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Thermal-break aluminium vs PVC windows

Move the temperature or tap Heat up and watch what happens to a PVC window and to a thermal-break aluminium one, from summer sun to a fire.

Everyday temperature: both profiles are as good as new.

Cross-section of a PVC profile and a thermal-break aluminium one at the chosen temperature

PVC

Rigid, as always.

Thermal-break aluminium

Solid, unchanged.

Polyamide strips: firm.

Other materials

  • WoodUnchanged
  • Float glassFirm
  • CopperSolid
  • SteelSolid

Move the slider or use the arrow keys

Values for guidance only: they vary with the PVC formulation, the alloy, how long the heat lasts and the loads. The drawing is schematic and does not replace the tests or the data sheet of each system.

What happens at each temperature

Tap a temperature to see it in the drawing.

  1. A dark profile in summer sun

    A dark surface in the sun gets much hotter than the air: according to the ASTM D4803 standard, a black vertical face can be about 40 °C above it.

  2. PVC softens

    It does not melt: it loses stiffness and can deform. The EN 12608 standard requires window PVC not to soften below 75 °C (Vicat temperature).

  3. PVC decomposes

    It darkens and gives off hydrogen chloride (HCl), a toxic and corrosive gas. Then it chars.

  4. Wood catches fire

    Depending on how much heat it receives, wood can start to ignite from about 250 °C; with a direct flame, generally above 300 °C.

  5. The thermal break’s polyamide melts

    The polyamide 6.6 strips with 25% glass fibre, which join the inner and outer halves of the aluminium profile, melt at about 260 °C.

  6. Flashover in a fire

    When the hot gases at the ceiling reach about 500–600 °C, everything in the room ignites at once.

  7. 6063 aluminium melts

    The window-and-door alloy starts to melt at 615 °C and is liquid at 655 °C. Before that it has already lost strength: at 300 °C it keeps about 20%.

  8. Float glass softens

    Ordinary window glass softens at about 720 °C. In a fire it usually cracks before that, because of the temperature difference between the centre and the edge.

  9. A fully developed fire

    A fully developed fire runs from 700 to over 1,000 °C; the test curve in the ISO 834 standard reaches 945 °C after one hour.

  10. Copper melts

    Pure copper melts at 1,084.62 °C, one of the fixed points of the international temperature scale.

  11. Steel melts

    It depends on the type: 304 stainless steel melts between 1,399 and 1,454 °C, and carbon steels a little higher.

Thermal-break aluminium and PVC, point by point

Heat is only one part. These are the facts that matter most when choosing, with what each one has in its favour.

Frame insulation

PVC It insulates better: a PVC frame usually has a Uf of 1.1 to 1.6 W/m²K.

Thermal-break aluminium Between 1.8 and 3.5 W/m²K depending on the system: the wider the thermal break, the better it insulates.

In the complete window the glass also matters a lot: compare the Uw each manufacturer declares.

Heat and sun

PVC It softens from about 75 °C and expands about three times as much as aluminium (70 µm per metre per °C). Dark colours use foils or pigments that reflect heat.

Thermal-break aluminium The sun does not soften it: at 100 °C it keeps more than 90% of its strength and only melts at 615–655 °C. It expands 23.4 µm per metre per °C.

Stiffness and window size

PVC With an elastic modulus of 3 GPa, large panes or dark colours need steel reinforcement inside, and its profiles are usually wider.

Thermal-break aluminium It is more than 20 times stiffer (69 GPa): it allows slimmer profiles and larger panes.

Fire

PVC It is self-extinguishing: it needs about 390 °C to ignite and stops burning if the flame is removed. In a fire it decomposes and gives off hydrogen chloride.

Thermal-break aluminium Non-combustible: class A1 under EN 13501-1, without the need for testing. The polyamide strips and the paint are combustible, in small amounts.

Neither is a fire-resistant window: for that there are special systems, tested and certified.

Long-term colour

PVC It contains UV stabilisers, and EN 12608 limits how much its colour can change after accelerated ageing.

Thermal-break aluminium Anodising or powder coating, with their own quality labels (Qualanod and Qualicoat) that check their weather resistance.

Upkeep

PVC Water and neutral soap. It does not corrode: it performs well near the sea.

Thermal-break aluminium Water and neutral soap. Near the sea, a finish suited to marine environments is advisable.

Recycling

PVC It can be recycled and re-extruded; in Europe the industry has a programme for this (VinylPlus).

