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Permeability Makes Perfect

Posted by Bijan Mansouri, Guest Columnist

Oct 11, 2019 5:24:21 PM

Advances in technology and increased builder awareness are making the fight against water leakage more effective than ever.

In the building industry’s ever-increasing pursuit of tighter and more waterproof structures, what are we at risk of losing? Is there a point at which we build a wall that is too tight? While a water-tight assembly is vitally important for controlling issues such as mold growth and protecting indoor air quality, some building practices may be inadvertently making it easier for moisture-related issues to fester. After all, no matter how tightly a wall is constructed, water is inevitably going to find its way in. There’s no such thing as a “waterproof” wall, just one that is built so tightly it is almost guaranteed to get wet and stay wet.

Breathable - building-web

There are a growing number of methods for managing moisture, driven by advances in material technology, evolving building codes, and a growing awareness among end users for mold prevention, indoor air quality and energy efficiency.

The highest-performing wall assemblies are ones that have been designed to realistically manage moisture and dry out, not those designed with the unachievable goal of completely locking out all moisture. The good news is that there are a growing number of methods for managing moisture, driven by advances in material technology, evolving building codes, and a growing awareness among end users for mold prevention, indoor air quality and energy efficiency.

Managing Water in many Forms

Water can find its way into a wall in numerous ways. High humidity and extreme temperatures can cause vapor diffusion when warm indoor air causes condensation on colder outside surfaces. Wind-driven rain can be forced into small openings in the exterior cladding at joints, laps and utility cutouts, and wind blowing around the building can create a negative pressure within the wall assembly which siphons water into the wall.

When reservoir claddings become wet from rainwater or condensation and are then warmed by the sun, the vapor pressure of the stored water increases, driving it inward and outward from the cladding material. Where that moisture goes from there—and how quickly it gets there—is largely a function of how permeable the adjacent building materials are within the assembly.

Breathable - Installation-web

Typar Drainable Wrap features an additional layer of integrated polypropylene fibers that create a highly efficient gap for shedding excess water.

Permeability measures the amount of vapor transmission that a building material will allow over a period of time. ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, addresses two testing procedures for measuring permeability: the Desiccant Method and the Water Method.

In the Desiccant Method, the material to be tested is sealed to a test dish containing a desiccant, or drying agent, and the assembly is placed in a controlled atmosphere. Periodic weighing determines the rate at which water vapor has moved through the specimen into the desiccant. In the Water Method, the dish instead contains distilled water, and periodic weighing determines the rate of vapor movement through the specimen from the water.

In most wall assemblies, outwardly driven moisture won’t cause many problems (unless you’re dealing with a material like stucco that’s been painted with a low-perm paint, in which case you would see bubbling and cracking). But the inwardly driven moisture presents a problem, especially in situations where conditioned indoor air is much cooler than the warm, moist exterior.

Typically, this inwardly driven moisture vapor is managed by separating the cladding from the rest of the assembly with a capillary break, which can be a gap or a sheathing material that sheds water or does not absorb or pass water. Impermeable sheathing, such as extruded polystyrene (XPS), is one option for halting inward vapor drive. In these types of assemblies, the inwardly driven moisture condenses on the surface of the XPS sheathing and drains downward.

But in situations where a reservoir cladding is paired with a highly permeable sheathing like gypsum board (which can be as high as 50 perms) or a moisture-retentive material like oriented strand board (OSB), an air gap may not be enough to slow down inward moisture intrusion. In these applications, an added weather resistant barrier (WRB)—commonly referred to as a building or house wrap—is needed to reduce unwanted moisture intrusion.

In the recent paper, “Inward Drive – Outward Drying,” building scientist Joseph Lstiburek identifies the “sweet spot” for the permeance of this WRB layer as between 10 and 20 perms. Too high, he writes, and the moisture driven out of the back side of the reservoir cladding into the air space will blow through the layer, through the permeable sheathing and into the wall cavity. Too low, and the outward drying potential of the cavity is compromised. Thankfully, advances in building wrap technology are adapting to meet this need.

Breathable - Typar Drainable Wrap-web

Changing building codes and greater adoption of certain cladding materials have caused specifiers to take a closer look at how they manage moisture in the wall assembly.

Evaluating WRBs

Due to their durability and ease of installation, plastic building wraps made of polyethylene or polypropylene fabric have been a popular method of protecting against moisture intrusion since the 1970s. But as building assemblies have gotten tighter, building wraps have taken on a new function—helping to remove trapped water from the building enclosure. Their unique functionality enables them to block moisture from the outside while also allowing walls to “breathe” to prevent vapor buildup. And the very latest innovations in building wrap technology is also taking this moisture removal function one step further to incorporate drainage capabilities.

