Currently in Europe there is a growing trend towards low energy and passive houses where natural materials are often used. These materials are very promising in terms of environmental protection. The use of green thermal insulation materials brings a significant reduction of energy requirements of buildings which is their major benefit. Previous research shows that crop-based thermal insulating materials exhibit very good thermal insulation and acoustic properties. However, their structure is different from conventional insulation materials (EPS, XPS, PUR, and MW) and their hygrothermal behaviour is much different. This article focuses on studying the behaviour of fibrous green insulations under humidity and moisture load. The test samples of the materials were treated with different types of hydrophobic agents before testing. The goal of the experiments was to reduce the sensitivity of the insulation to humidity and moisture (i.e. reduce their hygroscopicity). The chemical treatment of the fibres should improve the practical properties of the thermal insulation materials and extend their service life after they have been incorporated in a structure and attacked by moisture.