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Using finite elements, the authors studied temperatures and thermal stresses in 1.41 m wide by 2.12 m high sealed double-glazed windows of 6-mm glass with a 12-mm air space between the panes. The purpose was to find factors significantly influencing thermal stresses. Three types of commercially used windows subject to winter conditions (day and night) were analysed: both panes clear glass; outer pane heat absorbing and inner clear; and outer clear with a metallic film on its room-side surface and inner clear. A sensitivity analysis included variations in size of window, gasket and sealant stiffnesses, frame absorption, solar heat flux, exterior air film conductance, and outdoor air temperatures (−5 and −25 °C). Further, the influence of horizontal and vertical shadows was studied. For the calculation of thermal stresses, the authors found that the windows could be analyzed pane by pane rather than as a three-dimensional structure. Solar radiation and particularly shadows had a major influence on thermal stress, while window size and aspect ratio did not. A case study in which the predicted temperatures were compared with those measured by Sasaki in 1974 showed good agreement and indicated that a simpler steady-state rather than transient thermal analysis gave a close estimate of the temperature difference between the centre and edges of panes for design against thermal breakage. Key words: window breakage, thermal stresses, glass temperatures, shadows, double-glazing, finite elements.

General performance requirements for sealed double-glazed windows and the need for performance tests are discussed. The possibility of condensation of moisture and clouding or scumming on the inside glass surfaces is emphasized. The results of some exploratory studies on the degree of sealing achieved in a number of units and on condensation on the interior surface are given. These results are based on measurements of the dew point of the air between the panes and on surface temperature measurements taken with various temperature differences across the windows. Factors involved in determining the adequacy of the sealing of the space between the panes are considered and lines of study suggested. General requirements of a test procedure for determining the degree of sealing achieved and the relative life of seals are discussed and accelerated aging apparatus proposed for this purpose is described.

Many house owners are puzzled by the appearance of moisture as a film of water or as frost on their window panes in winter. This moisture comes from the warm air in the home and is deposited on the cold window surface by a process called condensation. This is the same action that causes moisture to collect on the surface of a glass containing ice water.