Advanced Thermal Insulation Plasters Derived from Hazelnut Shell Waste: A Comprehensive Experimental Research

Reducing thermal losses through building envelopes remains a key strategy in the pursuit of low-carbon, energy-efficient buildings. This study presents an innovative and sustainable retrofitting approach involving thermal insulation plaster modified with finely ground hazelnut shells, an abundant agricultural by-product in Türkiye. The modified plaster is applied symmetrically on both sides of standard masonry briquettes in varying proportions (2%, 4%, and 6%), and its thermal performance is experimentally assessed via the laboratory-scale coheating test method. The results reveal a substantial reduction in U-values compared to the uninsulated briquette (5.5 W/m 2 K): the 2% shell-modified plaster achieves a U-value of 2.40 W/m 2 K (56.4% improvement), the 4% variant achieves 2.14 W/m 2 K (61.1%), and the 6% formulation performs best at 2.04 W/m 2 K (62.9%). In terms of effective thermal conductivity, the modified plasters exhibit values in the range of 0.0408–0.04856 W/mK. Additionally, the 6% composition exhibits enhanced thermal inertia, delaying internal heat loss and offering extended indoor comfort. All samples demonstrate exceptional measurement repeatability, with day-to-day U-value variation below 2%. These findings surpass thermal performance benchmarks reported in previous studies using bamboo or plaster thickness alterations, and position hazelnut shell-modified plaster as a high-potential solution for sustainable building retrofits. The outcomes offer practical implications for low-cost housing, rural construction, and building refurbishment programmes, while also informing policymakers and material standardisation bodies about scalable bio-based alternatives that align with circular economy and decarbonisation goals.