This research analyses straw degradation inside straw bale walls in the region and develops the prediction of degradation inside straw bale walls. The results show that the straw inside straw bale walls have no serious concerns of degradation in the high hygrothermal environment in the region with only moderate concerns of degradation in the area 2–3 cm deep behind the lime render. The onsite investigations indicate that the degradation isopleth model can only predict straw conditions behind the rendering layer, whereas the isothermal model fits the complete situation inside straw bale walls. This research develops the models for predicting straw degradation levels inside a straw bale building in a warm (humid) continental climate. The impact of this research will help the growth of low carbon energy efficient straw bale construction with confidence pertaining to its long-term durability characteristics both in the region and regions sharing similar climatic features globally.