Water-Managed Wall Systems, continued
Drainage Planes and Air
There are many choices of drainage plane material, and there's
a lot of discussion over which is best. The most common
drainage planes on houses are still the various building papers
-- asphalt-saturated felt, plastic housewraps, and coated
papers. Housewrap makers like to stress the advantages of their
brands, but the difference between one paper and another is
really a minor issue. It's how you use them that counts, and
the key factors are the air space and the flashings.
Let's take an example. Figure 3 shows a classic drainage
plane: tar paper installed shingle fashion behind the cladding.
In this case, the cladding is stucco. Do you see the air space?
No, because there isn't one. But there's supposed to be one,
and in the old days there usually was. The reason we often
don't get a drainage space behind stucco today is that tar
paper has changed.
Figure 3.Portland cement stucco bonds tightly to
modern asphalt felts, denying water the drainage space it needs
to flow down and escape the wall. Moisture that penetrates
stucco will saturate the paper and destroy its water
repellency, wetting the wall structure beneath unless an air
space is somehow created.
Decades ago, there was a product called 15-pound felt --
asphalt-impregnated rag felt paper that weighed 15 pounds per
100 square feet. Today, in its place we have "#15 felt," which
weighs less than 7 pounds per 100 square feet. In place of the
old 30-pound felt, we now have #30 felt, which weighs 16.5 or
17 pounds. The papers are lighter and contain less
The old heavy felts absorbed water and swelled up when we
applied a scratch coat of stucco over them. They were intended
to swell. Then as the felt paper dried, it wrinkled, shrank,
and debonded from the back of the stucco, creating a thin,
convoluted drainage space. If you tear into an old stucco wall,
you'll see the space. But that process doesn't happen with
modern asphalt felt papers -- instead, the paper bonds to the
stucco and sticks. It gets wet and stays wet, there's no
drainage, and the paper starts to rot. Then your wall is
West of the Mississippi, they figured it out. Western stucco
applicators use two layers of Type D coated paper under their
stucco instead of one layer of asphalt felt, and drainage
occurs between the two layers of paper. The stucco might bond
to the top layer, but the layer underneath stays free. Some
brands of Type D paper even come with pieces of grit stuck on
them, which helps to hold the two layers of paper apart.
My point is, any building paper will fail if it bonds to the
cladding, and the system can't drain without an air space. With
stucco, we create the air space by using two layers of
Building Papers and Drainage
Figure 4 shows a collection of engineers and Ph.D. scientists
setting up an eight-sided test building in my backyard at the
Building Science Corporation. The reason we're working in the
dark is that we spent the whole day arguing about how to do
this experiment. It wasn't until much later that we realized it
would have been a lot quicker to build two four-sided
structures, but that's another story.
Figure 4.The author and his colleagues tested 21
different combinations of building papers and claddings on this
eight-sided structure (top). Measuring the added water (center)
provided a basis for comparing results across wall systems. The
blower door apparatus (bottom) turned out not to matter,
because leaks were apparent without any air pressure
We tested 21 different configurations of
cladding-and-building-paper combinations for water leakage: 14
different combinations of vinyl siding over various sheathings
and papers, and 7 combinations of hard-coat stuccos over
various sheathings and papers. The holes in the wall are
viewports that let us see what happens when we add water.
We depressurized the enclosure to simulate a 100-mph wind
pressure, but we needn't have bothered: Any system that leaked
leaked with no pressure difference. A garden hose, it turns
out, is plenty high tech.
We added a measured amount of water each time. That way, you
can measure how much goes in and how much comes out; you can
find the difference; and if you want, you can even weigh the
different materials before and after to find out how much water
was absorbed. Remarkably, no one has done this kind of test for
most of the wall systems on the market. With all the changes
we've made in materials, no one has ever checked some of these
systems to see if they work. So we tried this experiment with a
few systems, and we learned some interesting things.
Stucco and housewrap. In
Figure 5, we see what happened when we applied a hard-coat
stucco over a housewrap. The stucco bonded so tightly to the
housewrap that when we peeled the paper back, the advertising
transferred to the back of the stucco.
Figure 5.Portland cement stucco stuck tightly to
housewrap (left) and felt paper (right) in the author's tests,
destroying the effective water repellency of the building
That tight contact destroys the water repellency of housewrap,
which works like a tent in a rainstorm: If you touch the wall
of the tent, you cause a leak. Here, the stucco has established
what we call "capillary continuity" by bonding to the
housewrap, and water repellency is lost. That's why you should
never put hard-coat stucco on any plastic housewrap -- the
stucco defeats the housewrap.
