
Compatibility of different systems.
Is a rainscreen present?
What is the cladding material?
Holistic cost of the WRB/AB system.
Climate of the project.
Is there fresh air intake present such as an ERV system?
Is exterior continuous insulation required or desired?
Often, all these questions have competing answers and it’s design teams job to synthesize that information and recommend to our clients which system has the best compromise.
Based on the criteria above, we narrow down to the four main types of WRB/AB systems:
Sheet Applied
Common Products: DuPont® Tyvek Housewrap & Commercial, DuPont Tyvek Drainwrap, Benjamin Obdyke HydroGap®
Positives: Commonly used - contractor familiarity and affordable material costs. Sealants and tapes are often interchangeable with other manufactures if specified correctly. High vapor permeance. UV stable products for open joint rainscreens are available. Product can be installed in almost any weather condition and season.
Negatives: Mechanical attachment - which reduces air barrier continuity. Requires sealing at all sheet seams and metal flashing with self-adhered tape, which adds time and can be installed incorrectly. Difficult to achieve an effective Air Barrier. If no rainscreen, cap fasteners can conflict with paneled siding. Often times not installed correctly, which leads to building performance issues.
Rolls of sheet-applied WRB are the most widely available, and general contractors are typically the most familiar with their installation requirements. Furthermore, most sheet-applied products do not require certification training to maintain product warranties from the manufacturer. Most sheet-applied WRBs have high vapor permeance, allowing the WRB layer to dry effectively. Cap fasteners should be used to reduce water infiltration risk and to maintain air barrier continuity.
The biggest limitation of sheet-applied systems is their effectiveness as air barriers. The primary method to improve their effectiveness is to use approved self-adhered tape at all sheet seams, tying the WRB/AB into the framing and flashings. For high-performance goals, the most effective way to enhance the air barrier is to seal the exterior sheathing seams with a compatible product before installing the sheet WRB. A liquid-applied or self-adhered tape can be used for this purpose.
Some sheet products have integrated dimples to create a drainage plane. Benjamin Obdyke’s HydroGap WRB is a commonly used sheet-applied WRB of this type. However, some building envelope engineers argue that a 1 mm gap is insufficient for effective drying. In practice, if the cladding attachment is over-fastened, it can squash the dimples, making the drainage system ineffective. Despite these concerns, we recommend this system if the project cannot achieve a traditional rainscreen with furring strips.
Self-Adhered Sheet Applied
Common Products: VaproSheild® WrapSheild IT, Henry® Blueskin VP100, Benjamin Obdyke HydroGap® SA Housewrap
Positives: WRB seams are inherently sealed. If fully-adhered, meaning whole sheet is sticky, attachment penetrations eliminated. Similar performance to liquid applied, but often less costly. Better Air Barrier continuity. Can get UV stable products for open joint rainscreens.
Negatives: Harder to install than sheet applied. Often tapes and sealants are not cross compatible between manufacturers. More expensive than sheet applied.
Self-adhered systems require more skill and precision to install, which is why some general contractors may discourage owners from choosing them. However, the advantages are significant. When detailed and installed correctly, self-adhered WRBs provide better water resistance and serve as a more effective air barrier than mechanically applied sheets. The WRB is stored in rolls, similar to non-self-adhered sheet products.
As with sheet-applied systems, there are drainable self-adhered products available. Benjamin Obdyke’s HydroGap® SA Housewrap is one of the most commonly used options of this type.
Integration with Panels
Common Products: ZIP®
Positives: Better Air Barrier continuity, exterior thermal insulation, and can be a reduction in install time.
Negatives: Must carefully tape all sheathing panel seams and attachment fasteners. More care has to be taken while storing and installing sheathing as not damage the integrated WRB.
This system Integrates the structural sheathing OSB, continuous rigid insulation, and the WRB into standard 4’ x 8’ panels. Major benefit of continuous insulation to reduce thermal bridging thru the framing. The exterior sided rigid insulation can also act as air barrier once taped and sealed. In construction, we often observe that the attachment nails are not spot sealed with liquid applied WRB, but this is critical for WRB and AB continuity.
Liquid Applied
Common Products: Prosoco® Cat 5 & MVP, SEMCOworks® , Poly Wall ® Blue Barrier Liquid Wrap 2300.
Positives: Best for air barrier continuity, and waterproofing. No attachment fasteners as its sprayed or rolled on, and works on many different substrates.
Negatives: Most expensive material costs, and usually requires specialized certified installers. Often needs primed coat, and needs to be installed in good weather conditions, which can limit the installation time frame.
The most robust performing system since its easier to catch all the weak points of building with WRB and AB material, but the most expensive overall system. Most products require a primer coat and a final WRB coat. The primer coat works on many various substrates.
Can mix and match application systems - can use a liquid applied products at rough openings and use another type of WRB around the rest of the structure. This is good cost saving measures as it uses the better performing system at a higher risk area, and less expensive everywhere else.
Summary
Balancing budget, performance, and risk is important when deciding on which WRB and AB system to specify. At PLOT we communicate advantages and disadvantages to each system and select the right system for your project.
