Thermal Bridging in ASHRAE 90.1-2022
ASHRAE 90.1-2022 introduced new rules for dealing with thermal bridges, those spots in your building envelope where heat sneaks through faster than it should. Think of shelf angles, floor slab edges, parapets, and balcony connections. These areas have always been energy weak points, but until now, the standard mostly ignored them.
That's changed. Section 5.5.5 now requires you to account for thermal bridging, and this guide breaks down what you need to know, including the specific calculation methods for different framing types.
How to Use This Guide
Thermal bridging requirements touch multiple sections of 90.1-2022 and reference external standards. Use this decision tree to find your path:
Step 2: What's your framing type?
CFS attic roof (≤600mm spacing, insulation at ceiling)
AISI S250 (required)
CFS Attic Roofs
CFS wall — covered by Sections A2–A8
Pre-calculated tables
Appendix A
CFS wall — not in A2–A8 tables
AISI S250 (with conditions)
Steel-Framed Walls
Wood framing
Parallel path calculation
Section A9
Mass wall
Isothermal planes or 2D calc
Section A9
Metal building
Section A9.4.6 equations
Section A9
Other / non-standard
Section A9 procedures
Section A9
Step 3: Which compliance path? (for thermal bridges)
Prescriptive
Meet Section 5.5.5 requirements directly
Section 5.5.5
Trade-off
Use Psi/Chi values from Table A10.1 in Appendix C calculation
Table A10.1
Performance
Adjust insulation conductance in energy model
Section A10.2
Quick Reference
Thermal bridge Psi/Chi values
Section 5.5.5, Table A10.1
CFS wall U-factors
AISI S250-21 or Section A9
CFS attic roof U-factors
AISI S250-21 (required)
Calculation method by assembly type
Section A9.2
Air film and material R-values
Section A9.4
Pre-calculated assembly U-factors
Sections A2–A8
Alternative assembly procedures
Section A9
Why This Matters
For years, energy models assumed building assemblies connected perfectly, no thermal shortcuts, no weak spots. But real buildings don't work that way. A steel beam punching through your insulation creates a heat highway that your U-factor calculations never accounted for.
As insulation requirements got stricter over successive 90.1 editions, this gap between modeled and actual performance grew wider. The new thermal bridging provisions close that gap.
The Three Types of Thermal Bridges
Clear-Field Thermal Bridges
These are the repeating elements spread across your assembly, studs, masonry ties, fasteners, metal girts and purlins. Appendix A handles these through assembly U-factor calculations, so Section 5.5.5 doesn't cover them.
Linear Thermal Bridges
Any element running along a line (horizontal, vertical, or diagonal) that cuts through your insulation layer. Floor edges, parapets, shelf angles, and window perimeters all fall here.
You'll calculate these using the Psi-factor (ψ):
Psi-factor = thermal transmittance per unit length
IP units: Btu/(h·ft·°F)
SI units: W/(m·K)
Point Thermal Bridges
Discrete penetrations — a beam poking through a wall, a column through a roof, structural anchors. You measure the cross-section at the outer face of your outermost insulation layer.
You'll calculate these using the Chi-factor (χ):
Chi-factor = thermal transmittance of the point bridge
IP units: Btu/(h·°F)
SI units: W/K
Compliance Paths
You've got three ways to comply, same as other envelope requirements.
Prescriptive Path (Section 5.5.5)
The simplest approach. The standard tells you exactly how much extra insulation to add at each thermal bridge location. No calculations required, just follow the requirements for your specific condition.
Section 5.5.5 covers five categories:
5.5.5.1
Roof/wall intersections (for insulation above deck)
5.5.5.2
Floor edges, with and without balconies/overhangs
5.5.5.3
Shelf angles and cladding support
5.5.5.4
Window-to-wall interfaces (four options based on window placement)
5.5.5.5
Everything else, requires area-weighted calculation
Wood gets a pass. Materials with thermal conductivity below 3.0 Btu·in/(h·ft²·°F) or 0.433 W/(m·K) are exempt. Wood falls under this threshold.
Trade-Off Path (Section 5.6 / Appendix C)
Can't meet prescriptive requirements at one location? Trade it off against better performance elsewhere in your envelope.
Here's how the math works:
For each thermal bridge that doesn't meet prescriptive requirements, use the Unmitigated Psi or Chi value from Table A10.1
For thermal bridges that do comply prescriptively, use the Default values from Table A10.1
For high-performance details that exceed requirements, enter actual calculated Psi/Chi values — you'll get credit toward overall performance
Each linear or point thermal bridge calculation gets assigned to its associated floor, wall, or roof assembly in the Appendix C calculation.
