Curtain wall thermal modeling is becoming a more important part of façade design as Canadian energy codes place greater emphasis on the real impact of thermal bridges. The 2025 edition of the National Energy Code of Canada for Buildings (NECB) updates the thermal-bridging requirements in Part 3, adds a reference to CSA Z5010:21 for thermal-bridge calculations, and requires point elements that penetrate insulation to be accounted for.
For curtain wall and building-envelope teams, the practical message is simple: nominal insulation values and centre-of-glass performance do not tell the whole story. Connections, anchors, slab edges, transitions and other highly conductive details can materially change effective envelope performance.
Important: the NECB is a model code. Its requirements become enforceable only when adopted by the applicable province, territory or other authority having jurisdiction. Always confirm the code edition and local amendments that apply to a specific project.
What changed in NECB 2025?
The Canadian Board for Harmonized Construction Codes identifies thermal bridging as one of the significant NECB 2025 building-envelope changes. The update:
- revises the thermal-bridging requirements in Part 3;
- references CSA Z5010:21, Thermal bridging calculation methodology, as a recognized calculation framework; and
- requires thermal bridging from fasteners and other point elements penetrating the insulation layer to be included in assembly thermal-resistance calculations.
The full NECB 2025 is published by the National Research Council of Canada.
Why this matters for curtain wall design
Curtain wall façades are highly three-dimensional systems. Aluminium framing, pressure plates, fasteners, anchors, slab-edge interfaces, spandrel zones and transitions to adjacent wall or roof assemblies all create paths for heat flow. A detail that appears acceptable when reviewed only by nominal R-value can perform very differently when those conductive paths are included.
That is why thermal analysis should be treated as an integrated façade-design task rather than a late energy-model input. The earlier the team identifies high-loss details, the easier it is to improve them without redesigning anchors, panel geometry or architectural interfaces.
Curtain wall thermal modeling is not one single calculation
A useful distinction is between fenestration-system performance and building-envelope thermal bridges.
1. Fenestration-system performance
This includes the thermal performance of glazing, frames, mullions, transoms, spandrel zones and other components that make up the curtain wall system. These calculations are generally handled using recognized fenestration simulation and testing methods.
2. Building-envelope thermal bridges
These occur where conductive components or interfaces bypass the primary insulation layer. Typical façade examples include slab edges, shelf angles, parapets, anchors, brackets, transitions between curtain wall and opaque wall systems, and other connection details.
CSA Z5010 is particularly relevant to this second category. Its scope provides requirements and guidance for two- and three-dimensional thermal simulation of building-envelope thermal bridges. It does not replace the separate methods used to determine fenestration-product U-values.
Linear and point thermal bridges
Thermal bridges are commonly represented in two ways:
- Linear thermal transmittance (ψ-value): heat flow associated with a repeating junction or interface, expressed per unit length.
- Point thermal transmittance (χ-value): heat flow associated with a discrete penetration or connection, such as a localized fastener or bracket.
For façade design, both can matter. A modest ψ-value repeated around every slab edge can create a large whole-building impact, while a highly conductive point connection may be significant when repeated hundreds or thousands of times.
How CSA Z5010 fits into the workflow
CSA Z5010 provides a consistent methodology for numerical thermal-bridge calculations. In practice, it helps teams define model geometry, material properties, boundary conditions and reporting in a repeatable way.
The standard is useful because thermal modeling results are sensitive to assumptions. Two analyses of the same detail can produce different results if they use different cut-off planes, conductivities, boundary conditions or treatment of adjacent assemblies. A standardized methodology improves consistency and makes the results easier to review.
A practical curtain wall thermal modeling workflow
- Confirm the governing code and performance target. Determine which NECB or provincial code edition applies and whether the project is using a prescriptive, trade-off or performance path.
- Map the façade thermal boundary. Identify glazing, spandrel, slab edges, anchors, parapets and transitions to adjacent assemblies.
- Separate system U-value calculations from thermal-bridge calculations. Use the appropriate methodology for each rather than forcing every façade condition into one model.
- Prioritize repeated details. Slab edges, mullion zones, perimeter transitions and common anchors can dominate total heat loss because they occur so frequently.
- Model critical 2D and 3D details. Use the required geometry, material properties and boundary conditions, and document the assumptions clearly.
- Translate results into whole-envelope performance. Incorporate linear and point transmittances into effective assembly or building-level calculations as required by the compliance approach.
- Iterate before drawings are frozen. Compare alternatives for thermal breaks, anchor geometry, insulation continuity and interface details while design changes are still practical.
Common mistakes to avoid
Ignoring the curtain wall-to-slab interface
Even when the curtain wall system itself performs well, the slab edge and anchorage zone can create a major thermal bypass.
Using nominal insulation values as effective performance
Continuous insulation on a drawing does not automatically mean continuous thermal resistance. Fasteners, brackets, steel plates and aluminium components can substantially reduce effective performance.
Counting linear bridges but overlooking point elements
NECB 2025 specifically highlights point elements that penetrate insulation. Small components can become important when repeated across a large façade.
Waiting until the energy model is complete
Thermal modeling is most valuable when the results can still influence the detail. Late analysis often turns an optimization exercise into a compliance problem.
What this means for façade teams
Architects, façade consultants, manufacturers and contractors should expect thermal performance to become more closely tied to actual connection details and interfaces. The design conversation is shifting from “How much insulation is shown?” toward “How does heat actually flow through the complete assembly?”
For curtain wall projects, that means thermal review should sit alongside structural, air/water and constructability reviews—not after them.
Frequently asked questions
Does NECB 2025 automatically apply to every Canadian project?
No. The NECB is a national model code and must be adopted by the applicable authority having jurisdiction. Project teams should verify the legally adopted code and any local amendments.
Does CSA Z5010 replace curtain wall U-factor simulation?
No. CSA Z5010 addresses thermal-bridge simulation for building enclosures. Fenestration-system U-values are determined using the applicable fenestration methods. The two analyses are complementary.
When should thermal modeling start?
Ideally during design development, once the primary curtain wall system, slab-edge condition and major interfaces are known but before details are fully frozen.
Bottom line
Curtain wall thermal modeling is becoming a more important bridge between façade detailing and energy-code compliance. NECB 2025’s stronger treatment of thermal bridging and its reference to CSA Z5010 make it increasingly important to quantify repeated linear and point losses, document assumptions and address weak details early.
For more façade-engineering material, visit the Curtainwall.ca Resources page or explore Calculation Training.
Featured image credit: Christian Ladewig via Unsplash.