Some Common Misconceptions About Building Enclosures

It's easy for small amounts of water to enter a home's wall cavity, both during and after construction. When water evaporates it becomes a gas (water vapor) that needs to escape. If the walls can't completely dry, a home is more likely to experience mold and rot.

A building material's ability to let water vapor pass through is called permeance, measured in units called “perms.” There are two ways that the American Society of Testing and Materials (ASTM) measures permeance in its E96 test: Procedure A (desiccant or “dry cup” method) and Procedure B (water cup or “wet cup” method).

Here are some common misconceptions about building enclosure performance and the truths behind them:

• Myth: High-perm products are the most effective way to improve drying rates

TRUTH: “Whether a vapor-control material is 5 perms or 35 perms, the drying rate is usually limited by other materials like the cladding,” says John Straube, a principal at RDH Building Science and an associate professor at the School of Architecture at the University of Waterloo in Canada.

• Myth: The “dry cup” test is the superior way to measure permeance

TRUTH: “The house wrap industry emphasizes the ASTM E96 Procedure A or ‘dry cup' test,” adds Straube. “But the test that's more appropriate for in-service use of wood-framed buildings is really Procedure B or the ‘wet cup' test. I have a colleague who says, ‘Procedure A measures how much water vapor passes through when there isn't any water vapor, while Procedure B measures how much passes through when there is water vapor.'”

• MYTH: An air/water barrier can prevent all moisture problems

TRUTH: It defeats the purpose of an air/water barrier if a home's exterior flashing isn't done properly. “There are far greater mold risks when sheathing gets wet after the home is occupied than when it's being built,” says Straube. “If windows leak into the wall, that's really scary because you can't see it.”

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