Abstract In order to meet the energy performance regulations for new buildings and to improve the energy performance of the existing building stock, a high amount of insulation materials will be needed in the coming years. Meanwhile, the demand for renewable materials and materials with a low environmental impact is increasing. In previous research, the ecological and hygrothermal potential of lime hemp as a building material has been shown. Besides incorporating a large share of renewable and fast-growing hemp shives, lime hemp combines thermal insulating properties and a high thermal inertia with a good moisture regulating behaviour. However, despite the growing interest in the material and promising lab results, the thermal performance in real conditions is still uncertain. Due to the lack of validated data for in-situ cast walls, rather conservative λ-values for lime hemp are currently compulsory used in energy performance certification calculations in Flanders, the Flemish region of Belgium. In this paper, the results of a case study analysis of four real-life lime hemp projects are presented. The thermal performance of four dwellings with in-situ cast lime hemp walls (both new construction and renovation) is determined by in-situ measurements of the U-value. The measurements show that the in-situ U-value of the lime hemp walls is significantly better than the U-value that is calculated with the λ-value that is currently accepted by the Flemish Energy Agency. However, drying of lime is a slow process, resulting in initially higher U-values that gradually decrease in the first months after lime hemp application. Once dried, in-situ cast lime hemp walls can meet the Flemish energy performance requirements.