Key takeaways
- A thermal break lowers an aluminium frame's U-value from about 5.5–7.0 W/m²K to 1.0–2.4 W/m²K by separating the inner and outer metal with a barrier roughly 600 times less conductive.
- Polyamide PA66 GF25 strips of 24–34 mm with foam-filled chambers typically reach Uf 1.0–1.6 W/m²K, while poured-and-debridged resin gaps of 6–13 mm typically reach Uf 2.8–4.0 W/m²K.
- At −10 °C outside and 20 °C / 50% RH inside, the frame surface must stay above about 9.3 °C to avoid condensation, which needs a temperature factor fRsi of at least about 0.65; non-thermally-broken frames typically fall well below this.
- Non-thermally-broken aluminium is acceptable mainly in naturally ventilated tropical buildings where indoor and outdoor temperatures rarely differ by more than about 10 K, and for internal partitions.
- Always ask for the profile Uf from an EN ISO 10077-2 simulation and an EN 14024 shear test report, because a quotation that just says thermal break can mean anything from 14.8 mm to 34 mm.
A thermal break is a strip of low-conductivity material, usually glass-fibre-reinforced polyamide (PA66 GF25) or poured polyurethane resin, that splits an aluminium profile into separate inner and outer shells so heat cannot flow straight through the metal. It lowers the frame U-value (Uf) from roughly 5.5–7.0 W/m²K for solid aluminium to 1.0–2.4 W/m²K. It also keeps the inside frame surface warm enough to prevent condensation in most heated buildings.
Why aluminium needs a thermal break
6063 aluminium conducts heat at about 200 W/mK, compared with about 0.3 W/mK for PA66 GF25 polyamide, 0.17 W/mK for uPVC and 0.13 W/mK for softwood. A continuous aluminium frame therefore acts as a cold bridge from outside to inside. In winter the inner face approaches outdoor temperature, and in hot climates the frame conducts solar heat indoors. A barrier about 600 times less conductive, placed across the full depth of the profile, interrupts that path.
The width of the break is only part of the story. Frame Uf also depends on chamber geometry, foam inserts, gasket design, glazing rebate depth and how far the glass sits inside the insulating zone. Two profiles with the same 24 mm strip can differ by 0.5 W/m²K.
Two ways to make a thermal break
Insulating strips (crimped polyamide)
The inner and outer shells are extruded separately. Their receiving grooves are knurled, PA66 GF25 strips are inserted, and rollers close the aluminium onto the strips to create a mechanical lock. Strip widths range from 14.8 mm in basic systems to 34 mm and more in high-performance windows. Because the shells can be finished before assembly, the inside and outside can be different colours. Mechanical strength is verified by transverse shear and tensile tests to EN 14024 at low, room and high temperatures, typically −20 °C, +23 °C and +80 °C.
Poured and debridged resin
A single extrusion is made with a channel that is filled with two-part polyurethane resin. Once the resin cures, the aluminium bridge beneath it is cut away (debridged). The method is fast and common in North American storefront and commercial windows. The insulating gap is narrow, typically 6–13 mm (¼–½ in), which limits thermal performance. If the channel is not mechanically locked, the resin can shrink and slip over time.
| Attribute | Polyamide strip (PA66 GF25) | Poured & debridged polyurethane |
|---|---|---|
| Typical insulating width | 14.8–34 mm (wider in Passive House systems) | 6–13 mm (¼–½ in); dual-pour is wider |
| Barrier conductivity | ≈ 0.3 W/mK | ≈ 0.1–0.2 W/mK |
| Typical frame Uf achieved | 1.0–3.0 W/m²K | 2.8–4.0 W/m²K |
| Different inside / outside colours | Yes; shells finished before assembly | Usually no; finished as one piece |
| Key test references | EN 14024 shear and tensile tests | AAMA TIR-A8, AAMA 505 |
| Typical markets | Europe, Australia, Middle East, Asia, high-performance residential | North American storefront and commercial windows |
| Main quality risk | Poor knurling or rolling causes strip slip under load or heat | Dry shrinkage, inaccurate debridge cut |
Frame U-values with and without a thermal break
The table below shows indicative values for a casement at the EN 14351-1 reference size of 1.23 × 1.48 m. It uses a 30% frame fraction and the hand-calculation method of EN ISO 10077-1. Warm-edge spacers are assumed for the better frames. Surface temperatures assume +20 °C inside and −10 °C outside.
| Frame type | Uf (W/m²K) | Glazing Ug | Uw (W/m²K) | Inner frame surface |
|---|---|---|---|---|
| No thermal break | 5.5–7.0 | 1.1 double | ≈ 2.6–3.0 | ≈ 2–6 °C |
| Narrow break (14.8 mm strip or pour & debridge) | 2.8–3.8 | 1.1 double | ≈ 1.8–2.1 | ≈ 8–11 °C |
| 24 mm PA66 strip, standard chambers | 2.0–2.6 | 1.1 double | ≈ 1.5–1.7 | ≈ 11–13 °C |
| 34 mm PA66 strip, foam-filled | 1.0–1.6 | 1.1 double | ≈ 1.2–1.4 | ≈ 13–15 °C |
| 34 mm PA66 strip, foam-filled | 1.0–1.3 | 0.6 triple | ≈ 0.8–1.0 | ≈ 13–15 °C |
For US readers, Uw 1.2 W/m²K is about U-0.21 Btu/h·ft²·°F, and 2.8 W/m²K is about U-0.49. The differences in whole-window U-value, condensation and comfort are explained further in U-value and SHGC explained.
