Aluminium Replacement Profiles: Competing on the Thermal Break

What a profile has to beat

Aluminium extrusions dominate window frames, curtain walls, machine guarding and structural profiles for reasons that have nothing to do with thermal performance: they are stiff, dimensionally stable, cheap to extrude, easy to anodise, and every fabricator in the world knows how to cut and assemble them. A plastic profile that wants to replace aluminium has to beat it on one of those axes without losing too badly on the others.

In practice the axis it beats aluminium on is thermal. Aluminium conducts heat at around 200 watts per metre kelvin, which is why a plain aluminium frame is effectively a thermal short circuit through the building envelope. A plastic profile conducts at 0.2, three orders of magnitude lower, and the difference shows up directly in the U-value of the frame.

The other axes are where the argument gets interesting. Aluminium has a stiffness of 70 gigapascals; PVC has about 2.5 and glass-fibre polyamide about 10. So a plastic profile needs more section, more chambers and often a steel or aluminium reinforcement inside it to reach the same structural performance, and that reinforcement is a thermal bridge that has to be designed around.

The thermal break is the argument

The numbers are stark and they are the reason the market shifted. A plain aluminium frame has a U-value of about 5.6 W/m²K. A thermally broken aluminium frame, with a polyamide strip separating the inner and outer halves, comes down to about 3.1. A glass-fibre polyamide profile is around 1.5. A multi-chamber PVC-U profile is around 1.3.

For a whole window, the frame is only part of the area, so the whole-window U-value moves less dramatically than the frame number suggests. But in a cold climate the frame is where the condensation forms, and a frame at 5.6 will run below the dew point on a cold night no matter how good the glass is. That is the practical argument that sells plastic profiles: not the energy bill, the black mould on the reveal.

The thermally broken aluminium option is the interesting competitor, because it captures most of the thermal benefit while keeping aluminium's stiffness and stability. It does so at a cost: the polyamide strip is a separate extrusion, the assembly is a crimped or rolled joint, and the resulting profile is more expensive per metre than either a plain aluminium profile or a PVC one.

Table 1 — The four materials, compared on the four axes that decide a frame.

Where PVC wins and where it does not

PVC wins on cost, on thermal performance, on chemical resistance and on the fact that it can be welded into a frame with a simple heating plate. A PVC window frame is welded at the corners, which is faster and cheaper than the mechanical corner cleats and gaskets an aluminium frame needs, and the weld is also a thermal and water barrier.

PVC loses on three things that matter in specific applications. It creeps under sustained load, so it is a poor choice for a structural member carrying a permanent bending moment. It softens above about 60 °C, which rules it out of anything near a heat source or in a dark colour in a hot climate. And its stiffness is low enough that a window profile above a certain size needs a steel reinforcement inside the chambers, which is a cost and a thermal bridge.

The temperature limit is the one that catches designers out. A white PVC profile in direct sun reaches perhaps 50 °C on the surface. The same profile in a dark grey or bronze reaches 75 to 80 °C, at which point the material is noticeably softer and the expansion is beyond what the design allowed for. This is why dark PVC profiles exist but are always a special grade, and why the fixing details change.

Glass-fibre polyamide: the engineering answer

Glass-fibre reinforced polyamide sits between PVC and aluminium and occupies a useful niche that neither fills. At 10 GPa it is four times stiffer than PVC and a seventh as stiff as aluminium, and at 1.5 W/m²K it is nearly as good a thermal break as PVC. Its expansion, at 0.015 mm per metre over 50 K, is the lowest of the three, which matters in a long profile.

The application where it earns its price is the structural thermal break in a curtain wall. A GF-PA profile connects the inner and outer aluminium sections, carries the structural load between them, and interrupts the thermal path. It has to be strong, stiff, dimensionally stable and bondable to aluminium, and it has to do all of that while being a poor conductor. Very few materials can.

GF-PA is also used for machine frames, guarding and structural profiles where aluminium would be used but where the thermal bridging or the electrical conductivity is a problem. Its limitation is price and process: the material is expensive, it needs to be dried carefully before extrusion, and it is abrasive on the die, so tool life is shorter than for PVC.

One property needs to be designed around. Polyamide absorbs moisture, and absorbed moisture both plasticises the material and changes its dimensions. A GF-PA profile that is dry when it is extruded will grow slightly as it equilibrates, and a profile that is machined to a tight tolerance when wet will shrink when it dries. Conditioning before machining is not optional.

Designing a profile that replaces aluminium

The design language of a plastic profile is different from an aluminium one, and copying an aluminium section in PVC is the most common way to fail. Aluminium sections are thin-walled and rely on the material's stiffness; plastic sections are thick-walled, multi-chambered, and rely on geometry.

• Chambers, not walls. A multi-chamber profile gets its stiffness from the number and depth of chambers, and each chamber is also a convection barrier that improves the U-value.
• Wall thickness of 2.5 to 3.5 mm for a window profile. Thinner walls are hard to fill and hard to calibrate; thicker walls waste material and sink.
• Ribs and webs at 0.6 of the outer wall, for the same sink reason as any injection moulded part.
• Drainage and ventilation slots designed in from the start, because water will get into the chambers and has to get out.
• Reinforcement chambers sized for a standard steel section, because a large window will need one and retrofitting is not possible.
• Draft angles of 1 to 2 degrees on every internal surface, and generous radii at every corner.

The reinforcement question is the one that decides whether a PVC window profile is viable at all. Above roughly 1.5 metres in either dimension, the profile needs a steel or aluminium section inside the main chamber to carry wind load. That section is a thermal bridge, so the design has to place it where it does the least harm, which usually means in the room-side chamber rather than the outside one.

Expansion, colour and the sun

Thermal expansion is the detail that fails on site rather than in the design office. PVC moves 0.035 mm per metre for every 50 K, which sounds small until a 6 metre profile in a dark colour sees a 70 K swing and moves 3 mm. If both ends are fixed rigidly, that 3 mm becomes a buckling load, and the profile bows or the weld cracks.

Table 2 — Colour against surface temperature and the fixing allowance it forces.

The rule in practice is that any run over about two metres needs one end free to move, and any dark colour needs both ends free regardless of length. The fixing detail is a slotted hole or a clip that allows axial movement, and it costs almost nothing if it is designed in and a great deal if it is discovered after the first hot day.

The commercial comparison

On a per-metre basis a PVC window profile is usually cheaper than a thermally broken aluminium one and more expensive than a plain aluminium one. The comparison that matters is not per metre but per completed window, and there the picture changes, because the fabrication cost differs.

Table 3 — Where the cost actually sits in the two routes.

The recyclability line is the one that is changing fastest. Aluminium has a well-established, high-value recycling stream and a strong environmental story. PVC recycling is established in Europe for window profiles specifically, with take-back schemes and a closed-loop stream, but it is less universal. A buyer who is choosing on embodied carbon will look at both numbers, and the answer depends heavily on where the profile is made and where it is installed.