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Water-Managed Wall Systems, continued

Drainage Planes and Air Spaces

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 asphalt.

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 unprotected.

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 paper.

Building Papers and Drainage Planes

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 difference.

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 papers.

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 assembly.

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 (, 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, 2¢/sq.ft. paper.

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 Plane

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 surface.

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 (bottom).