The use of hemp-lime as a construction material combines renewable low carbon materials with exceptional hygrothermal performance. The hemp plant can grow up to 4 m in four months, with low fertilizer and irrigation demand, making it very efficient in the use of time and material resources. All parts of the plant can be used – the seed for food stuffs, the fibre surrounding the stem for paper, clothing and resin reinforcement, and the woody core of the stem as animal bedding and aggregate in hemp-lime construction. The unique pore structure of the woody core (shiv) confers relatively low thermal conductivity and hygric buffering. The construction technique promotes good air tightness and minimal thermal bridging of the building envelope. All these factors combine to produce low carbon, hygrothermally efficient buildings that are low energy both in construction and in use, and offer opportunities for recycling at end of life. This paper presents the hygrothermal performance of an experimental hemp-lime building and compares the results of steady-state co-heating tests with laboratory tests and computer simulations of transient performance.