All tested materials were permeable to water vapour, so moisture can move inside the insulation layer in form of water and water vapour. Due to variations in thermal conductivity values, it must be pointed out that insulation capacity of a product depends on the manufacturing process. The smaller variations the quality of insulation production has - the better.
Moisture content of insulation layer also affects the insulation capacity, and therefore the structures should be designed so that the relative humidity inside the structure is as low as possible. There is a growing interest for diffusion-open ("breathable") structures in Finland. These are insulated with hygroscopic insulation, and hemp is one possible material for them. Hygroscopic insulation has been considered as protection against condensation. However, there is always a risk of structural failure or other unpredictable event, and therefore an organic insulation must be treated with fungicide.
Choosing the proper type of fungicide is a challenging task. Manufacturing techniques should be developed to spread and attach the chemical to the insulation product. Using a windshield with thermal insulation capacity, e.g. mineral wool board, decreases moulding risk by improving conditions inside the structure.
Air permeability was not measured during this study. Visual inspections indicate that air permeability of hemp is at the same magnitude as other fibrous insulation materials with same density. Therefore, a windshield must be used in external walls insulated with any type of hemp material.