Sealants, Thermal Break Profiles & Curtain Wall Joint Performance

Published 8 October 2026 — Hanart Facade Engineering Team

Joint Types in Curtain Wall Systems

Curtain wall joints serve four distinct functions: structural bonding, weather sealing, thermal separation and movement accommodation.

Structural silicone joints. In four-sided SSG systems, the silicone adhesive carries glass dead weight and resists wind loads. The joint is designed with a width-to-depth ratio between 2:1 and 3:1, typically 12 mm wide by 6 mm deep. Structural silicone must achieve minimum tensile strength of 0.5 MPa and elongation at break above 150 percent per ASTM C1184. Substrates must be primed with manufacturer-approved primer to ensure adhesion to anodized aluminum and ceramic-fritted glass.

Weatherseal joints. Weatherseal sealants fill gaps between adjacent panels or between the curtain wall and building structure. Movement capability is classified per ASTM C920: Class 25 accommodates plus or minus 25 percent joint movement, while Class 50 accommodates plus or minus 50 percent.

EPDM gasket seals. EPDM rubber gaskets provide continuous compression seals in unitized systems. Compression set per ASTM D395 must be below 25 percent after 22 hours at 70 degrees Celsius to ensure long-term recovery.

Mechanical cap seals. Pressure plate systems compress a dry gasket with a metal plate screwed to the frame at 250 to 300 mm on centre for high-wind zones.

Structural Silicone Glazing (SSG) Adhesives

Structural silicone is the most critical adhesive in modern curtain walls. A failure risks glass fallout, air infiltration and water penetration.

Neutral-cure versus acetoxy-cure. Neutral-cure silicones release alcohol during curing and are compatible with metals, coated glass and stone substrates. Acetoxy-cure silicones release acetic acid that corrodes anodized aluminum. Hanart exclusively specifies neutral-cure structural silicones, typically two-part for factory assembly and one-part for field repairs.

Compatibility testing. Hanart conducts full compatibility and adhesion tests per ASTM C1087 and ASTM C794, including 21-day water immersion at 50 degrees Celsius, 5,000 hours UV exposure per ASTM G155, and cyclic shear testing at 25 percent strain for 2,000 cycles.

Strength requirements. Hanart designs SSG joints with a safety factor of 5.0 on characteristic tensile strength. For a typical four-sided SSG panel 1,500 mm by 2,500 mm weighing 200 kg, dead load requires 9 mm joint width; wind suction of 2,000 Pa brings the required width to 12 mm.

Durability. Hanart requires structural silicones to maintain at least 80 percent of initial tensile strength and elongation after 5,000 hours of xenon-arc exposure, corresponding to approximately 25 years of outdoor exposure.

Weatherseal Sealants & Movement Capability

Weatherseal sealants protect curtain wall joints from air and water infiltration while accommodating the movement that structural silicone cannot absorb. The selection of weatherseal sealant is governed by joint width, expected movement, environmental exposure and aesthetic requirements.

Movement capability: Class 25 versus Class 50. Class 25 sealants accommodate plus or minus 25 percent of the joint width. Class 50 sealants accommodate plus or minus 50 percent. Hanart specifies Class 25 for most panel-to-panel joints in unitized systems, where thermal movement is moderate. Class 50 is used for expansion joints, structural movement joints and interfaces with dissimilar materials such as concrete or steel, where differential movement is large.

Modulus selection. Low-modulus sealants (stress below 0.4 MPa at 100 percent elongation) place less stress on the bond faces and are preferred for sensitive substrates such as stone, coated glass and thin aluminum sections. High-modulus sealants (stress above 0.6 MPa) resist greater loads and are used in joints that must also carry limited structural loads. Hanart selects modulus based on substrate sensitivity and load requirements, with a default preference for low-modulus products to minimize adhesive failure risk.

Colour matching and UV stability. Weatherseal sealants are exposed to direct sunlight and must resist yellowing and chalking. Hanart requires that custom-matched sealants maintain Delta E colour shift below 3.0 after 2,000 hours of UV exposure, verified by spectrophotometric measurement per ASTM D2244. Certain sealant formulations with high plasticizer content attract atmospheric dust and develop a visible dirt line at the joint face; Hanart specifies low-dirt-pickup formulations for all visible facade joints.

EPDM Gaskets & Compression Seals

EPDM (ethylene-propylene-diene monomer) rubber is the dominant gasket material in curtain wall systems. Its combination of ozone resistance, UV stability, low compression set and cost effectiveness makes it ideal for continuous seals in unitized panels, stick-built pressure plates and sliding window systems.

