Thermal break
A thermal break is a strip of low-conductivity material that splits a metal window frame into an inside half and an outside half, so heat and cold cannot travel straight through the metal. Aluminum windows need one to perform in any climate with a real winter.
Aluminum carries heat very well. That is why it makes a strong, slim window frame, and also why a solid aluminum frame is a problem in winter. Cold passes straight through the metal. The inside face of the frame becomes the coldest surface in the room, and condensation forms on it.
A thermal break fixes this. The frame is made as two aluminum shells, one outside and one inside, joined by a material that conducts heat several hundred times less than aluminum. Heat trying to cross the frame has to pass through that material, and most of it does not.
The two ways to build one
| Method | How it is made | Typical break width |
|---|---|---|
| Polyamide strips | Two glass-reinforced polyamide strips crimped between separate inside and outside profiles | about 14-45 mm (9/16 in. to 1-3/4 in.) |
| Pour-and-debridge | Polyurethane poured into a channel in one extrusion, then the aluminum bridge under it is cut away | about 6-10 mm (1/4 in. to 3/8 in.) |
Both methods are legitimate. Polyamide strips are standard on European systems. Pour-and-debridge is common on US frames. What matters most is the width of the break: a wider break gives heat a longer path through insulating material. Pour-and-debridge is cheap and proven. Strips can be made much wider, and the gap between them can hold a separate insulated cavity.
The European aluminum systems in my estimator list whole-window U-factors between 0.22 and 0.29 with double glazing. Most of that comes from a wide polyamide break.
What to ask for
Asking "is it thermally broken?" is not enough, because nearly everything sold today is. Ask these three questions:
- How wide is the break, in millimeters or inches?
- What is the whole-window U-factor for the exact size and glass on your schedule?
- What spacer is in the glass? An aluminum spacer undoes part of what a good break achieves.
The full comparison, with real numbers for each method, is in thermally broken aluminum windows explained.
Common questions
- Do uPVC or wood windows need a thermal break?
- No. uPVC and wood are poor conductors on their own, so the frame material already does the job. The term is about metal frames - in practice, aluminum and steel.
- Is a thermally broken window the same as a thermal break window?
- Yes. The two phrases describe the same product. The frame either has an insulating barrier between the outside and inside metal, or it does not.
Used in