You can also use Psi and Chi values from other recognized sources per Section A10.1, you're not limited to Table A10.1.
Performance Path (Section A10.2)
For whole-building energy models using the Energy Cost Budget Method (Section 12) or Performance Rating Method (Appendix G).
The modelling approach:
Adjust the conductance of insulation layers in your modeled assemblies to account for thermal bridging. You're not changing material properties — you're modifying layer conductance to reflect real-world thermal bridge impacts.
Same rules apply: indicate whether each intersection complies prescriptively, then input unmitigated or mitigated Psi/Chi values with quantities.
U-Factor Calculation Methods by Assembly Type
Section A9 spells out exactly which calculation methods you can use for each type of construction. If your assembly isn't covered by the pre-calculated tables in Sections A2–A8, you'll need to use Section A9 procedures.
Alternative Assemblies (Section A1.2): For assemblies not addressed by, or different from the specifications in Sections A2–A8, use the testing, calculation, and modeling procedures in Section A9 to determine U-factors, C-factors, F-factors, or heat capacities.
Acceptable Methods by Construction Type
Two- or three-dimensional finite difference/finite volume computer models work for everything. Beyond that, here's what's allowed:
Roofs
Insulation entirely above deck
Testing or series calculation
Metal building roofs
Testing, or calculation per Section A9.4.6 (single/double layer)
Attic roofs, wood joists
Testing or parallel path calculation
Attic roofs, steel joists
Testing, parallel path with Table A9.2-1 adjustment factors, or modified zone method
Attic roofs, concrete joists
Testing, parallel path (solid concrete), or isothermal planes (hollow sections)
Other roofs
Testing or 2D calculation
Above-Grade Walls
Mass walls
Testing, isothermal planes, or 2D calculation. Parallel path not acceptable.
Metal building walls
Testing, or calculation per Section A9.4.6
Steel-framed walls
Testing, series path with Table A9.2-2 adjustment factors, or AISI S250 (see conditions below)
Wood-framed walls
Testing or parallel path calculation
Other walls
Testing or 2D calculation
Below-Grade Walls
Mass walls
Testing, isothermal planes, or 2D calculation. Parallel path not acceptable.
Other walls
Testing or 2D calculation
Floors
Mass floors
Testing, parallel path (solid concrete), or isothermal planes (hollow sections)
Steel-joist floors
Testing or modified zone calculation
Wood-joist floors
Testing, parallel path, or isothermal planes
Other floors
Testing or 2D calculation
Slab-on-Grade Floors
No testing or calculations allowed — use the prescribed values.
Steel-Framed Walls: AISI S250 Requirements
For steel-framed walls, you can use AISI S250-21, but there are specific conditions that affect how you apply it.
When AISI S250 Applies Without Modification
Use AISI S250 directly (no adjustments) when:
The wall has no cavity insulation and relies on continuous insulation only, framing can be at any spacing
The wall has less than 23% framing factor
When You Need to Adjust AISI S250 Inputs
If your steel-framed wall has higher framing factors, you'll need to use more conservative inputs:
600 mm (24") o.c. with 23% framing factor
Use next lower spacing input values
400 mm (16") o.c. with 25% framing factor
Use next lower spacing input values
Non-Standard C-Shape Framing
For steel framing members that aren't standard C-shapes, AISI S250's "calculation option for other than standard C-shape framing" is permitted. This involves testing per ASTM C1363 and developing correction factors.
Cold-Formed Steel Attic Roofs: AISI S250 Required
Mandatory: U-factors for attic roofs with CFS conventional C-shape framing or CFS trusses, where insulation sits at the ceiling joist or bottom chord and framing spacing ≤ 600 mm (24") on-center, shall be determined in accordance with AISI S250.
Joist and Rafter Framing (Section B4.1)
For steep-pitched roofs with C-shape ceiling joists or rafters:
Where:
Rs-roof= Cumulative R-value of roof components (excluding steel and cavity insulation)Rins= R-value of cavity insulation between joistsFc= Correction factor from Table B4.1-1
Correction Factors (Fc) for Joist Framing:
3.5" to 4"
16" o.c.
0.94
0.95
0.96
6"
16" o.c.
0.81
0.85
0.88
8"
16" o.c.
0.65
0.72
0.78
10"
16" o.c.
0.27
0.62
0.70
12"
16" o.c.
0.27
0.51
0.62
3.5" to 4"
24" o.c.
