After world war two, cavity walls became a widespread external wall type in the cool but humid climate of North Western Europe. Moisture tolerance of the unfilled construction was excellent. Instead, calculation and testing underlined that the unfilled cavity walls of that era performed poorly from a thermal insulation point of view. After the energy crisis of 1973, cavity filling was therefore introduced as the main upgrade. Hence, extensive testing revealed upgrading was less simple than expected. Air in- and exfiltration through the wall, wind washing behind the fill, thermal stack induced air looping around the fill and thermal bridging all cooperated in lowering expected thermal quality of the filled walls. Anyhow, at the same time testing underlined that moisture tolerance remained outstanding in the cool, humid climate of North Western Europe also without cavity ventilation. That resulted in a set of recommendations how to construct high performing filled cavity walls. SAGE Publications 2007.