Figure 6 shows DuPont's new StuccoWrap, which is a wrinkled
Tyvek. In spite of the name, the only product StuccoWrap
doesn't seem to work with is stucco. When the stucco bonds to
the StuccoWrap, drainage is lost along with water repellency.
Stucco applied to StuccoWrap produced the second worst
performance of the systems in our test (the worst was stucco
applied directly to a nonwrinkled plastic housewrap).
Figure 6.Wrinkled StuccoWrap adhered as tightly to
stucco as did the other papers in the test, losing its ability
to hold out water (left). However, the wrinkles worked
effectively when a layer of inexpensive felt was installed over
the StuccoWrap before the stucco was applied (right). The two
papers in combination provided outstanding drainage and water
protection. For stucco walls, the author recommends
paper-backed stucco lath over wrinkle wrap as a drainage
But StuccoWrap was also part of the best-performing system we
tested. When we added a cheap felt paper over the StuccoWrap
and then applied the stucco, the system worked perfectly. The
cheap paper was a bond break, the StuccoWrap remained free, and
then we saw tremendous drainage in the grooves.
So whenever somebody asks me, "How do you apply stucco to a
building on the East Coast?" I tell them this: Put up OSB,
staple up DuPont Tyvek StuccoWrap, and then put a paper-backed
stucco lath over it -- a product such as Tilath from the
Alabama Metal Industries Company
(www.amico-lath.com), metal lath that comes
with the paper stuck to it. The paper backing creates a bond
break to provide the drainage. The difference between total
success and major failure is that top layer of low-grade,
Comparing housewraps. In
general, it's no use comparing one housewrap to another, at
least in terms of water holdout characteristics. It's how you
use them that counts.
The housewrap marketing people love to compare performance.
But they do it based on a totally meaningless test. The
standard test method is to fasten the housewrap over the bottom
of a glass cylinder and see how high a column of water you can
pour into the cylinder before the water comes through the wrap
(every inch of water is equivalent to 250 pascals of wind
pressure). The lab comes up with some figure, and then the
marketing people go out and brag about it.
That's great, but in the real world we don't build houses with
little glass cylinders. We staple the housewrap to the wall and
nail siding over it. We put thousands of holes in it. That
means the real performance of the system is about the holes --
and whatever the ads say about housewrap performance, when you
put nail holes in them, they're all the same.
And for the system as a whole, the key thing to understand is
that the air space makes all the difference. Water is like a
politician: It always does the easiest thing possible. Whatever
housewrap you have, and however many nail holes there are in
it, if water can go down, it will go down, instead of sideways
through the nail holes. As long as there is an air space, every
housewrap works and every felt paper works. So forget about
comparing housewraps: Make sure there's an air space, and then
concentrate on the flashings.
Insulated Sheathing as a Drainage
Housewrap and felt are not the only drainage plane materials.
Foam sheathing also works well, if you detail it carefully. In
the Building America program
we've had good success using foil-faced rigid polyisocyanurate
as the drainage plane, taping the joints with red Tyvek tape.
We've also used extruded polystyrene boards such as Dow
Styrofoam the same way. It's an economical way to build a
well-insulated wall that performs well.
As in any water-managed system, with foam sheathing the
drainage space is critical. Foam-sheathing drainage planes work
well with vinyl, because you create the air space just by
putting the vinyl up. Vinyl is also self-ventilating -- I don't
happen to like its looks, but it performs beautifully. Brick
also works well over foam sheathing.
Wood siding will not work well over a foam drainage plane
unless you space the wood away from the wall with furring
strips. In fact, you should fur out wood clapboards over
housewrap and asphalt paper as well. It's the only way to allow
the wood to dry evenly, and to prevent substances in the wood
from degrading the building paper. Wood siding should also be
primed on the back and ends, not just painted on the weather
I have a bias against tapes and sealants. I don't like to rely
on adhesives. So we have developed a way of building a
foam-sheathing drainage plane where you don't have to tape the
joints. We use a shiplap vertical joint where the boards butt
together, and at the horizontal joints we attach a strip of
poly that acts as a Z-flashing (Figure 7, page 6). That poly,
with the help of gravity, keeps the water on top of the
drainage plane.Figure 7.Extruded polystyrene or foil-faced
polyethylene can be an effective drainage plane material.
Joints in the foam can be taped with red Tyvek tape (top). To
reduce reliance on tape or sealant, the author likes to apply
the foam with shiplap joints at the vertical seams and a black
poly strip as a Z-flashing at horizontal joints