Solid mass masonry buildings are a significant fraction of the existing building stock, and many contribute to the historic fabric of neighborhoods. However, with wall R-values of R-3 to R-5, they do not meet modern standards for energy efficiency and comfort. Insulating these buildings successfully—without causing long-term damage—is a vital part of the ‘toolkit’ for meeting energy and climate goals. This session will cover potential pitfalls and risks of interior insulation, including interstitial condensation, freeze-thaw damage, decay of embedded wood members, and surface water concentrations. We will then cover assemblies and details that work to control these risks.