Microbial growth often thrives in humid conditions, at high relative humidity. Moulds are complex organisms; many types of mould are able to survive strong variations in humidity and temperature, such as those on building façades. For some building materials a critical relative humidity is determined, which functions as a theoretical threshold; at this (or lower) relative humidity microbial growth will likely not occur. Hemp-lime is a building material that consists of hemp shiv (the woody core parts of the hemp stem) and building lime. It is a material that can be used for walls, and even though it has been used for more than 20 years, thusfar little is known about its critical moisture levels for microbial growth. The aim of this research was therefore to determine at what relative humidity microbial growth occurs on carbonated hemp-lime material, and to study if there is a protective influence of a carbonated lime binder on the hemp shiv. The objective was to study microbial growth on hemp shiv, hemp-lime and on hemp with a thin layer of lime at three relative humidity (75 %, 85 % and 95 %) and at two different temperatures (15°C and 23°C); conditions that could occur naturally in a hemp-lime façade exposed to high rain loads in a northern European climate. Hemp shiv seems to have a relatively low resistance to microbial growth, similar to that of wood. However, because the hemp is protected by lime it can withstand much higher relative humidity without microbial growth occurring on the material. The critical moisture level for hemp-lime seemed to occur between 75 and 85 % RH, while the material was completely without microbial growth at 75 % RH. The lime had a protective effect on the hemp and acted as a mould inhibitor, both over time and with varying temperature and humidity.

Thermally insulating historic buildings is imperative in order to reduce energy demands of the existing building stock. Therefore an insulation material is needed that improves energy efficiency while being compatible with the existing structure from a hygrothermal, aesthetic and cultural heritage perspective. Hemp-lime is a building material that consists of on a combination of hemp shiv, the woody core part of the hemp stem, and building limes. The aim of this study was to determine if hemp-lime could be a feasible option for thermally insulating historic masonry walls in Sweden. The objectives were to measure energy performance of full-scale masonry façades insulated with hemp-lime and to monitor moisture levels inside the masonry walls. Three small single leaf masonry façades were constructed. One façade was uninsulated, the other had internal hemp-lime insulation and a third had external hemp-lime insulation. Energy use for space heating as well as temperature and relative humidity in the walls and rooms were monitored. Results show that thermally insulating historic masonry walls with hemp-lime can lead to an improvement in energy performance of 44-53 % compared to uninsulated single-leaf masonry. However, moisture levels were higher in the masonry façades that were insulated with hemp-lime.