Condensation: the problem a thermal break really solves
Indoor air at 20 °C and 50% relative humidity has a dew point of about 9.3 °C. At 60% RH, common in kitchens, bathrooms and crowded apartments, the dew point rises to about 12 °C. Any surface colder than the dew point collects water. A non-thermally-broken frame at −10 °C outside sits at roughly 2–6 °C on its inside face, and the results are running water, mould on reveals and swollen sills.
The risk is expressed as the temperature factor fRsi = (Tsurface − Toutside) / (Tinside − Toutside). The Passive House Institute requires fRsi ≥ 0.70 for cool-temperate climates. Our C80 tilt-turn is certified at fRsi 0.73, which gives a surface of about 11.9 °C at −10 °C outside. In North America the equivalent measure is the NFRC Condensation Resistance (CR) rating. The C80 tilt-turn rates CR 68 in the configuration supplied for a −35 °C Toronto project.
When a non-thermally-broken frame is acceptable
The rule of thumb is that a thermal break is optional when the indoor–outdoor temperature difference rarely exceeds about 10 K and the interior is not kept cold and dry by air conditioning. In those conditions, heat flow through the frame is small compared with solar gain through the glass, so the glass SHGC matters more than the frame Uf. Non-thermally-broken systems typically cost 10–25% less. Acceptable cases include:
- Naturally ventilated homes in tropical climates such as Manila, Lagos or Colombo, where outdoor temperatures stay at 24–33 °C year-round. See our Philippines market page.
- Internal partitions, shopfronts opening into conditioned malls, unheated garages and storerooms.
- Budget projects in mild climates where the local energy code allows Uw above 3.0 W/m²K. Check the code first.
There is an important caveat for hot-humid climates. Hotels and offices running air conditioning 24 hours a day at 22–24 °C cool the frame below the outdoor dew point of 24–26 °C, so condensation can form on the exterior face and inside frame cavities. In the Gulf, summer temperature differences reach 25 K (48 °C outside, 23 °C inside), and a thermal break pays back through lower cooling loads. For both cases we specify thermally broken frames.
How ARCVIEW builds its thermal breaks
Our C70 / C80 casement and tilt-turn and lift & slide systems use 24–34 mm PA66 GF25 polyamide strips with foam-filled chambers. Knurling, strip insertion and rolling are automated, and shear strength is tested on every batch. The 34 mm C80 reaches Uw 0.9 W/m²K with triple glazing and is used for Canadian projects down to −35 °C. Test reports are in our certificate library.
We do not manufacture poured-and-debridged profiles. If a North American storefront specification requires that method, a domestic storefront supplier will be a better fit. For a broader comparison of frame materials, see aluminium vs uPVC windows.
Frequently asked questions
- What is a thermal break in an aluminium window?
- A thermal break is an insulating barrier, usually glass-fibre-reinforced polyamide (PA66 GF25) strips or poured polyurethane resin, that separates the inner and outer aluminium parts of a frame. Aluminium conducts heat about 600 times better than polyamide, so the barrier cuts frame heat loss from Uf ≈ 5.5–7.0 W/m²K to 1.0–2.4 W/m²K and keeps the inner surface warm enough to resist condensation.
- Is a wider thermal break always better?
- Generally a wider break lowers Uf, but width alone does not determine performance. Chamber geometry, foam inserts, gasket design and how deep the glass sits in the frame all matter, and two profiles with the same 24 mm strip can differ by about 0.5 W/m²K. Compare certified Uf values from an EN ISO 10077-2 simulation rather than strip widths.
- Do I need thermally broken windows in a tropical climate?
- Not always. In naturally ventilated tropical buildings, where indoor and outdoor temperatures differ by only a few degrees, non-thermally-broken aluminium is acceptable and typically costs 10–25% less. Solar-control glass with a low SHGC has more effect on comfort. For buildings air-conditioned to 22–24 °C around the clock, specify a thermal break to avoid condensation forming on cold frames in humid air.
- Are polyamide strips or pour-and-debridge thermal breaks better?
- Polyamide strips are wider, typically 14.8–34 mm, and reach lower Uf values of about 1.0–3.0 W/m²K. They also allow different inside and outside colours. Pour-and-debridge breaks are typically 6–13 mm wide and reach about 2.8–4.0 W/m²K, but they are fast to produce and well established in North American commercial storefront. For residential and high-performance windows, polyamide strips are the usual choice.
- Will a thermal break stop all window condensation?
- No. It keeps the frame warm, but condensation still depends on indoor humidity, glass edge temperature and installation. At 20 °C and 60% RH the dew point is about 12 °C, so even good frames can sweat in poorly ventilated rooms. Combine a thermally broken frame with warm-edge spacers, Low-E glazing, an insulated installation joint and adequate ventilation.
Sources & standards
- 01ISO 10077-2: Thermal performance of windows, doors and shutters — Numerical method for frames — International Organization for Standardization (ISO)
- 02EN 14024: Metal profiles with thermal barrier — Mechanical performance — Requirements, proof and tests — European Committee for Standardization (CEN)
- 03Passive House certified component criteria for windows, including the fRsi hygiene criterion — Passive House Institute
- 04NFRC 500: Condensation resistance rating procedure — National Fenestration Rating Council
- 05Aluminium in buildings: windows, doors and façades — European Aluminium
