Fire performance of multi-pane low-energy windows in post-flashover fires

The effect of ventilation conditions on the development of compartment fires is a well-described phenomenon, and the impact on the structural response of a building or on the spread of fire to and from façades can be significant. However, most, if not all, fire models such as the Eurocode parametric fire (EN 1991-1-2, annex A) [2] do not consider that new ventilation openings may be created during the fire as window glass breaks and falls out. This is despite fire-induced glass breakage being identified as an important topic of interest for the broad community of fire engineers and fire scientists all the way back in 1985, when Prof. H. W. Emmons presented at the first IAFSS symposium [3]. It is not for the lack of trying, though. Since Prof. Emmon’s presentation, several studies have investigated window breakage in fires both in relation to the thermal and mechanical effects on the material level (see an overview in e.g. [4], [5], [6]) and to a lesser extent in relation to the derived effects on the fire (see e.g. [7]).

Most of our knowledge and understanding of the performance of windows in fires relate to legacy windows with single or double layers of glass panes. But modern windows should not be expected to behave like these for several reasons;

i. modern windows are typically larger to allow more sunlight into the building,
ii. they need toughened glass to withstand the mechanical forces they experience,
iii. they use low-energy coatings to increase their energy efficiency, and
iv. their cavities between panes are filled with argon instead of air to decrease the conductivity and internal convection.

Additionally, at least in the Nordic regions, modern windows have triple layers of glass panes, and often there is a requirement to use laminated safety glass for personal safety. All these factors affect the fire performance in different ways, but the general trend seems to be that the fire resistance of windows has increased over the last 30 years. In relation to windows, fire resistance means both the time until the window cracks and the amount of glass that falls out of the frame, which then forms the actual ventilation opening.

Despite the overall trend and our knowledge of the important impact ventilation conditions have on fire development, our fundamental understanding of glass breakage in modern windows is still limited. Therefore, we decided to look more closely into the subject through a series of tests conducted at the Danish Institute for Fire and Security Technology (DBI).