PRINCIPLES/DEFECTS
Roofing Systems
.
Thermal bridges: penetrations, structural elements
.
Insulation defects: gaps
.
Air leakage: penetrations, structural elements, flutes in corrugated steel decking,
incomplete attachment of loose-laid membranes
The thermal performance of roofing systems can be reduced by thermal defects including insulation
defects, thermal bridges and air leakage. The insulation defects include those discussed
previously, with gaps between insulation boards and batts being a particular problem. Childs
studied three thermal bridges caused by high conductivity components penetrating the insulation of
a roofing system consisting of lightweight concrete on a metal deck. These penetrations include a
pipe used to support rooftop mechanical equipment, a steel l-beam also used as an equipment
support, and a concrete pillar used to support a window washing system. Steven Winter Associates
also discusses thermal bridges associated with equipment supports and roof railings, and
calculates the effect of these bridges and alternative, nonbridging designs on the heat transmission
rates.
One of the most serious thermal performance problems in roofing systems is air leakage. Air
leakage through or around the insulation decreases the thermal effectiveness of the system. In
cold climates the leakage of moist air from the building interior into the roofing system will cause
condensation within the roofing system, leading to increased heat flow through moist insulation and
possibly the degradation of roofing materials. While vapor retarders are effective in controlling the
diffusion of moisture into the roofing system, it has been repeatedly pointed out that convection due
to air leakage is the predominant mechanism for moisture transport into roofing systems (Tobiasson
1985, 1989). Such air leakage arises from improper sealing of roofing system penetrations, i.e.,
pipes, plumbing vent stacks and structural supports for rooftop equipment. Other air leakage sites
are associated with structural features such as expansion joints, incomplete attachment of loose-
laid membranes, and unsealed penetrations through flutes in corrugated steel decking. Many air
leakage sites are associated with the connection of the roofing system and the exterior walls.
PAGE 2.3-8
Previous Page
