Aluminium window frames dominate the global fenestration market. But their fundamental weakness — thermal conductivity of 160 W/m·K — creates thermal bridges that undermine building envelope performance. Pultruded FRP (fiber reinforced polymer) window frames, with thermal conductivity of just 0.3 W/m·K, eliminate this problem entirely.
The Physics of Thermal Bridging
A thermal bridge is a pathway through the building envelope where heat transfers more readily than through the surrounding insulated assembly. In a wall with R-20 insulation, an aluminium window frame acts as a thermal short circuit, conducting heat at a rate approximately 500 times higher than the adjacent insulation. Even with polyamide thermal break strips, aluminium frames typically achieve U-values of 2.0-3.5 W/m²·K for the frame alone.
FRP Frame Performance
Pultruded FRP window frames achieve frame U-values of 0.8-1.2 W/m²·K — without thermal breaks, because the entire frame is inherently insulating. When combined with triple-glazed insulating glass units, FRP fenestration systems routinely achieve overall window U-values below 0.8 W/m²·K, meeting Passivhaus certification requirements.
Dimensional Stability
Unlike PVC frames that expand and contract significantly with temperature changes, pultruded FRP profiles have a coefficient of thermal expansion similar to glass (6-10 × 10⁻⁶/°C vs glass at 9 × 10⁻⁶/°C). This thermal compatibility reduces seal stress and maintains long-term air tightness — critical for energy-efficient building envelopes.
Structural Strength
FRP window frames offer tensile strength exceeding 240 MPa — comparable to aluminium alloys — while being 35% lighter. This allows larger spans and narrower sight lines without structural compromise. The high stiffness-to-weight ratio of pultruded FRP enables slender frame profiles that maximize glazing area and natural light.
F1 Composite Fenestration Systems
F1 Composite offers 70, 80, and 90-series pultruded FRP fenestration profiles designed for casement, tilt-and-turn, sliding, and fixed window configurations. All systems are designed for compatibility with standard hardware and weatherseal systems, making specification and installation straightforward for fabricators and installers.
Contact our fenestration engineering team for detailed U-value calculations, section drawings, and glass configuration recommendations for your project.