A comparative Life Cycle Assessment (LCA) was performed on two natural fibre thermal insulation products made from hemp and sheep wool and a benchmark mineral wool product. The assessment revealed both advantages and some disadvantages with the natural fibre materials. A major finding was the particularly low or absent impact in terms of global warming potential shown by the natural fibre materials. This was caused by the renewable carbon sequestered in the material withdrawing carbon dioxide from the atmosphere. With regard to the end of the product life, the study showed that only a proportion of the total amount of carbon dioxide was released in landfilling and composting scenarios. Dynamic vapour sorption analysis was conducted on varied natural fibres in order to develop a protocol for studying moisture sorption in natural fibres. Data from these studies were subsequently used in calculating the integral heat of wetting in hemp fibres. The energy release caused by the quantity of material studied in the LCA during predicted relative humidity fluctuations was found unlikely to affect the results of the LCA. However, where large quantities of natural fibres are used throughout a construction the heat of wetting may produce a notable difference in the internal temperature of a building and provide a degree of passive environmental control.