The spacer bar is one of the least visible but most important components in a sealed unit. It performs three critical functions: maintaining the cavity width between panes, containing the desiccant that absorbs moisture, and forming part of the primary and secondary seals that keep the unit airtight.
The cold bridge problem: Traditional aluminium spacer bars conduct heat approximately 1,000 times faster than glass. This creates a cold strip around the edge of the unit where internal condensation forms — visible as water droplets at the bottom of the window, particularly in cold weather.
Warm-edge spacers use materials with much lower thermal conductivity — typically stainless steel (15 W/mK vs aluminium's 200 W/mK) or composite materials (0.2-1.0 W/mK). This dramatically reduces the cold bridge effect, improving both comfort and the overall U-value of the window.
Impact on U-value: Switching from an aluminium spacer to a warm-edge spacer improves the overall window U-value by approximately 0.1-0.2 W/m²K. This seems small, but when Part L compliance margins are tight, it can make the difference between passing and failing.