ASHRAE Journal - August 2019 - 61

COLUMN BUILDING SCIENCES

PHOTO 1 Thousands of
Holes. Cedar shingles nailed
through a house wrap water
control layer resulting in
thousands of holes on a
large house on the ocean
in New England. But the
wall assembly does not leak
because the hydrostatic
pressure driving force is
controlled by the drainage
mat installed between the cedar shingles and the mechanically attached water
control layer resulting in, wait for it, "drainage."

controlled by the drainage mat installed between the
cedar shingles and the mechanically attached water control layer resulting in, wait for it, "drainage."
It was clear that if you drained the water penetrating
the cladding you would control the hydrostatic pressure
and small holes would not matter. This is not new news.
I think Vitruvius got there two millennia ago.†
The obvious "ah-ha" for more than two millennia is
that you can't drain water on a drainage plane (aka,
"water control layer" or "water resistant barrier [WRB])
without a drainage space or gap. The issue for quite
some time has been twofold: how big of a space or gap do
you need and when do you need one.
So where am I going with this? Well, I think we now
have a reasonable metric on drainage (an "assembly" metric), and that gives us lots and lots of options
because with good drainage pretty much everything
works. I still don't think we have a reasonable metric for water control layer materials (the "material"
metric), more on that later. The good news is that
assembly performance trumps material performance.
Always.
Time for some heresy: with a good drainage metric
I don't think we need much of a material metric for
water control layers. The goal should be to get rid of
the crazy metrics that restrict materials and innovative
approaches-if you have good drainage.
Although we now have a reasonable metric on drainage (in my opinion) and the necessary associated drainage space or gap (in my opinion) we do not have much
agreement on when we need the drainage and the gap.
Some background is necessary here.
†"De architectura," Marcus Vitruvius Pollio, around 25 BC and yes,
we were here before ("Vitruvius Does Veneers," ASHRAE Journal,
May 2015).

Photo 2 Stucco Failure.
Not a pretty sight. No drainage coupled with moisture
sensitive sheathing-oriented
strand board (OSB)-and
high levels of thermal resistance (not enough energy
flow to facilitate drying).

Photo 3 EIFS. There were
thousands of EIFS buildings
with failures due to the lack
of drainage between adhered
expanded polystyrene (EPS)
rigid insulation and sheathings, both oriented strand
board (OSB) and gypsum
board. Today, "water managed EIFS" is an excellent
system; robust, durable
and energy efficient. EIFS went through a "near death" market experience and
came roaring back with a high performance product. "Hard coat" stucco folks
take note. You need to do to stucco what got done to EIFS, drainage, drainage,
drainage.

We have thousands of buildings with stucco failures
to thank for the metric on drainage (Photo 2). Yes, we
were also here before ("The Coming Stucco-Pocalypse,"
ASHRAE Journal, January 2018 and "The Perfect Storm
Over Stucco, ASHRAE Journal, February 2008).
The model building codes (International Building
Code, aka "IBC" and the International Residential Code,
aka "IRC") will both have the following requirement
for stucco installed over wood based sheathings in the
imminent future:
"a minimum 3/16 inch space shall be provided between
the stucco and water-resistive barrier or a space having a drainage efﬁciency of not less than 90 percent, as
measured in accordance with ASTM E2273 or Annex A2
of ASTM E2925, shall be added to the exterior side of the
water-resistive barrier."
Yeah! Finally! About time! We also have to thank
American Society for Testing and Materials (ASTM).
Let's look a little more closely at the code referenced
ASTM Standard. Guess what? ASTM E2273 is "Standard
Test Method for Determining the Drainage Efﬁciency
of Exterior Insulation and Finish Systems (EIFS) Clad
Wall Assemblies." Guess why we needed a test method
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.
A U G U S T 2 0 19

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