Low-emissivity (low-E) windows, a popular green technology for keeping buildings warm in winter and cool in summer, have a rare side-effect: they can set neighbours' property on fire. The glass is coated with a thin layer of metal or metal oxide that lets visible light through but acts like an infrared mirror. Heat from the interior is reflected back in during winter, while unwanted solar radiation is repelled in summer.
How Bowed Glass Creates a Fire Risk
However, that reflected heat must go somewhere, which becomes problematic if the glass is slightly bowed. The curved window acts like a magnifying glass, focusing heat onto a spot or along a line. Instances have been reported where concentrated reflected sunlight has scorched wooden decking, melted artificial turf, and damaged plants.
In one extreme case, a series of fires was traced to newly fitted low-E windows in a neighbour's home. After three fires in one week, the affected homeowner moved a propane tank that was close to the focal point. According to experts, any low-E window could potentially be a firestarter if it is poorly manufactured.
Architects Warn of 'Skyscraper Death Rays'
Several high-profile cases have made architects aware of the dangers of curved reflecting glass in larger buildings, where they can produce what are colloquially called “skyscraper death rays”. However, the small domestic version may be less obvious, as any low-E window could be a hazard if the glass is bowed.
The problem arises when sunlight reflects off the window and is concentrated onto a nearby surface. The coating on low-E glass is designed to reflect infrared radiation, but if the glass is not perfectly flat, the reflected rays can converge. This concentrated energy can raise the temperature of materials like wood or plastic to their ignition point.
Preventive Measures and Industry Awareness
Architects and builders are now advised to specify flat low-E glass and to avoid curved designs that could focus sunlight. Homeowners should inspect windows for any visible bowing and consider the placement of combustible materials near windows. The construction industry is also urged to improve quality control during manufacturing to prevent defects that could lead to dangerous focusing effects.