Material composition and hardness. EPDM gaskets for curtain walls are typically compounded with 50 to 70 Shore A hardness, balancing compression force against insertion resistance. Hanart selects 60 Shore A as the default for unitized panel gaskets, with 50 Shore A for glass-to-frame seals where low compression force protects the glass edge.

Compression set and long-term recovery. Compression set measures the permanent deformation remaining after a gasket has been compressed and then released. Hanart requires EPDM gaskets with compression set below 20 percent after 70 hours at 100 degrees Celsius per ASTM D395 Method B. This ensures that after 25 years of compression, the gasket still exerts sufficient force to maintain air and water tightness.

Ozone and UV ageing resistance. Hanart requires EPDM gaskets to pass 100 pphm ozone exposure for 100 hours at 40 degrees Celsius per ASTM D1149 without cracking. UV resistance is verified by 2,000 hours in a xenon-arc chamber per ASTM G155 with no surface crazing or hardness change exceeding 5 Shore A points. Certain compound additives can leach plasticizers that stain light-coloured aluminum finishes; Hanart requires aluminium compatibility testing per ETAG 002 with no visible staining after 30 days at 70 degrees Celsius.

Polyamide Thermal Break Profiles

Thermal break profiles separate the exterior and interior aluminum extrusions in a thermally improved curtain wall frame. The thermal break material must insulate thermally while bonding structurally to the aluminum, maintaining shear and bending strength across the temperature range from minus 40 to plus 80 degrees Celsius.

PA66 GF25 composition. Polyamide 66 with 25 percent glass fibre reinforcement (PA66 GF25) is the industry standard for thermal breaks. The glass fibre increases tensile strength from 80 MPa to 180 MPa, while reducing thermal expansion from 80 microstrains per degree Celsius to 30 microstrains per degree Celsius. This coefficient is close to that of aluminum (24 microstrains per degree Celsius), minimizing thermally induced shear stress at the aluminum-polyamide interface.

Profile geometry. I-shape profiles provide the simplest thermal separation with a straight web between inner and outer frames, achieving a frame U-value of approximately 2.2 W per square metre-kelvin with a 34 mm break width. C-shape profiles add a return leg that increases the heat flow path, reducing U-value to approximately 1.8 W per square metre-kelvin. Cavity profiles incorporate hollow chambers filled with insulating air or polyurethane foam, achieving U-values below 1.5 W per square metre-kelvin. Hanart selects the profile type based on the project's thermal performance target, with C-shape profiles as the default for high-performance facades.

Linear expansion coefficient matching. The thermal break must expand and contract with the aluminum frame without delaminating. Over a temperature swing of 70 degrees Celsius, a 3,000 mm long thermal break will expand 6.3 mm while the aluminum expands 5.0 mm, creating a differential movement of 1.3 mm. Hanart designs the rolled-in knurl geometry to accommodate this differential movement through micro-slip at the interface, verified by 1,000 thermal cycles from minus 30 to plus 80 degrees Celsius per EN 14024.

Shear strength and structural integrity. The thermal break must transfer wind loads and dead loads from the outer frame to the inner frame without shear failure. The minimum shear strength requirement per EN 14024 is 25 MPa. Hanart requires a tested shear strength above 35 MPa with a safety factor of 3.0 on the design load. For a typical wind load of 2,000 Pa on a 1,500 mm wide mullion, the shear stress is 88 MPa, well below the allowable limit of 117 MPa.

Joint Failure Modes & Prevention

Joint failures in curtain walls follow predictable patterns. Understanding these failure modes allows Hanart to specify materials, detail joints and implement quality controls that prevent failures before they occur.

Sealant cohesive cracking. Cohesive failure occurs when the sealant tears within its own body, leaving adhesive bond to both substrates intact. It is caused by cyclic movement exceeding the sealant's fatigue limit, UV degradation of the polymer backbone, or excessive depth-to-width ratio that concentrates stress. Prevention requires correct joint sizing, Class 50 sealants in high-movement joints, and UV-stable formulations. Hanart verifies sealant fatigue life by 2,000-cycle movement testing per ISO 9047.

Adhesive failure and debonding. Adhesive failure occurs when the sealant separates from the substrate. The most common cause is inadequate surface preparation: anodized aluminum that has not been primed, glass with residual cutting oil, or concrete with loose laitance. Hanart prevents adhesive failure by mandating primer on all anodized aluminum, solvent cleaning of glass edges, and adhesion testing on every batch of substrate material.

Thermal bridge condensation. When the thermal break is too narrow, poorly fitted, or missing entirely, the interior aluminum surface becomes a cold bridge. In winter conditions, the surface temperature drops below the indoor dew point, causing condensation that stains finishes, promotes mould growth and degrades indoor air quality. Prevention requires thermal break widths of at least 34 mm in high-performance systems, continuous thermal break profiles without thermal short circuits at brackets or anchors, and dew point analysis per ISO 13788 for every project.

