Traditionally, spacers made of aluminium or galvanised steel are used for insulating glass. However, these create thermal bridges due to their thermal conductivity. Spacers with low thermal conductivity (low Psi-value, heat-relevant values) such as those made of plastic or stainless steel are therefore used to reduce heat loss in the edge area. The UW value (heat transfer coefficient of the window, heat-relevant values) can thus be improved by approx. 0.1 W/m²K. In addition, condensation in the edge area is avoided.

In the case of multiple glazing, the warm edge is a spacer in the transition area from the glass to the frame (edge seal), which keeps the individual glass panes permanently at a distance and is made of a material with very low thermal conductivity. Originally, the spacer was made of aluminium, which, however, has a high thermal conductivity and thus causes heat loss in the edge area of the glazing. Increased requirements for energy efficiency have led to the development of spacers made of highly insulating plastic, which is characterised by very low thermal conductivity. The use of the new material resulted in an increase in temperature at the inner edge of the window, which led to the name “warm edge” being coined.