Solid masonry walling was the accepted form of wall construction until it was superseded in the early 1900s by twin-leaf (cavity) masonry construction and recently lightweight framed construction. However, solid masonry construction is estimated to represent some 15% of the existing domestic stock, a considerable number of dwellings which could be thermally upgraded. Additionally, the increased levels of insulation proposed by building regulations may present, in some instances, technical challenges to cavity and framed construction. In these cases, it may be appropriate to consider specifying solid masonry construction for new build. The objective of this Good Building Guide is to highlight some of the risks associated with solid masonry walling and to provide solutions for improving the thermal performance of this construction form. 8 pages.