During housing rehabilitation, wall insulation is often considered as a means of improving thermal performance and reducing the risk of condensation and mould growth.  Although the three approaches to installing insulation (cavity, internal and external) achieve similar improvements in thermal performance, a specification which is inappropriate for a particular property may have serious implications for weathertightness, durability, maintenance and fire safety. This guide is for use during the planning stages of rehabilitation projects. It summarises the key decisions when choosing a method of insulation and identifies the principal features of a property which influence the choice of a particular approach.