First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 43

Rain penetration
control elements

Rain penetration
control elements

Rain penetration
control elements

First line of
defence

Second line
of defence

First line of
defence

Second line
of defence

First line of
defence

Second line
of defence

Inner
boundary of
second line
of defence
( e.g., asphait-
impregnated
paper of
polymeric
membrane )

Inner
boundary of
second line
of defence
( e.g., polystyrene
insulation with
capped joints )

Inner
boundary of
second line
of defence
( waterproof
membrane )

Heat, air and
vapour control
with structural and
other elements

Heat, air and
vapour control
with structural and
other elements

Heat, air and
vapour control
with structural and
other elements

(b) (c)

(a)

Figure SH2.1 Examples of a rain screen system and traditional exterior sheathing. Note how
the rain screen leaves an airgap to allow for moisture dissipation.

water that makes it past the first layer and to dissipate it adequately back to the
exterior.
The primary difference between a rain screen and a traditional exterior
sheathing is the anticipation that some water will penetrate the cladding
and will require dissipation back outside the wall. With a traditional exte-
rior sheathing, water will often penetrate a crack or other breaks between
two impermeable surfaces and become trapped inside a wall space, causing
long-term damage to the wall, as well as an ideal condition for the growth of
mold and bacteria.
Because buildings with rain screens anticipate penetration of water to their
second line of defense, it is very important to select a competent product based
on its ability to repel water on a regular basis. This second line of defense is
called a building wrap , a flexible sheet that can be molded to the contours of a
building and easily cut to specific customized shapes.
Examples of building wraps that were considered for use in the Home
Depot Smart Home at Duke University are Vaproshield, Tyvek Homewrap,
Tyvek Commercialwrap, Grace Ice and Water Shield, and No. 15 asphalt felt.
Important factors for selecting a building wrap include air penetration, bulk
water holdout, vapor permeability, ultraviolet (UV) exposure tolerance, and