Lessons From Florida's Hurricanes: Why Stucco Walls Got Wet - Continued
On the other hand, our testing of windows and window assemblies indicates that many of them leaked under conditions well below their listed, rated value. In fact, many tested windows leak under a simple water spray with no wind pressure at all. Factory testing of windows seems to be missing a widespread incidence of leakage at the window-assembly corners. Also, windows are tested as single units, but are often sold as preassembled "mulled" units, with two or more windows combined in a composite arrangement. Every preassembled mulled unit we tested leaked at zero wind pressure (Figure 7).
Figure 7.Factory-assembled windows like this double unit (top left) are frequently found to leak at the corners (top right) and the center joint (bottom).
From our visual inspections of windows in the field, and from a closer look at some randomly selected windows that we took apart, it seems clear that there is a widespread problem with the connections between the windowsill and the window jamb: Windows delivered to the site are likely to be leak-prone before installation.
Window installation is also an ongoing concern. The methods used in Florida, as elsewhere, often don't ensure reliable water management. In particular, precast concrete windowsill components sold into the Florida market are shaped in a way that directs some leakage into — rather than out of — the building (Figure 8).
Figure 8.Precast concrete windowsills for block walls (top two photos) have a profiled flange attachment rib that directs some leakage toward the interior (middle). A better windowsill design would direct water toward the outside (bottom).
It would be good if windows delivered to the job could be made to hold water out more effectively. But in the meantime, builders have to be aware of the limited water resistance of window units, and design walls that are tolerant of window leakage.
Paints and Coatings
When we first went to Florida, some people had the idea that we would focus on paints and coatings. Many observers had noticed that homes only a year or two old had shown more leakage than homes that had been around for five or 10 years, and they thought perhaps this was because successive repaintings had sealed all the microcracks in the stucco. If we required a high-build elastomeric paint on new homes in the first place, the reasoning went, maybe we could prevent the whole problem.
That idea makes sense, but it doesn't hold up completely. For one thing, all those older buildings were repainted (and the cracks patched using other means as well) only after they had been through the process of shifting and had settled down to some kind of equilibrium. There are very few paints and coatings around that can span the shrinkage cracks in a new building and also stay intact as the building shifts and cracks over its first few years. So while patching and painting a stucco wall is a good idea — in fact, it's necessary maintenance — and it has to be done continuously over the life of a building, it is particularly important during the first two or three years.
Also, high-build paints and elastomeric coatings span microcracks most effectively when the surface is smooth. On rough-surfaced stucco, which is a very common finish in the industry, coatings are much less effective at sealing surfaces (Figure 9).
Figure 9.Paints and coatings are not highly effective at spanning cracks and pinholes in a rough stucco coating. Ordinary paint (top left), a 3-mil high-build coating (top right), and a 10-mil elastomeric coating (bottom left) all leave pinholes that can be seen under magnification. Thick elastomeric coatings over stucco can also be prone to blistering (bottom right).
And to gain flexibility and crack-spanning ability in a coating, you often have to give up vapor permeability, so that the coating may tend to trap moisture within the stucco as well as keep bulk water out. That trapped moisture can cause coatings to blister. Modified stucco mixes may even re-emulsify and turn to goo when you trap moisture in them with a low-permeability coating.
That said, specialty elastomeric coatings hold great promise. The "holy grail" of coatings research has always been to develop a highly elastic coating that also breathes. We're not sure how much it needs to breathe, but generally I think the perm rating should be 10 or higher. And, for the present, specialty coatings should be applied to stucco only by knowledgeable installers. "High build" acrylic paints that get you 5 to 6 mils of thickness at permeabilities of greater than 10 perms are pretty much the optimum performance limit with conventional coating systems.
Improving Masonry Mass Walls
Clearly, masonry-block walls — as commonly built in Florida and other places — have a limited capacity to hold and drain water. When cracks and crevices in the wall assembly are full, water trickles onto interior floors at the base of walls. Saturation of walls also leads to humid conditions on interior wall faces, sometimes allowing mold or mildew to grow.
Two proven methods would improve the performance of these walls. First, the foundation slab or footing should be built with a stepped-down seat or shelf where the first course of masonry block is set, as in Figure 10 (previous page). This will direct water that reaches the base of the wall outward to the outdoors, rather than inward, where it can damage floors or cause humidity problems.
Figure 10.To help block walls store greater amounts of incidental water and to direct the moisture back outside rather than allowing it to trickle into the home, foundations should have a stepped-down shelf at the slab edge. A weep screed at the lower termination of the stucco will allow accumulated water to get out.
The interior-wall face will perform better if covered with a continuous layer of semipermeable rigid insulation, such as commonly available extruded polystyrene (Figure 11). This will reduce vapor migration into the home as well as condensation, preventing moisture from accumulating in the home's drywall.
Figure 11.Extruded polystyrene insulation applied to the inside face of the block wall, to the outboard of the interior furring, or to the wall framing will allow some drying by vapor diffusion to the interior, but will direct bulk water downward toward the base of the wall, where it can escape to the outside.
Refining Drained Frame Walls
Wood-frame stucco-clad walls should have a bond break layer installed between the stucco rendering and the drainage plane.
In practice, this means applying two layers of building paper, or one layer of building paper over a layer of plastic housewrap, before applying stucco. At the joint between drained upper-story assemblies and mass-wall lower-story assemblies, a weep-screed flashing should be installed, as in Figure 12.
Figure 12.So that horizontal control joints between floors direct water to the outside of the wall, first apply a bridge flashing between the wood frame wall and the block wall below, and then attach a weep-screed flashing that overlaps the lower wall (top left). The primary drainage-plane membrane should then be installed overlapping the weep screed (top right) before a bond-break paper layer and wire lath (bottom left) are put in place. Finally, stucco can be applied to both walls (bottom right).
Window and Flashing Recommendations
There is a problem in Florida and other states where high-wind codes are taking effect that is caused by the contradictions that arise between two imperatives: the obligation to ensure structural integrity and the need to keep out water. In many cases, building officials are enforcing fastening schedules and structural connections at the expense of proper flashing and drainage details.
To fix this problem, all of us — including builders, code officials, and manufacturers — need to think through what we're doing when we attach a window or other component to a wall assembly. Here are a few things to consider:
First of all, whether it's a window, a dryer vent, or a hose bibb, when you install something through a wall, you have to flash the opening. Second, drainage assemblies for windows have to extend all the way to the back of the window, because windows can leak at any point. And, finally, flashings above windows and other penetrations have to catch water from all the way to the back of the cladding system, and have to direct it all the way to the exterior of the building.
Maintenance and Crack Repair
Stucco cracks have to be addressed with ongoing homeowner maintenance. The best practice is to allow stucco walls a reasonable "breaking-in" period, from one to two years. By then, most if not all of the cracks that are going to appear will already be evident. At that point, cracks should be individually sealed with caulking or a brush-in cementitious crack-repair formula, and then the walls can be repainted.
Inspecting walls every few years, and repairing them in this fashion as needed, should be enough to keep a stucco wall performing well for many decades.Joe Lstiburek, P.Eng.,is a principal of Building Science Corp. in Westford, Mass., and an investigator of moisture-related building problems.