Embodied energy is a measure of the energy used in producing, transporting and assembling the materials for a building. Operational energy is the energy used to moderate the indoor environment to make it functional or comfortable—primarily, to heat or cool the building. For many building geometries, the walls make the most significant contribution to the embodied energy of the building, and they are also the path of greatest heat loss or gain through the fabric, as they often have a greater surface area than the roof or floor. Adding insulation reduces the heat flow through the wall, reducing the energy used during operation, but this adds to the embodied energy. The operational energy is not only a function of the wall buildup, but also depends on the climate, occupancy pattern, and heating strategy, making an optimisation for minimum overall energy use non-trivial. This study presents a comparison of typical wall construction types and heating strategies in a temperate maritime climate. The transient energy ratio method is a means to abstract the heat flow through the walls (operational energy for heating), allowing assessment of the influence of walls in isolation (i.e. in a general sense, without being restricted to particular building geometries). Three retrofit scenarios for a solid wall are considered. At very low U-values, overall energy use can increase as the embodied energy can exceed the operational energy; current best practice walls coupled with low building lifetimes mean that this point may be reached in the near future. Substantial uncertainty is present in existing embodied energy data, and given its contribution to total energy use, this is a topic of urgent concern.