Historically, sea grass has been known and used for years as thermal insulation material. However, it is not recognized as a contemporary building material, and thus information regarding its hygrothermal properties and performance in modern constructions is scarce. In the attempt to create an image of the hygrothermal properties of sea grass as thermal insulation, experiments have been carried out in the laboratory to determine the thermal conductivity, the sorption curves and the water vapour permeability of the material. In order to investigate the hygrothermal performance a number of test walls were built and humidity in a construction containing sea grass was measured.