What a thermal break window profile actually is

A thermal break window profile is two aluminum extrusions joined only by a polyamide strip, so inner and outer faces are not one piece of metal.

A standard hollow window section is a single extrusion. Heat moves from the warm face to the cold face through the metal itself, and aluminum is very good at that. The result is a frame that behaves like a radiator between inside and outside, however thick the glass.

In a thermal break section the profile is no longer one piece. The outer part carries the weather face, the glazing rebate and the drainage path. The inner part carries the interior face and the hardware. Between them sits a strip of glass-fibre reinforced polyamide, PA66-GF25, and that strip is the only structural connection between the two halves.

Because the metal is interrupted, heat crossing the frame has to pass through the polyamide. Polyamide conducts heat roughly 600 times more slowly than aluminum — around 0.3 W/(m·K) against roughly 200 W/(m·K) for the alloy. The frame stops short-circuiting the glass.

What the polyamide bridge has to do — and how wide it is

The strip is structural, not a gasket: it carries wind-load shear and holds the two halves in register, and its width is what sets the bridge width.

This is the part most drawings get wrong. The bridge is not an insulator pushed into a slot as an afterthought. It is the joint. Every wind load on the frame arrives as shear across that strip, and the strip has to hold the two aluminum halves in alignment for the service life of the window.

Plain polyamide would creep under that load and the joint would open. PA66 with 25 percent glass fibre keeps the joint stiff, keeps the two halves in register, and stays dimensionally stable across the temperature range a facade actually sees.

Nearly all thermal break window profiles are made by crimping the strip in: the polyamide is locked between the two aluminum extrusions, so the finished section is two aluminum parts joined only by the strip. The older cast-in route, where the polyamide is poured into a channel and the metal web milled away, is rare today. Whichever route was used, the drawing has to end with one continuous non-metallic path across the section. If a metal web survives anywhere along that path, that point conducts.

The bridge width is not a free design number. It is the width of the strip you specify, and polyamide strips are extruded in a fixed range of widths rather than made to any size. 14.8 mm is the width in widest use across window and door work, and the width is matched to the depth of the frame series rather than chosen for effect. Wider is not automatically better: beyond a point the gain in frame U-value flattens out, while the section gets heavier, less stiff and more expensive. Tell us the series you are working to and we will quote the strip width with the section.

What the strip does not do is rescue poor glazing. A frame with a correct break, a single pane and an aluminum spacer still condenses, because the frame is only one of three paths between inside and outside. Frame, glass and spacer have to be dealt with together.

Where the break sits, and why the section shape matters

The break must run continuously around the frame: any point where metal crosses it cancels insulation there, whatever the rest of the section does.

Position matters as much as presence. The break belongs on the boundary between the weather side and the interior side, so that the condensation plane stays on the warm side of the barrier. Put it too far out and the inner aluminum half becomes a cold surface that collects water on a winter morning.

Chamber design works with the break rather than instead of it. A multi-chamber section breaks the cavity into smaller cells so air inside cannot circulate freely and carry heat across the frame. You can see this in our sections: the hollow cells behind the break are there to stop internal convection, not to add stiffness.

Drainage has to be arranged so that water leaves the section on the outdoor side and never crosses the barrier. A drain slot cut straight through the break is a metal-and-water shortcut, and it is easy to miss on a drawing.

Finally, the T-slots and hook slots running along the rebate are for EPDM gaskets and glazing seals. They are sized so the seal compresses against the glass and the sash without being over-compressed, which is what keeps wind-driven rain out of the cavity.

The traps that quietly cancel a thermal break

Fixings, corner joints, sills and hardware bridge a break more often than the profile fails, so check the joints before arguing about cost.

A correctly broken section can still perform like a solid one, and the reasons are almost always at the joints rather than in the profile:

• Screws and fixing ports that pass straight through the break. A screw crossing both aluminum halves is a direct metal path. Where the hardware has to fix through, the port should be arranged so the screw lands in one half only.
• Corner joints without a thermal break. Crimped or cleated corners connect the two halves of the frame. A corner that is mechanically continuous from inside face to outside face undoes the break at the most exposed point of the window.
• Unbroken sills and thresholds. Door thresholds are the usual casualty, because they are asked to take traffic and the break is sacrificed to stiffness.
• The glazing spacer. An aluminum spacer between the panes conducts around the edge of the glass and can dominate the whole window, particularly in a small unit where edge area is large relative to the centre.
• Metal hinge and handle plates. Hardware fixed to both halves of the frame is a bridge, and it is also the part a buyer never thinks to check.

