ASHRAE Journal - June 2023 - 43

TECHNICAL FEATURE
they are prescriptive and " tradeable " as opposed to the
mandatory criteria in Section 5.4. The newly introduced
Section 5.5.5 begins with a scoping paragraph
that outlines that two types of thermal bridges-point
and linear-need to be addressed. The third type
of thermal bridge-clear fi eld thermal bridge-is
addressed by Appendix A; Section 5.5.5 only addresses
linear and point thermal bridges. In addition, Section
3 now includes defi nitions to describe the types of
thermal bridges and associated calculation factors.
Graphic examples of the types of thermal bridges
are shown in Table 1.
Details of New Thermal Bridges Content
The thermal bridging requirements include various
exceptions for warmer climates and semiheated
spaces. One notable exception is for materials with a
thermal conductivity (k) less than 3.0 Btu·in/h·ft2 ·°F
(0.433 W/[m·K]), which is characteristic of wood.
The provisions proceed through a series of prescriptive
requirements, the envelope trade-off
method (Section 5.6 and Appendix C), and modeling
protocols (Section 12 and Appendix G).
The prescriptive option (Section 5.5.5) requires
mitigation of thermal bridges in opaque assembly
constructions. By design, the prescriptive path provides
a straightforward approach to compliance.
It is intended to encourage user understanding
by defi ning thermal bridge mitigation in terms of
the required increase in insulation (R-value). This
approach eliminates the need for more complex calculations
that may be applicable under certain situations
and that have been accomodated under the trade-off
approaches described below.
The following paragraphs describe specifi c criteria in
Section 5.5.5 for fi ve common types of thermal bridges.
The discussion then moves to trade-off and performance
options and helpful fi gures in the Standard.
Roof and Wall Intersections
Section 5.5.5.1, Roof and Wall Intersections, applies
to roofs with insulation entirely above deck. It explains
when insulation can be interrupted by the structural
elements or members of the building envelope.
Subsections explain what to do for each category, such
as roof edges that intersect with walls, parapets at the
building perimeter, and parapets within the fi eld of a
roof (e.g., fi re walls). Examples can be seen in the section
on Informative Appendix K (Table 1).
Floor to Wall
Section 5.5.5.2, Intermediate Floor Intersections,
explains how to address fl oors that intersect with
the building envelope wall. It distinguishes between
requirements for fl oor edges with and without projections
(e.g., balconies or overhangs that penetrate the
TABLE 1 Example of figures contained in Standard 90.1's Informative Appendix K.
CATEGORY
SUBCATEGORY
Roof
edges with
continuous
insulation
Roof edges
with integral
or cavity
insulation
With or
without
parapet
With or
without
parapet
IInterinterior
Framing
Interior
Interior
Roof edges
with interior
insulation
With or
without
parapet
Interior
Parapets
within the
fi eld of a roof
Interior
Interior
building envelope). Each subsection of Section 5.5.5.2
describes how the insulation is to be installed and
adjusted to mitigate thermal bridging, depending on
the type of insulation and how it is incorporated into the
building envelope.
Cladding Support
Section 5.5.5.3, Exterior Cladding Support, covers the
allowances for thermal bridges related to shelf angles
that support masonry exterior cladding. The contents
of Section 5.5.5.3 discuss the maximum allowances
for point thermal bridging and the minimum offset
for the support system to allow continuous insulation
to bypass the support and mitigate the thermal
bridge. In addition, the provisions discuss the option to
choose approved designs contained in Standard 90.1's
Normative Appendix A, " Rated R-Value of Insulation
J U N E 2 0 2 3 ashrae .o rg ASHRAE JOURNAL
43
Coping
R-5 min
or
Exterior
R-5 min
Exterior
Parapet
R-5 min
Exterior
Structure
Exterior
2 ft min
Continuous
Insulation
Cavity
Insulation
FIGURE
Exterior
KEY
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ASHRAE Journal - June 2023

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