3 Forgotten Variables to Consider When Choosing a Weather Resistant Barrier

Codewatcher Typar

In the third of a four-part series, a building science expert calls out important areas specifiers must consider when selecting a weather resistant barrier.

The highest performing wall assemblies are the ones that have been designed to realistically manage moisture and dry out, not those designed with the unachievable goal of completely blocking out all moisture.

We drilled into this concept in previous blogs “The Sweet Spot For Building Wrap Permeability,” and “Weather Resistant Versus Water Resistant Barrier,” where Bijan Mansouri, Technical Manager, TYPAR Construction Products, discusses the building science behind choosing the right weather resistant barrier (WRB) for a project.

“There are a growing number of methods for managing moisture, all being driven by advances in material technology, evolving building codes, and a growing awareness in the industry regarding mold prevention, indoor air quality and energy efficiency,” Mansouri says.  However, outside of the general advice and evaluation criteria many building professionals already adhere to when it comes to selecting a WRB, there are three other variables that must remain top of mind, emphasizes Mansouri:

  1. Cladding. In addition to the challenges associated with reservoir claddings, other types of cladding require careful consideration when it comes to managing moisture. For example, tightly fastened cladding such as cedar siding or fiber cement board might allow water trapped between the siding and a smooth building wrap to pool and could eventually make its way through the wrap and into the framing. These are cases where a drainable building wrap would provide the needed capillary break to allow water to drain out of the assembly.
  1. Surfactant Resistance. The water resistance of a building wrap can be degraded by chemicals known as surfactants (or surface-active agents), often found in cladding materials such as cedar and stucco as well as in solutions used to power wash siding. These chemicals reduce the surface tension of water, easing its ability to pass through microscopic openings in the membrane. Some building wraps offer added protection against the harmful effects of these chemicals, which might be an important consideration for assemblies constructed using these materials.
  1. Geography and Climate. The International Building Code mandates that an exterior wall assembly incorporate “a means for draining water that enters the assembly through the exterior.” However, a growing number of states have added even more prescriptive measures to their codes. Oregon, for example, requires that the building envelope shall consist of an exterior veneer, a water-resistive barrier, a minimum 1/8” (3mm) space between the WRB and the exterior veneer, and integrated flashings. The envelope must provide proper integration of flashings with the water-resistive barrier, the space provided, and the exterior veneer. In lieu of providing the 1/8” space between the exterior veneer and the WRB, builders can install the exterior veneer over a water-resistive barrier that is manufactured to enhance drainage and meets the 75% drainage efficiency requirement of ASTM E2273 or other recognized national standards. TYPAR Drainable Wrap, which has a drainage efficiency of 94.8%, would meet that requirement.

As a rule of thumb, the Building Enclosure Moisture Management Institute recommends that any area receiving more than 20 inches of annual rainfall should incorporate enhanced drainage techniques in the wall system, especially if using an absorptive cladding material. Areas receiving 40 inches or more should utilize rainscreen design regardless of cladding material. The orientation of the wall, amount of overhang, altitude, and even nearby trees can also have an impact on how much water intrusion can be expected and how likely it is to dry.

“No matter how advanced a WRB material is, it alone cannot be counted on to protect a structure from unwanted air and moisture intrusion without taking the whole assembly into consideration,” Mansouri reminds. “It is important to specify compatible materials to ensure all components work together and to make sure you know the specific manufacturer and code guidance for the region in which you are building.”

Click here to learn more about how TYPAR Drainable Wrap can help you better manage moisture in your homes.