A thermally broken aluminum window is three separately engineered parts joined into one structural assembly — and the part in the middle is doing more work than its name suggests.
Specify a "thermally broken aluminum window" and it is tempting to picture a single extrusion with a slot cut into it. It isn't. It is a composite assembly of three independently engineered components, each solving a different problem, mechanically locked together so they behave as one structural member while performing three unrelated jobs at once.
That distinction matters because every one of those jobs can fail independently. A frame can be structurally sound and still perform poorly thermally. It can be thermally excellent and structurally undersized. Understanding the assembly as three parts — not one material — is what lets a designer or estimator reason about where performance actually comes from, and where a cheaper substitution quietly costs you.
The outboard leg of the frame is the weather face. It carries the primary structural load of the assembly — wind pressure, its own dead weight, and the racking forces of the building moving around it — while being the surface that actually sees the climate: rain, UV exposure, and the daily and seasonal temperature cycling of an exterior wall. Because it is the load-bearing exterior member, its alloy, wall thickness, and finish (anodized or powder-coated) are selected as much for weatherability and structural capacity as for appearance.
Between the two aluminum members sits the zone that gives the assembly its name, and it is doing two jobs simultaneously, not one. Polyamide struts, insulating foam, and gasket seals interrupt the conductive path that would otherwise let heat move freely from the warm side of the frame to the cold side — aluminum is an excellent conductor, roughly 1,000 times more conductive than the polyamide used to break it. But that same zone still has to transfer every structural force — wind load, dead load, thermal expansion — from the exterior member to the interior member. A thermal break that only insulated and could not carry load would not survive a single storm; a thermal break that only carried load would defeat the entire point of the assembly.
The struts aren't insulation with a structural job on the side. They're a structural connection that happens to be a poor conductor — and both properties have to hold at once.
The inboard leg finishes the assembly on the occupied side of the wall. Here the priorities shift: it rarely sees weather, but it is touched, leaned on, and looked at every day, so durability, a clean finish, and dimensional consistency with the interior finishes matter more than climate resistance. Structurally, it closes the loop — the load path runs from exterior aluminum, through the thermal separator, into interior aluminum, and finally into the building structure.
Primary structural member. Resists wind, rain, UV, and thermal cycling on the weather side of the wall.
Polyamide struts, gaskets, and insulating foam. Interrupts conductive heat flow while transferring structural load between members.
Finished structural member on the occupied side. Prioritizes durability, appearance, and occupant contact.
Every component depends on the others to perform as intended. Undersize the interior member and the whole frame under-performs structurally even if the exterior member is rated correctly. Weaken the thermal separator — a thinner strut, a cut corner on the foam fill — and the frame's U-value climbs even though the aluminum on both sides is unchanged. This is also why a frame's overall thermal performance can't be judged from the glass alone: the frame U-value and the glass U-value are combined, area-weighted, into a single whole-window U-value, and a poor frame can drag down an otherwise excellent glazing package.
A non-thermally-broken aluminum frame typically runs a frame U-value in the range of 5.6–7.0 W/m²K. A well-detailed thermal break can bring that same frame down to roughly 2.0–2.8 W/m²K — without changing the glass at all. The calculator below lets you see exactly how much of a window's total heat loss that difference accounts for, at your own dimensions and glazing spec.
The companion U-value calculator combines your frame width, glass U-value, and frame type into a single whole-window rating — broken vs. non-broken, side by side.