Windows subjected to a radiant heat or direct flame contact exposure can fail if the framing material ignites or if the glass breaks.
Research has shown that glass is the most vulnerable part of the window. This finding only applies to the annealed and tempered glass commonly found in residential construction. If you have more expensive types of glass such as ceramic or a dual pane / intumescent filled unit, then failure from frame ignition would be a more likely scenario. From a fire performance perspective, tempered glass is much stronger and therefore performs much better than annealed glass when exposed to heat. Use of multi-layered glazing — for example, two or more pieces of glass in the window unit, such as a dual pane insulated glass unit — with tempered glass would be an affordable way to improve the ability of windows to better resist wildfire exposures. With glass being the most vulnerable part of the window, frame material can be selected based on aesthetic, cost or other performance issues.
SOURCE CREDIT: Surviving Wildfire