Water-Managed Wall Systems, continued
Figure 8 shows a system using housewrap as the drainage plane.
The builder has attached the window to the drainage plane with
a membrane strip over the nailing flange. Rainproof, right?
Sealing window flanges to the housewrap
(top left) is a common practice, but it does not protect
against the predictable leakage within the window assembly
itself. Only a pan flashing that dumps on top of the drainage
plane will effectively keep water out of the wall.
Peel-and-stick membrane applied over the top of the building
paper and wrapping the sides and bottom of the rough opening
(top right) accomplishes the task. Another option is Jamsill
Guard (center), an
injection-molded plastic sill pan with seamless corners. The
author highly recommends Tyvek FlexWrap for pan flashings,
whether the drainage plane is housewrap or rigid foam
In America, we build as if windows don't leak. We assume that
if we seal the window flange to the building paper, we're done.
But in fact, most of the window leakage happens behind that
seal, because the windows themselves come from the factory
leaking. All windows leak at the joints. The only things that
leak worse than windows are doors -- especially sliding
My colleagues and I conducted a survey of over 3,500 vinyl
windows that were less than two years old -- factory
manufactured, precision engineered. We found that 20% of them
had already begun to leak. So if you build a house with 20
vinyl windows, the odds are that 4 will leak right away (others
will leak later). Which 4 windows do you want leaking into the
wall? None of them, of course. So we have to assume that every
window leaks and build accordingly.
But what is the common practice? Cut a big X in the housewrap
and wrap it into the window. And where does that leave a hole?
Right at the corner, just where the window leaks. We line our
holes up on top of each other.
That's obviously not going to work. What we need is some kind
of gutter under the window that collects the water dripping
onto the sill and kicks it to the outside. There are a couple
of good ways to do this.
Flexible peel-and-stick. In
the bottom photo in Figure 8, we're demonstrating a really neat
product. I like to give the DuPont people a hard time
occasionally, but I love their new product Tyvek FlexWrap. It's
a formable flashing -- sort of an elastic ice and water shield
that you can stretch under a window and mold into a flashing.
It sticks beautifully. You put it in the opening and then
install a window. Now your wall's protected when the window
inevitably leaks. Ordinary, nonflexible peel-and-stick
membranes can be used for the same purpose, but it takes some
cutting and patching. In the upper right photo in Figure 8, the
builder has used a membrane called Blueskin, from Bakor, Inc.
With foam sheathing, it's particularly important to drain all
water on top of the foam, not behind it, because the foam can
trap the moisture inside the wall. In Figure 8, we're showing
how to install FlexWrap over foam. Tyvek doesn't recommend
that, maybe because if you stick the flashings right to the
foam, you might not buy any Tyvek. But I recommend FlexWrap as
a window pan flashing with foam-sheathing drainage planes (it
also works with other brands of housewrap).
Even water puddled on the sill will dry out better than water
absorbed by the wall. Some builders tack a small strip at the
back of the opening to block water from moving into the
interior, which is a good idea (Figure 9).
Figure 9.A strip of wood nailed at the back of the
rough opening sill forms a dam to prevent water from escaping
to the interior (top). Even better is a piece of wood bevel
siding nailed over the sill to create positive drainage toward
the exterior (bottom).
But it would be best to slope the sill, so the water will flow
out over the drainage plane. A quick trick is to tack a piece
of beveled siding over the sill to create a slope to the
outside, then apply the flexible membrane. If you need a flat
spot for your window to rest on, you can take small pieces of
the same beveled material and reverse them to create small
level pads on top of the membrane. The membrane seals around
Another option is what I call a "window booty." It's a premade
metal pan flashing that might cost $10 from a metal
fabricator's shop. In my opinion, windows should come with a
preformed plastic flashing in this shape. But that would be
admitting that the window might leak, and the lawyers don't
want that. So for now we have to make our own.
When you attach the pan, don't nail down through it into the
sill; fasten it through the vertical ears. Then wrap
peel-and-stick over it around the window jacks. (With the
FlexWrap pan flashings, I don't wrap the window sides. That's
not the big leak point anyway, so I trust the seal to stick to
Integrating the window. When
we install the window, we integrate it into the drainage plane.
The housewrap or felt above the window must lap over the top
flange or the top window flashing; we tape the side flanges of
the window to the housewrap on the side. But we don't seal the
bottom flange to the wall. That's the weep hole: We want water
to come out there if it has to. We will also provide weeps at
the bottom of the wall, though.
A window placed in a wall should have redundant drainage
systems. The window itself should drain; the opening the window
is in should drain; and the wall the opening is in should
drain. At every joint, flashings should kick water to the
exterior. If you don't provide drainage but trust to caulks and
sealants instead, you're asking for trouble.
Figure 10 shows what I'm talking about. We've torn the
cladding off beneath a window, exposing terminal rot in the
wood structure. It's evident from the dark decay pattern that
the water intrusion originated at the window's corner and
center mullion joints, then spread laterally and downward to
soak the whole wall area beneath the window. Areas away from
the window were not affected.
Figure 10.This wall shows characteristic damage
from leaks at the window frame joints. Good flashings and a
drainage plane would have saved the structure.
This is an adhesively attached EIFS wall, where the foam board
is a cladding and not a drainage plane. EIFS is a classic
nightmare, of course; but the drainage details and not the
cladding are to blame. EIFS systems that use a drainage plane
and good flashings beneath the foam board are quite effective
and reliable, and wood or vinyl siding installed without good
drainage systems can be a disaster just like EIFS.
Pan flashings for doors. I
have large builder clients who have experienced thousands of
callbacks and claims because wood floors were damaged by
leaking doors. When they started using pan flashings like the
metal one in Figure 8, the problem went away. Any door with
sidelights, and any sliding door, should have a good pan
flashing -- don't rely on the guarantee.
Here's a trick from the commercial building industry. For the
exterior doors of a slab-on-grade house, form a 3/4-inch
depression in the slab to create a seat (Figure 11). You're
building a preformed pan flashing right into the edge of your
slab. When that mulled front door or that slider leaks, this
little step-down will kick the water outside and save your
floors. The key is to shim the door up -- don't set it down
where it will be sitting in the puddle. (Use plastic shims, not
Figure 11.Forming a depression in a cast-in-place
floor slab creates a built-in drainage pan for mulled entry
doors and sliders, which are prone to leakage at
Rain and Vapor
Rain is the most important wetter of walls, but it's not the
only one. Occasionally, walls get wet from condensation, and
sometimes they start out wet because they're built with wet
materials. And sometimes our rain management systems aren't
perfect. So walls have to be designed to dry out -- to the
interior, to the exterior, or to both. To understand that, we'd
have to take a close look at the vapor permeability of building
materials and at the way vapor interacts with buildings. But
that's a subject for another article.
In any case, no matter how well a wall is detailed for drying,
there will be trouble if the wall is repeatedly subjected to
wetting by rain. On the other hand, if we eliminate rain as a
source of wetting, most walls will be fine. That's why it's so
important to drain everything and to focus on every element of
rainwater management: the drainage plane, the drainage space,
the flashings, and the weeps. If we pay proper attention to
those key details, we will have done most of what is necessary
to provide our buildings with dry walls.Joseph Lstiburek, B.A.Sc., M.Eng.,
Ph.D., P.Eng.,is a principal of
Building Science Corporation
(www.buildingscience.com). A forensic
engineer who investigates building failures, he is
internationally recognized as an authority on moisture-related
building problems and indoor air quality.