The 2018 International Building Code (IBC), Section 1402.2, Weather Protection, requires that exterior walls “provide the building with a weather-resistant exterior wall envelope…designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer…and a means for draining water that enters the assembly to the exterior.” This water-resistive barrier, as defined by Section 1403.2, is comprised of at least “one layer of No. 15 asphalt felt, complying with ASTM D226 for Type 1 felt or other approved materials…attached to the studs or sheathing.”

It is important to note the difference between a weather resistant barrier and a water resistant barrier, as they have distinct purposes yet are often confused with one another. The American Architectural Manufacturers Association (AAMA) defines weather-resistant barriers as a surface or a wall responsible for preventing air and water infiltration to the building interior. The differentiating factor is that a weather-resistant barrier must also preventing air infiltration, while water-resistant barriers are only responsible for preventing water intrusion.

The International Code Council Evaluation Service (ICC-ES) evaluates the following key performance characteristics for building wrap, which provide a valuable starting point for deciding which product best suits your project.

Water Resistance

As its most basic function, a building wrap must hold out liquid water. A premium building wrap will be able to pass both “water ponding” tests, which measures a house wrap’s resistance to a pond of one-inch water over two hours, and a more stringent hydrostatic pressure test where the wrap is subjected to a pressurized column of water for five hours.

Air Resistance

According to the Air Barrier Association of America (ABAA), an air barrier system is a system of building assemblies within the building enclosure—designed, installed and integrated in such a manner as to stop the uncontrolled flow of air into and out of the building enclosure. Because an air barrier isolates the indoor environment, it plays a major role in the overall energy efficiency, comfort and indoor air quality of a building. According to the U.S. Department of Energy, up to 40 percent of the energy used to heat and cool a building is due to uncontrolled air leakage. As such, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 and the IECC both include air barrier requirements.

For an individual building material to be classified as an air barrier, its air permeance must be equal to or less than 0.02 L/(s-m2) @ 75 Pa when tested in accordance with ASTM E2178. However, this air permeance test only measures the amount of air that migrates through the material itself and not through holes or gaps in the larger assembly. Therefore, it is important to keep in mind that a material’s effectiveness as an air barrier is largely dependent on proper installation and the use of compatible tapes, fasteners and sealants.


The ICC-ES looks at the tear resistance and tensile strength as the best measure of a building wrap’s durability, since it must be able to withstand the handling and application process without compromising its water resistance. UV and low temperature resistance are also important measures of durability because prolonged exposure to the elements can compromise the integrity of the product or cause it to crack.

Vapor Permeability

For a product to be considered a building wrap and not a vapor retarder, ICC-ES mandates the permeance rating must be higher than 5 perms. But there are a variety of ways permeability is achieved, and as noted by Lstiburek, a higher perm rating doesn’t always equal a better building wrap.

When selecting a building wrap, look for one that hits the “sweet spot” of 10 to 20 perms to achieve the desired balance of moisture protection and drying capacity. For example, some wraps have mechanical micro-perforations, which may allow the passage of more water vapor, but could also be more vulnerable to bulk water leakage. Generally, it’s better to go with a higher quality, non-perforated or micro-porous product, which allows for sufficient vapor mitigation while providing excellent resistance to bulk water.


Drainage is widely accepted as one of the most effective measures for reducing moisture damage due to rain penetration. Drainage is a critical component in allowing a building wrap to do its job, particularly in keeping walls dry. Usually this involves the use of furring strips that separate the wrap from the structural sheathing and framing, but new technologies have emerged that are helping to simplify this process.
No matter how advanced a WRB material is, it alone cannot be counted on to protect a structure from unwanted air and moisture movement without taking the whole assembly into consideration. It is important to specify compatible materials to ensure all components work together.
Changing building codes and greater adoption of certain cladding materials have caused specifiers to take a closer look at how they manage moisture in the wall assembly—and that’s a great thing. Advances in building wrap products have added a powerful tool to help achieve these goals. The smart way forward is to avoid waterproofing the wall to the detriment of breathability, but rather to take a holistic approach to designing a wall system that provides adequate protection against water yet can also dry out when it inevitably gets wet. \\

Bijan Mansouri is technical manager at TYPAR Construction Products. He is responsible for building code requirements, designing and development of new construction products, training builders and architects on their application, and creating and teaching proper practice and installation of building envelopes.

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