ASHRAE Journal - April 2020 - 78

COLUMN BUILDING SCIENCES

To state the obvious, wall assemblies are
FIGURE 6 Drained. Rainwater entry controlled by providing drainage (Courtesy of Straube, 2012).
constructed from materials that include
claddings, sheathings, insulations and components such as windows and doors. How
all these pieces come together lead to three
general approaches to dealing with rainwater. Some assemblies have more than one
layer dedicated to rainwater control and
provide drainage. Such assemblies are called
"drained." Other assemblies do not provide
Lap Siding
Pane Cladding Systems
Masonry Veneer
drainage. Such assemblies are either barriers or mass/storage assemblies. So at the end
FIGURE 7 Barrier. Rainwater entry prevented by providing a barrier (Courtesy of Straube, 2012).
of the day we have three approaches*:
* Drained (Figure 6);
* Barrier (Figure 7); and
* Mass/storage (Figure 8).
Personally, I think drained is absolutely
the best way ("Hockey Pucks and Hydrostatic
Pressure," ASHRAE Journal, January 2012 and
"Drain the Rain, On the Plane," ASHRAE
Journal, August 2019). Now it is not always
possible to drain rainwater that enters
buildings assemblies. A barrier is conStructural Glazing
Steel-Clad Foam Panels
Face-Sealed EIFS
structed from materials that are defined as
waterproof such as glass, steel, dense conFIGURE 8 Mass/Storage - Rainwater entry controlled by providing materials that can store, absorb,
crete or roof membranes. Mass/storage uses
redistribute and dry (Courtesy of Straube, 2012).
materials that can store, absorb, redistribute
and dry rainwater without being damaged
such as multi-wythe brick assemblies.
Drained assemblies have multiple lines of
defense for rainwater entry. In addition to
the exterior cladding, drained assemblies
layer building materials shingle fashion
inboard of the exterior cladding to direct
rainwater that penetrates the cladding
Rubble
Solid Masonry
Composite/Layered
back to the exterior. An inward layer whose
function is to control penetrating rainwater
is sometimes employed and is called the water control
are designed and constructed to drain the rainwater
layer. A space or air gap between the water control
that passes through the cladding. They are interconlayer and other materials promotes drainage, ventilanected with flashings, window and door openings and
tion and moisture redistribution.
other penetrations of the building enclosure to provide
Water control layers are typically water repellent
drainage of rainwater to the exterior of the building.
materials (building papers, house wraps, rigid insuThe materials that form the water control layer overlap
lations, fluid applied coatings, fully adhered memeach other shingle fashion or are sealed or are continubranes, sheet goods) located behind the cladding and
ous so that rainwater flow is down and out of the wall.
*The best, and I really mean best, discussion on all of this comes from John Straube. Check out the bibliography at the end of this column. He is even better in person; if you can see him present you are in for a treat.
78

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ASHRAE Journal - April 2020

Table of Contents for the Digital Edition of ASHRAE Journal - April 2020

Contents
ASHRAE Journal - April 2020 - Intro
ASHRAE Journal - April 2020 - Cover1
ASHRAE Journal - April 2020 - Cover2
ASHRAE Journal - April 2020 - 1
ASHRAE Journal - April 2020 - Contents
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ASHRAE Journal - April 2020 - Cover4
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