Page 44 - Green Builder Sept-Oct 2019 Issue

barriers as a surface or a wall responsible for preventing air and measures of durability because prolonged exposure to the elements
water infiltration to the building interior. The differentiating factor is can compromise the integrity of the product or cause it to crack.
that a weather-resistant barrier must also preventing air infiltration,
while water-resistant barriers are only responsible for preventing VAPOR PERMEABILITY
water intrusion. For a product to be considered a building wrap and not a vapor
The International Code Council Evaluation Service (ICC-ES) evalu- retarder, ICC-ES mandates the permeance rating must be higher than
ates the following key performance characteristics for building wrap, 5 perms. But there are a variety of ways permeability is achieved, and
which provide a valuable starting point for deciding which product as noted by Lstiburek, a higher perm rating doesn’t always equal a
best suits your project. better building wrap.
When selecting a building wrap, look for one that hits the “sweet
WATER RESISTANCE spot” of 10 to 20 perms to achieve the desired balance of moisture
As its most basic function, a building wrap must hold out liquid protection and drying capacity. For example, some wraps have
water. A premium building wrap will be able to pass both “water mechanical micro-perforations, which may allow the passage of more
ponding” tests, which measures a house wrap’s resistance to a pond water vapor, but could also be more vulnerable to bulk water leakage.
of one-inch water over two hours, and a more stringent hydrostatic Generally, it’s better to go with a higher quality, non-perforated or
pressure test where the wrap is subjected to a pressurized column micro-porous product, which allows for sufficient vapor mitigation
of water for five hours. while providing excellent resistance to bulk water.
AIR RESISTANCE DRAINAGE
According to the Air Barrier Association of America (ABAA), an air Drainage is widely accepted as one of the most effective measures
barrier system is a system of building assemblies within the build- for reducing moisture damage due to rain penetration. Drainage
ing enclosure—designed, installed and integrated in such a manner is a critical component in allowing a building wrap to do its job,
as to stop the uncontrolled flow of air into and out of the building particularly in keeping walls dry. Usually this involves the use of
enclosure. Because an air barrier isolates the indoor environment, furring strips that separate the wrap from the structural sheathing
it plays a major role in the overall energy efficiency, comfort and and framing, but new technologies have emerged that are helping
indoor air quality of a building. According to the U.S. Department to simplify this process.
of Energy, up to 40 percent of the energy used to heat and cool a No matter how advanced a WRB material is, it alone cannot be
building is due to uncontrolled air leakage. As such, the American counted on to protect a structure from unwanted air and moisture
Society of Heating, Refrigerating and Air-Conditioning Engineers movement without taking the whole assembly into consideration.
(ASHRAE) Standard 90.1 and the IECC both include air barrier It is important to specify compatible materials to ensure all compo-
requirements. nents work together.
For an individual building material Changing building codes and greater adoption of
to be classified as an air barrier, its certain cladding materials have caused speci-
air permeance must be equal to or fiers to take a closer look at how they
less than 0.02 L/(s-m2) @ 75 Pa when manage moisture in the wall
tested in accordance with ASTM E2178. assembly—and that’s a
However, this air permeance test great thing. Advances in
only measures the amount of air that building wrap products
migrates through the material itself have added a powerful tool
and not through holes or gaps in the COURTESY OF TYPAR CONSTRUCTION PRODUCTS to help achieve these goals.
larger assembly. Therefore, it is impor- The smart way forward is to
tant to keep in mind that a material’s avoid waterproofing the wall to
effectiveness as an air barrier is largely the detriment of breathability, but
dependent on proper installation and rather to take a holistic approach to
the use of compatible tapes, fasteners designing a wall system that provides
and sealants. adequate protection against water yet can
also dry out when it inevitably gets wet. GB
DURABILITY
The ICC-ES looks at the tear resistance Inspection clause. Bijan Mansouri is technical manager
and tensile strength as the best measure Changing building codes at TYPAR Construction Products. He is
and greater adoption of
of a building wrap’s durability, since it certain cladding materials responsible for building code requirements,
must be able to withstand the handling have caused specifiers to designing and development of new construction
and application process without compro- take a closer look at how products, training builders and architects on their
mising its water resistance. UV and low they manage moisture in application, and creating and teaching proper
temperature resistance are also important the wall assembly. practice and installation of building envelopes.

42 GREEN BUILDER September/October 2019 www.greenbuildermedia.com

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