Thermal-break aluminium It can be recycled again and again without losing its properties, using about 5% of the energy needed to produce new aluminium.

In short: a PVC frame insulates better and does not corrode; thermal-break aluminium is much stiffer, does not soften in the sun, is non-combustible and can be recycled without losing quality. To choose, look at the Uw of the complete window and where it will go: strong sun, dark colours, large panes or near the sea.

Sources

  1. EN 12608-1:2016+A1:2020, PVC-U profiles for windows and doors: Vicat softening temperature (EN ISO 306, method B50) of at least 75 °C.
  2. ASTM D4803, solar heat build-up in PVC building products: a black vertical face can be about 41 °C above the air.
  3. J. Yu et al., “Thermal degradation of PVC: A review”, Waste Management 48 (2016): PVC degradation begins with the release of hydrogen chloride, at around 250 °C.
  4. ILO and WHO, International Chemical Safety Card ICSC 1487 (PVC): when heated it decomposes and gives off toxic fumes, including hydrogen chloride.
  5. M. M. Hirschler, “Poly(vinyl chloride) and its fire properties”, Fire and Materials 41 (2017), and Vinyl Council of Australia, “PVC & Fire”: rigid PVC is self-extinguishing and needs about 390 °C to ignite.
  6. BASF, Ultramid A3EG5 data sheet (polyamide 66 with 25% glass fibre, the material of thermal-break strips): melting temperature of 260 °C (ISO 11357).
  7. The Aluminum Association, Aluminum Standards and Data: alloy 6063, melting range of 615 to 655 °C (1140–1210 °F).
  8. EN 1999-1-2 (Eurocode 9, structural fire design of aluminium structures): 6063-T6 keeps 71% of its yield strength at 200 °C and 19% at 300 °C.
  9. EN 13501-1 and European Commission Decision 96/603/EC: aluminium and its alloys are class A1, without the need for testing.
  10. V. Babrauskas, “Ignition of Wood: A Review of the State of the Art”, Journal of Fire Protection Engineering 12 (2002): wood ignites from about 250 °C.
  11. D. Drysdale, An Introduction to Fire Dynamics (Wiley): flashover occurs at about 500–600 °C in the hot gas layer.
  12. ISO 834-1, standard fire curve for fire resistance tests: 945 °C at 60 minutes.
  13. Pilkington, technical bulletin ATS-129 (properties of float glass): softening point of 715 °C (ASTM C338). O. Keski-Rahkonen, “Breaking of window glass close to fire”, Fire and Materials 12 (1988).
  14. International Temperature Scale of 1990 (ITS-90): freezing point of copper, 1,084.62 °C. 304 stainless steel data sheets: melting between 1,399 and 1,454 °C.
  15. EN ISO 10077-2, calculation of the thermal transmittance of frames (Uf). The Uf ranges are typical of the systems on the market: each system declares its own.
  16. International Aluminium Institute: recycling aluminium saves about 95% of the energy needed to produce primary aluminium. VinylPlus, the European PVC industry’s programme for recycling profiles.

Choosing windows and doors for your project?

We help you choose the thermal-break aluminium system, the finish and the profiles, and we quote you the material.

The message is put together with what you are looking at, with no obligation.

Frequently asked questions

At what temperature does a PVC window melt?

It does not melt: it softens. The EN 12608 standard requires the PVC of window profiles not to soften below 75 °C (Vicat temperature); above that it loses stiffness and can deform. From about 200 to 250 °C it starts to decompose and gives off hydrogen chloride, a toxic and corrosive gas.

At what temperature does an aluminium window melt?

The window-and-door alloy, 6063, melts between about 615 and 655 °C. Before that it loses strength: at 300 °C it keeps about 20% of its yield strength (EN 1999-1-2). In thermal-break windows, the polyamide strips that join the two halves of the profile melt at about 260 °C.

Which insulates better, thermal-break aluminium or PVC?

In general, the PVC frame: it usually has a Uf of 1.1 to 1.6 W/m²K, against 1.8 to 3.5 for thermal-break aluminium, depending on the system and the width of the break. In the complete window the glass also matters a lot: compare the Uw each manufacturer declares.

Does aluminium catch fire?

No: aluminium is non-combustible, class A1 under the EN 13501-1 standard. The polyamide strips of the thermal break and the paint are combustible, in small amounts. PVC is self-extinguishing: it is hard to ignite and stops burning if the flame is removed, but in a fire it decomposes.

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