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,"
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