Thermal Break

A thermal break is a strip of non-conductive material placed at the edges of a sandwich construction door section. It sits between the outer and inner steel skins where they would otherwise touch at the rails and stiles. Raynor defines it as "the separation between the outer and inner surfaces of a door section."

Steel conducts heat far faster than foam insulation. Without a thermal break, heat travels through the outer steel skin, along the steel rail at the section edge, and straight into the inner steel skin. The foam in the center is bypassed entirely. This is called thermal bridging.

The thermal break material is typically a rigid polymer or nylon extrusion. It fits between the top rail, bottom rail, and stiles at each section edge. It stops the direct steel-to-steel path.

The difference shows up in measured R-values. A sandwich door without a thermal break may rate R-9. Add a thermal break at the edges and the same door can reach R-12 or higher. Manufacturers like Clopay and Amarr publish R-values for the full door assembly. Those numbers assume the thermal break is in place.

A sign that the thermal break is missing or damaged: condensation forms on the interior liner of a door section along the rail lines during cold weather. The cold is traveling through the metal frame and chilling the inner surface.

The meeting rail joint seals air between sections. The thermal break is a separate issue: it controls heat conduction through the frame itself.