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At what temperature does windows break? Here is an interesting article that can answer this question:

Article By Dr. Vytenis Babrauskas, Fire Science and Technology Inc.

A theory exists for predicting the occurrence of the first crack in glazing, but this is not directly relevant to fire ventilation.

It is, in fact, very difficult to predict when glass will actually break enough to fall out in a real fire. The Russian recommendation of 300ºC appears to be a reasonable lower bound for the gas temperature required for breakage.

The BRI study can be taken to indicate that 3 mm window glass will break at a gas temperature of around 360ºC. For thicker, 4-6 mm glass, the mean temperature of breakage would appear to be around 450ºC, although the difference between the thinner and the thicker glass results seems rather larger than one would surmise. Double-glazed windows using 6 mm glass can be expected to break out at about 600ºC. Tempered-glass in not likely to break out until after room flashover has been reached.

In terms of external fires, at a heat flux of 9 kW m-2 some experimental results on ordinary glass showed the possibility of fallout, but the probability of fallout does not become high until about 35 kW m-2 is reached. Double-glazed windows can resist approximately 25 kW m-2 without fall-out. Tempered glass is able to resist fluxes of 43 kW m-2, at least under some conditions.

Factors such as window size, frame type, glass thickness, glass defects, and vertical temperature gradient may all be expected to have an effect on glass fall-out. Over-pressure due to gas explosions is an obvious glass failure mechanism. Yet, normal fires do show pressure variations and these could potentially affect the failure of glass panes.

All of these factors deserve some more study to obtain useful, quantitative guidance.

The above review has dealt only with the role of glass breakage in fire ventilation.

An extreme radiant exposure resulted in the failure of this window including both pieces of glass in a dual pane window, and the vinyl clad screen.

A window containing tempered glass. Note how tempered glass breaks into

small chunks.

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