None of this argues against thermal break sections. It argues for checking the whole assembly, because the frame's contribution is only worth what the joints allow.

Alloy, temper, wall thickness and finish we hold

We extrude these sections in 6063-T5 with a minimum 0.8 mm wall, and hold extrusion weight to within ±10 percent of theoretical weight.

6063-T5 is the standard choice for architectural sections: it takes anodizing and coating evenly, extrudes into the thin walls a window section needs, and holds strength after artificial ageing. Where a project needs more strength than 6063 gives in the section available, tell us the requirement at enquiry and we will say plainly whether the drawing is better served by a different alloy.

Wall thickness is quoted as the minimum on the drawing, not an average, and the sections we run for windows and doors are held at 0.8 mm and above. Weight tolerance of ±10 percent against theoretical weight is what the mill certificate shows, and it is the figure that tells you whether the die is running where it should.

Finishing is where most window and door projects differ. Mill finish suits sections that will be coated or wrapped downstream. Anodizing is our main architectural finish, in AA10 and AA15 film thicknesses for exterior use and thinner films for interior trim, and it is covered in detail on our anodized window and door frame profiles page. Powder coating to a RAL color gives the widest color range and is described on our powder coated window and door profiles page. PVDF coating for high UV exposure is available on request.

One point on ordering finishes: the two aluminum halves of a thermal break section are extruded and finished separately, then assembled. That makes the finish schedule part of the section definition, not a separate line item, and it is worth agreeing before the dies are made.

Lengths, containers and minimum order

No standard length — we cut to your list. A 20 ft container takes up to 5.85 m, a 40 ft up to 6.0 m. Minimum order is 500 kg per section.

There is no standard length for architectural profiles. Windows are cut from a cutting list, the list drives the bar length, and the bar length drives the container. Send the cutting list and we will work back from it.

The limits are worth knowing before you plan a project. A 20 ft container takes a maximum length of about 5.85 m. A 40 ft container takes about 6.0 m, which allows two lengths per bay in the standard arrangement. A 45 ft container can go to around 6.4 m, but these are less common on some trade lanes and should not be planned around.

Lengths under 5 m carry a sawing charge, because cutting short bars out of standard stock costs the same in handling as a long one. If your cutting list is dominated by short lengths, it is usually cheaper to take a longer bar and cut it yourself.

Minimum order is 500 kg per section, and that applies to new dies as well as to sections we already run. We do not publish die charges as a list; the cost depends on the section, so it is quoted with the section.

Standards we produce to

We produce to GB/T 5237, EN 755 and ASTM B221, and supply the inspection documents your project names — tell us which at enquiry.

GB/T 5237 is the Chinese standard for wrought aluminum alloy extruded profiles for architecture and is the base specification our domestic and much of our export work runs to. EN 755 covers extruded aluminum bar, tube and profile for the European market, and ASTM B221 covers extruded aluminum alloy bars, rods, shapes and tubes for North American projects.

These three differ in how they express tolerances, in temper designations and in the inspection documents supplied, so naming the standard at enquiry is what makes a quotation comparable between suppliers. If your project calls for a certification scheme we do not hold, we will say so at enquiry rather than at shipment.

Alongside the standard, every shipment carries the mill certificate for the heat concerned. Where a project needs an inspection document we do not normally issue, the time to raise it is with the enquiry, because it changes what has to happen at the press rather than at despatch.

What to send for a quotation you can compare

Send the section drawing, alloy and temper, finish, cutting list and quantity, and we will quote FOB Shenzhen or Guangzhou against those five items.

Quotations for window and door profiles diverge mostly because the enquiries did. Five items fix that:

• The section drawing, ideally as a dimensioned cross-section with wall thicknesses marked. If the drawing shows a thermal break, mark the break line and the strip width.
• Alloy and temper as the project specifies them, or the performance you need and we will say what section and alloy will deliver it.
• Finish — mill, anodized with the film thickness you want, or powder coated with the color reference.
• Cutting list, with quantities per length.
• Total quantity per section, so we can confirm it clears the 500 kg minimum.

Send those and you get a quotation with the section, length, finish, quantity and Incoterm stated, rather than a price per kilogram that leaves you guessing what it covers. If a section is new to us, we will tell you what happens at the die stage and how long it takes before you commit.

Thermal break window and door profiles are one part of what we extrude. The same plant runs architectural aluminum profiles across windows, doors, facades, handrails and industrial sections, in 26 extrusion presses and around 100,000 tonnes a year, all in-house. If you want to discuss a section or send a drawing, contact us and we will answer with specifics rather than a brochure.