Expert Says Poor Fenestration is Just One Cause for Building Envelope Condensation
April 20, 2010

Building enclosures are often designed without a proper understanding of the performance of the assembly when subjected to exterior weather and interior boundary conditions. This can result in condensation. That was the message Wagdy Anis, a partner with the Boston office of Wiss, Janey, Elstner Associates Inc., had for those attending the second Building Enclosure Science and Technology (BEST 2) conference, which took place last week in Portland, Ore. According to Anis there are six common ways that condensation can occur in a building-air leakage, diffusion, convection, thermal bridges, fenestration and ground contact-several relate specifically to glass and window systems.

Starting with air leakage, Anis said it's been called the biggest cause of condensation in buildings. He explained that moisture condensation in interstitial cavities from exfiltrating air in northern climates or from infiltrating hot humid air in southern climates, can cause problems such as mold growth.

Convection, said Anis, is the rotation of air into an assembly and then out again from the other side. He explained that cold air is heavier (than hot air) and sinks, pulling in warm, humid air replacing it and depositing moisture on the cold surface; this is especially true in vertical or sloping assemblies. Insulating glass, for example, can transfer energy through condensation, convection and radiation.

Speaking of fenestration, Anis said jokingly that it's poorly designed windows that keep him in business.

"I see horribly designed windows all the time, mostly sliders and double hungs, because the manufacturers do not know where to put the thermal break," he said, explaining that fenestration with a good thermal break that minimizes the amount of exterior metal exposed to the cold usually perform best, and from a condensation resistance perspective, may out-perform non-metal units.

Other common problems with the fenestration system can include: having the warm side of a thermally broken window frame exposed to cold temperatures; weepholes communicating between the indoor and outdoor environments resulting in air leakage of cold air into the window frame; and air leakage at the interface of the window frame to the opaque wall's air barrier causing cooling of the warm side of a thermally broken window.

In an effort to avoid condensation, regardless of the cause, Anis advised keeping enclosure component temperatures above the dew point of the air coming in contact with it. "Controlling air movement, vapor pressures and thermal bridges in building enclosure assemblies is critical to avoiding condensation in/on building enclosures," he said.

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