Development of a novel Fibre-Reinforced Polymer (FRP) wall panel system for thermally efficient residential construction in New Zealand

Abstract

This thesis investigates the viability of a novel Fibre Reinforced Polymer (FRP) wall panel system as an alternative to conventional lightweight timber-framed construction for residential buildings in New Zealand. This study is motivated by the widening gap between the thermal performance assumed at the building consent stage and the significantly lower as-built performance delivered by traditional timber-framed envelopes. Despite the existing insulation requirements under the New Zealand Building Code, timber-framed walls routinely fail to meet the minimum thermal thresholds because of systemic thermal bridging, discontinuous insulation, and high framing percentages arising from structural, weathertightness, and regulatory constraints. These factors collectively undermine compliance with Clauses H1 (Energy Efficiency) and E3 (Internal Moisture), indicating the need for alternative construction systems capable of delivering verifiable thermal performance.

The FRP wall panel system evaluated in this study was first implemented in two full-scale duplex dwellings (the case-study building) completed in Hamilton in 2024, providing an opportunity for a comprehensive structural and thermal assessment. The system comprises factory-produced FRP composite panels with integrated phenolic foam insulation, tongue-and-groove joints, and a cavity air layer, yielding an inherently continuous thermal envelope. A multi-method research framework was employed, integrating (1) a comparative literature review, (2) NZS 4214 isothermal-planes thermal-bridging analysis, (3) laboratory thermal conductivity testing, (4) BRANZ P21 structural verification of wall and lintel specimens, and (5) in-field heat-flux and U-value monitoring undertaken in both summer and spring across two units of the case-study building.

The experimental results confirmed that the FRP wall panel system exhibited markedly reduced thermal bridging compared to timber framing. Laboratory characterisation established an FRP skin thermal conductivity of 0.248 W/m·K, enabling accurate construction-level thermal resistance (R-value) calculation. When applying NZS 4214 isothermal planes method, the FRP wall panel system achieved a conservative construction R-value of R2.84 m²·K/W representing a 70.1% improvement over a typical timber-framed wall with a 30.95% framing fraction (R1.67), and a 77.5% improvement over a 34.12% framing fraction scenario (R1.60). These findings demonstrate that the FRP wall panel system comfortably exceeds the current H1/AS1 requirement of R2.0 for external walls without relying on the thermal-mitigation strategies required in timber framing. Structural testing further confirmed that the FRP walls and lintel configurations met or exceeded the NZBC Clause B1 performance when assessed against the AS/NZS 1170 load combinations and BRANZ P21 cyclic racking criteria.

In-field monitoring corroborated the analytical results. Continuous heat flux measurements recorded stable in-situ U-values consistent with theoretical modelling, while cavity and indoor temperatures remained resilient to outdoor fluctuations. The system effectively mitigates localised thermal bridging at corners, junctions, and lintels, where timber-framed envelopes typically experience significant insulation discontinuities. The analysis produced 12 representative U-value measurements over different diurnal periods. The individual values ranged from 0.060 to 0.387 W/m²K, while the average U-value across the study period was 0.206 W/m²K with typical indoor and outdoor temperatures of 22 °C and 17 °C, respectively. When compared with the theoretically derived U-value of 0.352 W/m²K (equivalent to R2.84 m²·K/W), the in-field results show that, on average, the measured U-value was 41% lower than the theoretical value, corresponding to an in-situ R-value of approximately R4.85 m²·K/W, which is approximately 70.8% higher than the analytical R2.84 value.

Collectively, these findings provide strong and defensible evidence that the FRP wall panel system offers a credible, high-performance alternative to conventional timber-framed construction. Through integrated structural and thermal functionality, the system addresses longstanding deficiencies associated with timber-framed envelopes, simplifies compliance pathways across Clauses H1, E2, and B1, and delivers a more thermally reliable building envelope under real New Zealand Zone 2 climatic conditions. Therefore, this study establishes the FRP wall panel system as a technically robust and practically viable construction methodology capable of supporting the delivery of warmer, drier, healthier, and more energy- efficient housing in New Zealand.