The Building Science Advisor (BSA) is a rule-based expert system webtool that was originally developed to assist building professionals to design energy efficient and durable wall systems for new construction. With the present focus being placed on upgrading the existing building stock, a retrofit module has been developed that, based on the location, existing construction, and planned retrofit strategy, provides recommendations on how to address the retrofit in a manner that will not create a durability problem. This webinar will describe the development of this tool and demonstrate its features and capabilities.