First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 44

whether they are certified as a weather-resistive barrier:
Air penetration . Rain screens benefit from having a building wrap with
low air penetration because when air penetrates the exterior sheathing
of a building, it can create a pressure gradient that drives water into and
through the exterior wall into areas not designed to handle water penetra-
tion. Rain screen systems therefore require a building wrap with low air
penetration. Air penetration rates lower than 0.004 cfm/ft 2 at 75 Pa are
considered acceptable. This is measured using standard ASTM E2178. *
Water resistance . The primary function of the building wrap is to stop
any water that reaches it from passing through to the other side. Water
resistance is measured as a function of the pressure required to drive water
through a fabric. Water resistances at hydrostatic pressures greater than
55 cm of water are considered acceptable. This is measured using AATCC
test method 127.
Water vapor transmission . The purpose of a rain screen system is to allow
for the dissipation of water back to the outside if it has penetrated.
High rates of water vapor transmission are therefore desired. Water vapor
transmission rates greater than 20 perms † are considered acceptable. This
is measured using standard ASTM E96M-05.
UV exposure tolerance . Because part of the Home Depot Smart Home
at Duke University building wrap might be exposed to sunlight, it was
important to use a material that has a high tolerance for ultraviolet (UV)
light exposure without breaking down. This was not measured using
a standardized test. Instead, manufacturer data were relied upon in the
comparative analysis.
Certification of building code compliance . Some, but not all, building wraps
are certified by a national organization to show that they meet building
codes as a water-resistive barrier.
These factors are compared in Table SH2.1 Tyvek Commercialwrap was
chosen for its superior air penetration rate, bulk water holdout rate, vapor
permeance rate, tolerance for UV exposure, and certification by a national
organization for meeting building code as a water-resistive barrier.
* ASTM International, formerly the American Society for Testing and Materials, is a voluntary
standards development organization.
† Permeability is a measure of the amount of water vapor (moisture) that can pass through a
specified material in a certain amount of time. The measure and degree of permeability is
expressed in units referred to as perms. A list of the various ASTM testing procedures can be
found at http://omnova.com/products/wallcovering/perm test.aspx