From Material Design to Building-Envelope Performance: Multiscale Evaluation of Phase-Change-Material-Enhanced Hemp-Lime Composites

Abstract

Hemp-lime composites (HLCs) are increasingly used in sustainable building envelopes because they combine bio-based insulation, moisture buffering, and carbon storage. Their wider use is nevertheless limited by modest thermal energy storage capacity. This thesis addresses that limitation by integrating shape-stabilised, bio-derived phase change materials (PCMs) into HLC to increase latent heat storage while retaining the hygrothermal functions required for building-envelope applications.

The scientific contribution of this work is to link PCM-impregnated hemp-shiv characterisation, binder design, composite performance, and envelope-scale modelling within one PCM-HLC framework. Unlike previous studies that mainly examined isolated PCM additions, this thesis generates new knowledge on how hemp-shiv size, high PCM substitution, SCM-modified binders, casting direction, specimen scale, and multilayer wall configuration jointly governs PCM-HLC performance. Material characterisation showed that smaller hemp shiv increased capillary entrapment and enabled PCM substitution levels of up to 70% under the tested preparation route. To support this high loading, the study developed a binder design incorporating supplementary cementitious materials (SCMs). Silica fume and metakaolin improved the hemp-binder interface and reduced mass loss to below 0.5%, indicating that the cohesive mix design can maintain composite stability during testing and operation.

Comprehensive characterisation of the 70% PCM-HLC revealed a clear multi-criteria performance trade-off. Specific heat capacity increased by up to 80% during phase transition, and baseline sensible heat capacity increased by 60%. These gains were accompanied by matrix densification to 796 kg/m³, a reduction in compressive strength to 0.18 MPa, and a 66% decrease in moisture buffering value. Even with these reductions, the composite retained sufficient strength for non-load-bearing insulation and remained within the Nordtest Good to Excellent moisture-buffering classifications.

To translate the material findings into building-envelope design, the thesis proposed and numerically optimised a multilayer PCM-HLC sandwich panel for cold climates using finite-volume simulations. The analysis showed that PCM effectiveness is strongly seasonal under the simulated Ottawa climate and the adopted HVAC control assumptions. A double-PCM-layer design reduced the modelled summer cooling load to zero for the representative summer week, but could act as a winter thermal bridge unless the envelope thickness reached at least 14 cm. For space-constrained applications, an optimised 10 cm configuration using a single interior PCM layer, an internal air-cavity thermal break, and exterior HLC insulation provided the most favourable balance of storage and resistance among the configurations tested. Under the same modelling conditions, this design reduced the simulated summer cooling demand to zero and lowered winter heating energy by 27% relative to a monolithic HLC baseline. Overall, the thesis advances understanding of how PCM loading, binder design, porosity, moisture buffering, and thermal resistance interact across material and envelope scales. It also establishes quantitative thresholds and design guidance for using PCM-HLC as a climate-responsive, bio-based building-envelope material.