How to Choose Thermal Break Strips for Curtain Wall Systems

1Understanding the Role of Thermal Break Geometry in Curtain Wall Frames

In a curtain wall mullion or transom, the thermal break strip does more than thermally decouple inner and outer aluminium shells — it also functions as a structural bridge that must transmit wind loads, dead loads from glazing and panel infills, and differential thermal movement across the frame. This dual structural-thermal role distinguishes curtain wall strip selection from the simpler requirements of window or door frame applications.

The critical geometric parameters are:

• Strip width (the insulating bridge dimension): wider strips reduce thermal conductance but reduce structural stiffness of the composite frame section; standard widths for curtain wall applications range from 24 mm to 34 mm, with wider sections used in passive or near-zero-energy building (NZEB) specifications
• Strip depth: matches the frame depth and determines the contact area with aluminium; insufficient depth creates stress concentrations at the aluminium-polymer interface under load
• Profile geometry (I, T, U, or custom extrusion): determines how the strip keys into the aluminium channel through roll-forming or polyurethane pouring processes; the interlocking geometry governs shear transfer and pull-out resistance

2Material Selection: Thermal Conductivity vs. Mechanical Demand

Polyamide 66 reinforced with glass fibre (PA66 GF) is the dominant material for curtain wall thermal break strips by a significant margin, and for good reason. Its combination of low thermal conductivity, high tensile and compressive strength, chemical resistance, and dimensional stability under sustained load makes it the only material currently qualified to EN 14024 structural requirements across the full range of curtain wall load cases.

The glass fibre content — typically between 25% and 30% — is a meaningful variable:

• PA66 GF25 Granules: offers a thermal conductivity of approximately 0.30 W/m·K and sufficient strength for the majority of standard curtain wall frames; it is the baseline specification for most commercial projects
• PA66 GF30 Granules: increases glass fibre content to raise tensile and compressive strength by approximately 15–20%, at a marginal increase in conductivity; appropriate for high-wind-load facades, tall buildings, or large-format panel systems where bending moments in the frame are elevated

Aerogel composite strips represent the emerging premium tier, offering thermal conductivities as low as 0.015–0.020 W/m·K — an order of magnitude below conventional PA66. Their application is currently concentrated in passive house, NZEB, and certified low-energy facade projects where the frame contribution to overall Uf value requires minimisation. Structural performance must be independently verified for each aerogel composite product, as mechanical properties vary significantly between manufacturers.

The table below summarises key material properties across the principal strip types used in curtain wall applications.

MATERIAL PROPERTIES COMPARISON

3Thermal Performance Metrics: What the Numbers Actually Mean

Specifying a strip by material alone is insufficient. The relevant performance metric for a curtain wall application is the linear thermal transmittance of the frame section (the Ψ-value, expressed in W/m·K), which accounts for the combined effect of strip width, material conductivity, aluminium section geometry, and frame depth. This value is used in energy modelling and compliance calculations under standards such as ISO 10077-2 or THERM simulation.

Key thermal parameters to verify with the strip manufacturer or system supplier:

• Declared thermal conductivity (λ): this must be the aged, stabilised value tested in accordance with EN ISO 10456, not the initial measured value, which may be up to 20% lower before thermal ageing stabilises the material
• Frame Uf value contribution: the strip's effect on frame thermal transmittance should be quantified for the specific aluminium section in question; a strip with lower λ does not automatically produce a lower Uf if the frame geometry creates significant two-dimensional heat flow effects around the strip
• Thermal cycling performance: the strip must maintain dimensional and mechanical stability across the service temperature range expected for the facade orientation and climate zone; for south-facing facades in high-solar-climates, surface temperatures can exceed 80°C, requiring materials with rated service temperatures of at least 100°C continuous

4Structural Integrity and Load Transfer Verification

A thermal break strip that softens, creeps, or delaminates under sustained mechanical load represents a facade safety risk, not merely a performance deficiency. The structural adequacy of the strip within the composite frame section must be verified against the design wind pressure and dead load requirements of the project.

The parameters governing structural suitability include:

• Shear strength of the strip-aluminium interface: verified through the roll-forming or pouring process qualification; EN 14024 requires a minimum shear strength of 3.5 kN per 100 mm of strip length under transverse load
• Creep resistance under sustained load: PA66-based strips must demonstrate less than 0.5% deformation under design service stress over a period of 10,000 hours; creep failure manifests as progressive frame deflection and eventual joint seal failure
• Impact resistance at low temperature: facades in cold climates subject to maintenance access or accidental impact must use strips that retain adequate toughness at temperatures down to −30°C or lower as specified
• Compatibility with frame assembly process: roll-crimping, knurling pattern, and surface treatment of the aluminium channel must be matched to the strip type; incompatible combinations produce inadequate mechanical interlock regardless of material quality

5Certification, Traceability, and Supply Chain Verification

Thermal break strips for curtain wall applications are structural components, and their procurement should be treated accordingly. Specifying a product by material description alone — without reference to certified test data and a defined supply chain — exposes the project to substitution risk during procurement and construction.

The minimum verification requirements for a robust specification are:

• Third-party certification to EN 14024 or equivalent: confirming that the specific strip profile has been tested as part of a qualified composite frame system, not in isolation
• Traceability documentation: batch-level records linking delivered product to certified test specimens; particularly important where aerogel or novel composite materials are specified
• Dimensional tolerances: manufacturer's declared tolerances for strip width, depth, and straightness, verified against the aluminium frame channel dimensions to ensure assembly compatibility
• Fire performance classification: relevant where the curtain wall system requires a declared fire resistance or reaction-to-fire class under EN 13501 or local building regulations