There is a growing need for climate-conscious constructions. Exterior walls insulated with straw bales are potentially a step in this direction. The straw is locally produced, carbon neutral, and has good energy efficiency. However, the susceptibility of straw to mold is a major concern in temperate climates. The purpose of this study was to investigate the risk of mold growth in exterior walls insulated with straw bales. Through computer simulations using the software Wärme Und Feuchte Instationär (WUFI), the relative humidity and temperature in vented and unvented exterior straw bale walls were examined under southern Swedish climactic conditions. The relative humidity and temperature were then applied to three different mold models: the Isopleth, the Folos 2D-, and the Mold 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 was any construction that had no risk of mold growth. The study showed that the common design solutions for straw bale constructions were likely to incur a risk of mold growth. Ventilated, infrequently used straw bale constructions incurred less risk of mold growth.