Jim Carfrae


This thesis is an investigation into the moisture performance of straw bales used in the construction of buildings. The principle of taking bales of straw off the field and stacking them up on themselves to form the walls of a simple building is a practise that started over a hundred years ago. The modern form of this building method is more sophisticated, and is spreading world wide from its origins in the arid regions of America. Despite advances in modern methods of construction there has been concern and doubt over the suitability of straw bale for use as a building material in a temperate maritime climate. The main concern being that the higher levels of environmental moisture will have the potential to damage the straw over time. In order to assess the moisture performance of the straw bales in the walls of a building in this damp climate, a simple and effective means of measuring the moisture in-situ has been developed as part of this research. The overarching methodology for this research is to develop a more accurate version of a probe that uses a block of wood to measure moisture. An environmental chamber in the laboratory has been used to establish the hygrothermal relationship between the timber to be used in the probe, and samples of the straw used in construction. This is the first time that a continuous set of sorption and desorption isotherms have been created for samples of straw and timber simultaneously, a process that took six months to complete. This data was used in the design of a new wood block probe, and examples of the new probes were installed in the walls of a straw bale house with a known moisture history. The resulting readings from the new probe were compared to those from a professional agricultural straw moisture probe. These results could be checked against the readings of the relative humidity and temperature in the wall. Forty-eight pairs of the new wood block probe were calibrated in the laboratory. Fourteen diverse examples of straw bale construction were selected as case study buildings. Having been surveyed for this research, a number were then selected to have the new probes installed, and evidence of their moisture performance was recorded. Sufficient data was acquired through this process to confirm the suitability of straw bales for use in the construction of buildings, in a temperate maritime climate.

Document Type


Publication Date