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by David Cook, RA
Photo © BigStockPhoto/Photo Roller
AMONG ITS MANY FUNCTIONS, THE
EXTERIOR ENVELOPE TYPICALLY
MUST CONTROL THE MOVEMENT
OF HEAT, AIR, BULK WATER, AND
WATER VAPOR BOTH IN AND OUT
OF A BUILDING. OVER TIME, VARIOUS
SYSTEMS AND COMPONENTS HAVE BEEN
DEVELOPED TO CONTROL THIS MOVEMENT.
In a properly functioning envelope, these systems
work together to maintain the building’s temperature
and allow components to dry quickly if they become
wet. While these systems contribute to overall
performance, they also contribute to the complexity
of the envelope design and sensitivity to error.
Recent editions of the model codes have increased
requirements for thermal insulation, and heightened
the base level of acceptable performance for air
barriers and vapor retarders. Increasing energy
costs have also caused building owners to demand
better performance. If one of an envelope’s
systems is improperly designed or installed, the
increased performance of the other systems may
reduce the resulting envelope’s capacity to dry.
This may heighten the potential for problems to
occur (Figure 1).
70 the construction speciﬁer | april 2013
3/14/13 11:50:04 AM