The overall performance of an envelope component such as a wall or roof is difficult to predict. Thermal andmoisture performance depend on the combination of materials used in the structure, thermal bridges, individualmaterials, construction workmanship, climate, orientation, air tightness, and indoor conditions. Modern buildingcodes are requiring more insulation and tighter construction to enable energy efficient buildings but provide littleguidance about how to ensure these assemblies remain moisture durable. Furthermore, as new products andmaterials are introduced, builders are increasingly uncertain about the long-term durability of their buildingenvelope designs.The Building Science Advisor (BSA) is a rule-based expert system webtool that is free for anyone to use and guidesthe user in selecting energy efficient and moisture durable wall designs tailored for their specific climate. Thecurrent state of the tool required the combination of expert opinion and hygrothermal simulations and thegrouping of building materials into different material property categories to build a concise rule-based database.We will present how this was accomplished as well as and different use cases of the tool. Then we will outline newfeatures that will be added to the BSA such as adding probabilistic hygrothermal simulation results for walls thatexperts have little experience with and the expansion of the tool to include retrofit capabilities.