BUILDING ENERGY - Fall 2016 - 38
show that it
or greater by
R-value or, roughly a 45 to 55 percent reduction in
performance depending on the insulation thickness.
Because of the intermittent nature of the rainscreen
clips, they performed better both in thermal images
and in the computer modeling than the continuous
Z-girts. The clip support system had half of the
heat flow of the Z-girts, or 25 percent of the design
intent. While the intermittent nature of the support
system certainly improved the performance, the
team investigated methods to further improve the
performance of rainscreen support systems (see
A number of thermally broken Z-girt and
rainscreen support systems currently exist on the
market. As part of the research project, the team
explored four thermally-broken options. All four of
the tested systems performed well. In general the
R-value of the assemblies was only reduced by 10
to 15 percent due to thermal bridging through their
support systems so that they achieved a minimum
of R SI-3.5 (RIP-20) with four inches of insulation.
The continuity of a thermal barrier across the
entire building envelope is fundamental to good
thermal performance. It reduces energy consumption,
increases thermal comfort and helps to prevent
condensation. While some thermal bridges may be
eliminated, the real goal of the research suggests that
thermal bridges can be effectively managed, and
that doing so will have a meaningful impact on the
performance of buildings.
The first priority should be to eliminate
continuous conductive elements, such as Z-girts or
masonry shelf angles that completely penetrate the
insulation layer. These systems are easily interrupted
by pulling them outboard of the thermal barrier
and using discontinuous supports to make required
connections back to structure. Second, try to utilize
available thermally broken products or strategies to
disconnect the heat flow through the thermal barrier.
Thermally broken rainscreen support systems,
brick ties and concrete slab connections are readily
available on the market. It is essential, however,
to ensure that the thermal break occurs within
the insulation boundary in the application of these
products. The research found some products that are
easily foiled by having breaks in undesirable locations
relative to the natural placement of insulation.
Finally, when the thermal bridge is a necessity, such
as when structure must penetrate uninterrupted
through the insulation, look for materials with the
lowest possible thermal conductivity. For example,
stainless steel has one-third the conductivity
of carbon steel, and fiberglass's conductivity is
significantly lower than that of stainless steel.
While this study is not intended to be an
exhaustive analysis of all thermal bridges, it does
identify the types of conditions that occur typically,
and helps quantify their localized impact. More than
anything, it is anticipated that this research will
help to develop an intuitive understanding of the
situations that lead to the thermal bridging regardless
of the specific project conditions, and provide the
tools for easily addressing them.
This research received financial support from the
American Institute of Architects Upjohn Research
Grant and Payette Associates.
ASHRAE. 2009 ASHRAE Handbook Fundamentals. Atlanta,
GA: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc., 2009.
ASHRAE. ANSI/ASHRAE/IESNA Standard 90.1-2007 Energy
Standard for Buildings Except Low-Rise Residential
Buildings. Atlanta, GA: ASHRAE, 2010.
A New Standard of Energy Efficiency
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