In recent decades there has been an increased focus on enhanced thermal resistance of building components and as a consequence, the relative importance of airtightness on the overall energy losses of buildings has increased significantly. The construction industry requires practical information on the airtightness of individual construction elements and building envelope interfaces. A literature review on the airtightness of window-wall interfaces has shown that no experimental data are available for masonry construction. This paper offers an investigative study on the airtightness of window-wall interfaces of masonry walls, for 13 different installation methods. The results show that the selected solutions cover a wide range of airtightness levels, from 0 m(3)/h m up to 31 m(3)/h m at 50 Pa. The experiments have permitted determining that a very good performance can be obtained by using polyurethane foam and caulking, airtight membranes, polyurethane foam and plywood framing, and plaster and caulking. On the contrary, mineral fibre insulation, a partial fill with polyurethane foam and plaster without caulking should be avoided when good airtightness is required. Furthermore, a comprehensive methodology for error calculation is offered, based on error propagation of partially correlated parameters, including the effect of measurement errors, extraneous air leakage and conversion to standard boundary conditions. (C) 2011 Elsevier B.V. All rights reserved.