ASHRAE Journal - August 2019 - 60

COLUMN BUILDING SCIENCES
Joseph W. Lstiburek

Drain the Rain, On the
Plane. The Drainage
*
Plane.
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Ten years ago ("Taming the Terminology," ASHRAE Journal, October 2009), I issued a
challenge regarding water control layers asking folks for ideas regarding a performance metric for water control layers. I suggested that we might need a material
metric or an assembly metric or perhaps both. At the time no rational performance
metric or metrics existed for a water control layer.
There were irrational ones. For example, how about
requiring a water control layer (a "material metric") to
resist 25 inches of standing water (6,200 Pa-the force
exerted by a 225 mph wind)? This is obviously madness
(yes, some products are rated this way) as house wrap
products that are rated this way and pass the test subsequently leak when they are installed on a wall with holes
created by staples and cap nails that are used to install
them and through nail holes penetrating them when
cladding is attached.
What is the point of having a metric that assumes
no holes? How would you install it? Yup, put holes in
it. Or if you can get it installed without holes how do
you attach stuff to it or through it? Yup, you put holes
in it. Couldn't you seal the holes after you put them

in? Sometimes, but mostly take "good luck" for an
answer.
But house wrap products-and water control layers in
general-with holes in them do not always leak. Huh?
Yes, you read that right. They don't always leak even
though they have holes-lots of holes. They can in fact
have thousands of holes and not leak. You also need a
driving force-hydrostatic pressure is the big one-air
pressure not so much-and yes we were here before
("Hockey Pucks and Hydrostatic Pressure," ASHRAE
Journal, January 2012). Check out Photo 1: 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

*My wife and I love musicals. When we were dating in early 1990, one evening we were both watching "My Fair Lady." Just after Eliza
Doolittle says "The rain in Spain stays mainly in the plain" I blurted out "You need to drain the rain on the plane" and the term "drainage
plane" was born. Yes, alcohol was involved. The next day I was lecturing on rain control and I got the class of builders and architects to
chant "you need to drain the rain on the plane." I pointed out that the rain draining on the "drainage plane" needed to be directed to a
ﬂashing, and that lead to the next spur of the moment chant "if you want to save cash, ﬂash...." followed by "don't be a dope, slope."
I have been milking these lines now for almost 30 years. I am more than somewhat bemused that codes and manufacturer's literature
today refer to "drainage planes"-if they only knew.

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ASHRAE Journal - August 2019

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