0.96
0.97
0.97
6"
24" o.c.
0.86
0.88
0.91
8"
24" o.c.
0.72
0.78
0.83
10"
24" o.c.
0.35
0.69
0.76
12"
24" o.c.
0.35
0.61
0.69
Linear interpolation between table values is permitted.
Truss Framing (Section B4.2)
For C-shape truss framing with ≥24" spacing and ≤3 web penetrations per 4 ft of truss:
Without rigid foam below truss:
With R-3 rigid foam between gypsum and bottom chord:
With R-5 rigid foam between gypsum and bottom chord:
Where Rins = R-value of cavity insulation at the bottom chord.
AISI S250 Wall Calculations: The OTZ Method
For walls with standard C-shape studs at 6", 12", 16", or 24" on-center spacing, AISI S250 uses the Overall Thermal Zone (OTZ) approach.
Step 1: Calculate the OTZ
Where:
Rcav= Total R-value of the wall cavity (insulation + any air space)Rshe= R-value of exterior continuous insulation (not including wood or gypsum)C₀throughC₅= Coefficients from Table B3.1.1-1 based on framing spacing and steel thickness
Step 2: Calculate the C-Shape Framing Factor
Step 3: Calculate Thermal Transmittance at Cavity
Step 4: Calculate Overall U-Factor
Shortcut: AISI S250 Appendix 1 provides pre-calculated U-factors for common wall assemblies with 43-mil steel at 16" and 24" spacing.
Section A9 Calculation Assumptions
When you're doing your own calculations, Section A9.4 specifies the values you must use.
Air Film R-Values
All exterior surfaces
0.03
All semi-exterior surfaces
0.08
Interior horizontal, heat flow up
0.11
Interior horizontal, heat flow down
0.17
Interior vertical surfaces
0.12
Air Space R-Values
Use Table A9.4.2-1 values based on effective emittance, provided:
Air space is enclosed and unventilated
Located interior of continuous air barrier
Reflective insulation (if used) is fitted closely and sealed
Air spaces < 13 mm have no R-value
Use 89 mm values for air spaces up to 300 mm
Insulation R-Values
Uncompressed: Use rated R-value
Uniformly compressed in confined cavities: Use Table A9.4.3
In steel joist attic roofs: Use Table A9.2-1 adjustment factors
In steel-framed walls: Use Table A9.2-2 adjustment factors
Testing Requirements
If you're going the testing route instead of calculations:
Material Properties (ASTM C177, C518, or C1363)
For concrete: multiply oven-dried conductivity by 1.2 to account for typical installed moisture content
Assembly U-Factors (ASTM C1363)
Use production-line or consumer-representative samples
Test must include: panel edges, joints, typical framing percentages, and thermal bridges
If assembly is too large, test representative portions or multiple portions with weighted average
Steel Thermal Properties
AISI S250 thermal conductivity values by steel thickness:
33
381
43
495
54
622
68
783
Conservative assumption: AISI S250 assumes C-shapes without web holes (punchouts). Framing with punchouts performs better than calculated.
Calculation Requirements Summary
Steel-framed walls
AISI S250 (with conditions)
Testing, series path + Table A9.2-2
CFS attic roofs (≤24" o.c.)
AISI S250 required
Testing
Steel-joist attic roofs
Parallel path + Table A9.2-1
Testing, modified zone
Wood-framed walls
Parallel path
Testing
Mass walls
Isothermal planes or 2D
Testing. Not parallel path.
Metal building
Section A9.4.6 equations
Testing
Linear
Psi (ψ)
Btu/(h·ft·°F)
W/(m·K)
Point
Chi (χ)
Btu/(h·°F)
W/K
Quick Reference: Table A10.1 Values
Doesn't meet prescriptive
Unmitigated
Meets prescriptive
Default
Exceeds prescriptive
Calculate actual value for credit
Training, If You Need It
To support practitioners navigating these requirements, we've developed on-demand training specifically focused on linear thermal bridging using LBNL THERM. The course walks through the calculation methods and modelling approaches you'll need for compliance, particularly when moving beyond prescriptive requirements. You can find details at https://docs.betterbuilding.io/sign-up-and-plans/news-and-updates/linear-thermal-bridging-lbnl-therm-on-demand-training.
As we continue developing our documentation and workflow tools, we want to ensure they actually support your practice. Are you finding the 50mm fenestration alignment manageable? How are the balcony percentage limits affecting your designs in colder zones? We'd love to hear how you're approaching these requirements and what would make compliance more efficient.
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