Electrochemical corrosion. Aluminum frames in contact with stainless steel anchors or copper flashings can suffer galvanic corrosion in the presence of moisture. The corrosion product occupies 2.5 times the volume of the original aluminum, exerting pressure that cracks sealant joints and deforms frames. Hanart prevents galvanic corrosion by inserting dielectric isolation layers between dissimilar metals, specifying compatible fastener materials per ISO 9227 salt spray testing, and designing drainage paths that prevent standing water at metal-to-metal interfaces.

Hanart Sealant Selection Protocol

Hanart operates a systematic sealant and gasket selection protocol that aligns international standards, project-specific conditions and factory quality control. The protocol ensures that every material in the curtain wall joint system is compatible, durable and correctly applied.

ASTM C920 and GB 16776 standard alignment. ASTM C920 classifies sealants by type, grade, class and use. Type S (single-component) and Type M (multi-component) define the cure chemistry. Grade NS (non-sag) and Grade P (pourable) define the viscosity. Class 25 and Class 50 define movement capability. Hanart maps these ASTM classifications to the Chinese national standard GB 16776, which specifies silicone sealants for building applications. GB 16776 requires tensile strength above 0.45 MPa, elongation at break above 100 percent, and adhesion after water immersion with no failure area exceeding 10 percent. Hanart requires all structural silicones to exceed both ASTM C920 and GB 16776 minimums by at least 25 percent.

Material compatibility pre-testing. Before production, Hanart conducts a three-phase compatibility test on the exact substrate combination for the project: 7-day ambient cure with daily visual inspection, 21-day water immersion at 50 degrees Celsius with adhesion testing per ASTM C794, and 30-day heat ageing at 80 degrees Celsius with hardness, tensile strength and elongation measurement. Only sealant-substrate combinations passing all three phases are approved. The results are documented in a project-specific compatibility certificate that travels with the panel through fabrication, shipment and installation.

Factory application quality control. Structural silicone is applied by automated dispensing robots in Hanart's unitized panel factory with positional accuracy of plus or minus 0.3 mm. Every panel receives visual inspection for voids and contamination, dimensional check of bead width and depth, 24-hour tack-free time verification, and a destructive adhesion peel test on 1 in 50 panels. Panels failing any check are stripped and resealed. For stick-built systems, Hanart deploys a sealant specialist who verifies substrate preparation, primer application, backer rod placement and tooling technique, with field adhesion tests at a rate of one per 20 metres of joint.

FAQ

What is the difference between structural silicone and weatherseal silicone?

Structural silicone carries wind and dead loads in SSG systems and must maintain adhesion under sustained stress. Weatherseal silicone only seals against air and water and is designed to accommodate larger movements. Structural silicone is tested to ASTM C1184 with higher strength requirements; weatherseal silicone is tested to ASTM C920 with emphasis on movement capability and durability. They are not interchangeable.

How wide should a thermal break be for a high-performance curtain wall?

Hanart specifies a minimum thermal break width of 34 mm for unitized systems targeting whole-wall U-values below 1.6 W per square metre-kelvin. Wider breaks of 44 mm or 52 mm are used for passive-house-grade facades. The break width must be continuous at anchors and brackets; any metal-to-metal contact creates a thermal short circuit that defeats the insulation.

Can EPDM gaskets be used with silicone sealant in the same joint?

Yes, but compatibility must be verified. Certain silicone plasticizers can migrate into EPDM and cause softening or swelling. Hanart tests the specific EPDM compound against the specific silicone formulation in a 30-day contact test at 70 degrees Celsius per ASTM C1087. Only compatible pairs are approved for production. In general, high-quality EPDM with peroxide curing and low plasticizer content is compatible with neutral-cure silicones.

What causes thermal break delamination and how is it prevented?

Delamination occurs when differential thermal expansion between aluminum and polyamide exceeds the bond strength of the rolled-in knurl interface. It is prevented by selecting PA66 GF25 with a thermal expansion coefficient close to aluminum, designing the knurl geometry to permit micro-slip, avoiding thermal break widths below 24 mm where shear stress is concentrated, and passing 1,000-cycle thermal shock testing per EN 14024.

How long do structural silicone joints last before replacement is needed?

Properly specified and applied structural silicone joints are designed for a 25 to 50 year service life. Accelerated ageing tests predict that high-quality structural silicones maintain at least 80 percent of initial strength after 25 years of outdoor exposure. Hanart warranties SSG systems for 10 years with an expected service life of 25 years. Routine inspection every 5 years is recommended to detect early signs of cohesive cracking, UV surface degradation or adhesive debonding.

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