A thermal break is an insulating material — usually rigid polyamide (nylon) or polyurethane foam — inserted into the cross-section of an aluminum or metal window frame to interrupt the direct conductive path between the cold exterior metal and the warm interior metal. Without a thermal break, an aluminum frame conducts heat (and cold) almost as efficiently as a solid aluminum bar, making it a major source of energy loss and a condensation point.
Why Metal Frames Need Thermal Breaks
Aluminum is an excellent conductor of heat — about 1,000 times better than the argon gas in an IGU. An aluminum frame without a thermal break has an effective U-factor around 1.9–2.4, worse than single-pane glass. A thermally broken aluminum frame drops to U 0.4–0.6, making it comparable to wood or vinyl. The thermal break is what makes modern aluminum-framed windows energy-efficient.
Types of Thermal Breaks
- Polyamide (nylon) strut — two aluminum extrusions mechanically connected by a nylon bridge. Very durable and structurally sound. The most common commercial window thermal break.
- Poured-and-debridged polyurethane — liquid polyurethane poured into a channel in the aluminum profile and cured, then the connecting aluminum bridge is removed. Very low thermal conductivity.
- Foam-filled profiles — hollow aluminum profiles filled with expanding foam; less common, lower performance.
Thermal Breaks in Vinyl and Wood Windows
Vinyl and wood frames are inherently poor heat conductors, so a separate thermal break material is not needed. The frame material itself acts as the thermal barrier. This is why vinyl-framed windows consistently outperform aluminum-framed windows of similar glass specifications on whole-window U-factor ratings.
