PRINCIPLES/DEFECTS
Wall Assemblies
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Airflow passages within the envelope
.
Poor material selection or attachment
Good thermal performance of a wall assembly requires the secure attachment of the elements
which make up the wall and the avoidance of unrestricted airflow passages within the system.
Failure to meet these requirements causes air movement within the wall, which can severely
degrade thermal performance and increase the potential for condensation within the system. While
envelope air leakage from inside to out is an obvious problem, other modes of air movement also
cause problems including air exchange between the building interior and the envelope system, air
exchange between the envelope system and the outdoors, and air movement within the envelope
system itself. Air movement within the envelope system degrades thermal performance due to
airflow around and through thermal insulation and due to self-contained convective loops within the
envelope system. Avoiding such air movement within the envelope requires a wall assembly that
does not contain extensive vertical airflow passages and that insures that the elements remain in
position over time. Vertical air spaces are sometimes designed into wall systems, for example
between the interior wallboard and the inner face of the backup wall, When such air spaces extend
over several stories of a building, the resultant air movement can be particularly significant. As
discussed earlier, when such a cavity exists next to a layer of fibrous insulation, the thermal
effectiveness of the insulation will be severely decreased. Almost any kind of wall system can
develop significant airflow paths within the envelope because of designs or materials that can not
resist wind pressures or structural movement or that lack adequate durability. The inadequate
support or attachment of envelope components can result in the repositioning of envelope elements
due to wind forces or the movement of structural components.
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