There is a growing need for climate conscious constructions. Exterior walls isolated with straw bales are potentially a step in this direction. The material straw is locally produced, carbon neutral with good energy efficiency. However, there are risks with straw as a building material and susceptibility to mould is a major concern in temperate climates. The purpose of this study was to investigate the risk for mould growth in exterior walls isolated with straw bales, which also entailed an investigation of the conditions for mould growth. Through computer simulations in the software Wärme und FeuchteInstationär (WUFI) the relative humidity and temperature in vented and unvented exterior straw bale walls were examined in a southern Swedish climate. The relative humidity and temperature where then applied to three different mould models:Isopleth-, Folos 2D- and Mould Resistance Design (MRD)-model. A parametric study was also conducted to ascertain the most sensitive parameters for straw bales.One of the primary objectives was to investigate whether there were any constructions that had no risk for mould growth. For the simulation of design solutions, 3 vented and 3 unvented constructions were used. The choice of design solutions was based on common straw bale constructions, but also unconventional solutions. The study shows that common design solutions of straw bale constructions are likely to have a risk for mould growth. Ventilated, infrequently used straw bale constructions have less risk for mould growth.