Wrap it up

You can’t stop the weather, but you can sell a barrier.

The debate over the necessity of weather barriers is over — they’re necessary. But the question remains: Which barrier is best suited for any given project?

“A house needs some kind of drainage plane,” said Allison A. Bailes III, PhD., president and founder of Energy Vanguard, a building science consulting firm in Decatur, Ga. “A housewrap’s main purpose is to serve as a drainage plane. When water gets behind the cladding, it drains the water down. And a housewrap is only one of a number of products that can do that.”

Generally speaking, weather barriers permit water vapor transmission from inside the house, but they also prevent bulk water from penetrating the building envelope, the Upper Marlboro, Md.-based Home Innovation Research Labs writes on its website. If moisture is allowed to build up in the wall cavity or between building layers, it becomes a breathing ground for mold and rot.

Products such as Tyvek (spun-bonded poly-olefin) from DuPont and Typar by Fiberweb have become popular choices for many single-family and multi-family builders. The manufacturers tout the products as lightweight and easy-to-install options and extol the virtues of their performance as moisture and air barriers.

But those types of housewraps aren’t the only game in town. “Other options are felt, which some builders still use; foam board, which is used for exterior sheathing and, if taped properly, can be used as a drainage plane; and there are products like the Zip Wall System [from Huber Engineered Woods],” Bailes said.

In recent years, the weather barrier category has swollen with new introductions and technology, such as Weather Tex by Fortifiber Building Systems Group. The manufacturer claims the two-ply product is the industry’s first hybrid weather-resistive barrier.

The drainage plane on a home is important, and choosing a product or materials should not be taken lightly. Plus, builders and architects need to consider a range of issues, such as the cladding material to be used.

Other issues include the rise and fall of temperatures within a certain range, the type of wall assemblies and, most importantly, climate. “In general, water vapor moves from the warm side of building assemblies to the cold side of building assemblies,” writes Building Science Corp. “This is simple to understand, except we have trouble deciding what side of a wall is the cold or warm side. Logically, this means we need different strategies for different climates.” 

The final piece of the puzzle, of course, is proper installation. A drainage plane should be installed before windows and doors, and it should be installed in a shingle lap fashion and have enough space to drain moisture down and out to the exterior. Horizontal joints should be lapped at least 6 ins. and vertical joints should be lapped 6 ins. to 12 ins. All joints should be taped. 

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