Water hasn't changed how it enters a house, but how we deal with it as builders certainly has. Let's start with an oft-mentioned truism: All wall claddings leak. What we might think of as a house's first line of defense against the elements turns out to be relatively defenseless. Regardless of your level of craftsmanship, water is going to find its way behind whatever you cover your exterior walls with—wood, masonry, metal, or vinyl.
In the not-so-distant past, when houses were not as well insulated or as tightly built as they are today, water that infiltrated behind the siding might have stained the flower-patterned wallpaper in the parlor, but chances are, the house didn't rot.
It's a different story with today's tighter and more energy-efficient homes. We began to understand this in the early 2000s, when mold was discovered in buildings throughout the U.S. With the mold came rot. Home builders remedied some of the problems by paying attention to things like ventilation and source control, which they previously had the leisure to ignore. But more than a few needed to buy back new homes from disgruntled homeowners.
And it got worse: As further measures were taken to improve building airtightness and energy efficiency, even more rot and mold problems arose.
But there are solutions. Building science has shown us what to do with water that leaks behind wall claddings. Proper installation of a housewrap—also known as a water-resistive barrier (WRB)—preferably with a rainscreen behind the wall cladding, enables us to build resilient homes that will last for generations
Water Watch
The main reason that rot and mold didn't substantially affect older homes is because the water that got past the leaky siding had a chance to dry. Before the days of plywood sheathing and drywall, not to mention insulation, caulking, and other airtightness measures, air flowed more freely through the building and any water that got in dried out.
Wood doesn't rot and mold doesn't grow when wood gets wet; it happens when the wood stays wet and can't dry. And engineered-wood products (OSB in particular) don't stand up to constant wetting as well as solid-wood framing and sheathing products do.
Joseph Lstiburek, the founding principal of Building Science Corporation, has said, "Rain is the single most important factor to control in order to construct a durable building." And the architectural design of a home can influence how much rainwater lands on the walls of a building. Here are some points to consider:
--Lack of overhangs. The absence of roof overhangs in many new houses allows more water to hit the walls. Here's an analogy: Roof overhangs are like an umbrella in a rainstorm. If you don't have an umbrella, you better make sure you're wearing a high-quality raincoat, or you're going to get wet. Think of a good WRB system as being like a raincoat for the house.
--Roof pitch. Shallow-pitch or flat roofs also contribute to the amount of water hitting a wall during rainy, windy conditions. A steeper roof, on the other hand, can act like an airfoil, pulling rain up over the roof and pushing it away from the walls.
--Wall height. The system that should be used is dependent on the location and design of the house you're building. It's a matter of the walls' exposure to water infiltration—more exposure will require a higher level of protection. For instance, a single-story house with a pitched roof and broad overhangs located in an inland location that doesn't receive much annual rainfall won't need nearly as much protection as a two-story building (which will have higher, more exposed walls) with a flat roof located on a windy, rainy seacoast.
Best Bets
I like to use a housewrap that has a textured surface as the first defense for draining out moisture that infiltrates behind wall claddings. The small gap it creates between the back surface of the cladding and the sheathing allows water to drain away to the ground. Manufacturers use a variety of patterns and technologies to hold the siding away from the sheathing when it's installed.
Dupont, the parent company of Tyvek, manufactures a family of small-gap housewraps. Two that I use are Tyvek Drainwrap and Commercial Wrap D. The main differences between them are that Commercial Wrap D is slightly tougher, and it has a longer UV-exposure time, so it will last longer if it's going to be a while before you install siding. Both have a slight, almost crinkled corrugation, which when installed properly creates vertical channels for water to run out. Regardless of how tightly the siding is attached to the sheathing, Drainwrap and Commercial Wrap D create enough of a space between the back of the siding and the wall for water to drain away.
Benjamin Obdyke's Hydrogap housewrap has 1-mm–thick blue plastic oblong dots bonded to its surface. The thickness of the dots holds cladding away from the wall and creates a space for water to drain away. One of the things I like about Hydrogap is that it can be installed in any direction, even rolled out vertically, without its draining abilities being affected.
All the housewrap manufacturers sell proprietary tapes and flashing materials to use with their products. I stick with the manufacturers' branded tapes that go with their housewraps and prefer ones with butyl adhesives. Tyvek's StraightFlash and FlexWrapare among the best in the market.
Not all brands of housewrap are available everywhere in the U.S. If you can't find one of the brands I prefer, look for a wrap with some kind of built-in air gap. Some housewraps can even be bought online. Benjamin Obdyke, for instance, will sell direct from its store website, and it offers free shipping.
This article originally appeared in THE JOURNAL OF LIGHT CONSTRUCTION, Check out the full site for more articles like this.
