Low-emissivity (low-E) window glass is a valuable green technology that helps keep buildings warm in winter and cool in summer. However, a rare side-effect can cause fires on neighbouring properties.
How low-E windows work
The glass is coated with a thin layer of metal or metal oxide that allows visible light to pass through but reflects infrared radiation. This means heat from the interior is reflected back inside, retaining warmth in winter, while unwanted solar radiation is repelled in summer.
The magnifying-glass effect
But that reflected heat must go somewhere, which can become a problem if the glass is slightly bowed. A curved window acts like a magnifying glass, focusing heat on a spot or along a line. There have been instances 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 a neighbour's newly fitted low-E windows. After three fires in one week, the affected homeowner moved a propane tank that was close to the focal point.
Architects and homeowners beware
Several high-profile cases mean architects now know to avoid the curved reflecting glass that can produce “skyscraper death rays” in larger buildings. But the small domestic version may be less obvious, as any low-E window could potentially be a firestarter if it is poorly manufactured.
