ASHRAE Journal - December 2020 - 59

COLUMN BUILDING SCIENCES

PHOTO 1 Water Test. I have tricked a youngster
into using an ordinary garden hose to spray water
against a brick veneer. This field test follows no
approved ASTM protocol.

PHOTO 2 Water Leakage. In about 20 to 30 seconds water can be seen penetrating and running
down the back of the brick. The water penetrates
the joints between the brick and the mortar. The
water does not pass through the brick or the mortar. The weak link is the connection between the
brick and the mortar.

FIGURE 3 " Down and Out. " Drain penetrating water
down and out of the wall. That flashing at the bottom of the wall is a big deal. One of its important
functions is to direct the penetrating water back to
the exterior.

Building Paper

Down
Building Paper
Flashing

Out

clearly a " hoser. " ‡ In about 20 to 30 seconds water can
be seen penetrating and running down the back of the
brick (Photo 2§). The youngster and everyone else watching turned into " true believers. " # This should, of course,
have been obvious, but folks are still stunned when they
actually see it.
So what do we do with this penetrating water? Easy.
Drain it down and out of the wall. Repeat after me
" down and out, down and out. " The principle is obvious
(Figure 3). Except not so obvious, as we shall see. Folks
forget the " out " part. Ouch. That ﬂashing at the bottom
of the wall is a big deal. One of its important functions is
to direct the penetrating water back to the exterior. Oh,
one more thing: don't forget to actually have a ﬂashing.
Let's look at Photo 3. The conversation started like
this: " Our bay bump outs seem to be leaking,but not all
of them. We don't understand. We build them all the
same way. We use the same roofer. " Whenever I hear
the words " we build them all the same way " my " Spidey
sense " II tingles. My response always is " let's go and take
‡Yes,

this uncontrolled, un-ASTM sanctioned experiment took place
in Canada.

§I did this at a garage wall. It tends to be difﬁcult to convince
someone to open up one of their walls just to show them how things
work. I did this at a builder's " model home " where the builder's construction ofﬁce was in the garage.
#They
IIThe

PHOTO 3 Bay Bump Outs. Two bay bump outs are shown. The vinyl siding has
not yet been installed above one bay bump out, but the brick has been installed
above the other. The metal roof flashing is sealed to the plastic membrane water
control layer in the wall where the vinyl siding is to be installed. On the brick
wall, the metal roof flashing is sealed to the face of the brick veneer. The flashing
does not penetrate through the brick and connect to the plastic membrane water
control layer behind the brick.

a look at a bunch of them. " Turns out that when the bay
bump outs had vinyl siding above them they did not
leak, but when they had brick above them they did. In
Photo 3, two bay bump outs are shown. The vinyl siding has not yet been installed above one bay bump out,
but the brick has been installed above the other. The
metal roof ﬂashing is sealed to the plastic membrane

joined the " cult of drainage. "

Amazing Spider-Man, a great, great comic book for a nerd in
the 1960s (I should know).

Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.

D ECEM BER 2020

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