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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? Wrong.




Figure 8.

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 (bottom).

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 doors.

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 nail holes.

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 the housewrap.)

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 wood.)


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 joints.

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 ( A forensic engineer who investigates building failures, he is internationally recognized as an authority on moisture-related building problems and indoor air quality.