Energy-efficiency of sustainable constructions and buildings are evaluated based upon the heating and cooling demands, but also according to the primary-energy demand, CO2 savings potential, and the ecological properties of building materials. To meet increasingly rigorous requirements, the demand for natural building materials is growing rapidly. The research objective of the here presented study is to stabilize soils with natural straw fibres to produce a composite, sustainable, non-toxic and locally sourced building material. The material appropriateness was determined by establishing the thermal conductivity of a selection of unfired earth bricks that were identified as potential new natural building materials. The thermal conductivity is an essential material characteristic to achieve the required insulation level and for market success as a new product. The earth bricks consist of soil, cement, gypsum and straw fibres. Straw was applied as fibre reinforcement for unfired bricks. Two fibre types were used: wheat and barley straw. The results indicated that the thermal conductivity of all investigated variants decreased with increasing fibre content while increasing with higher cement and gypsum contents. They also show that barley straw fibre reinforced bricks exhibited the highest thermal insulation values, The addition of fibre positively improves both, thermal and static properties. (C) 2015 Elsevier B.V. All rights reserved.