Technical information notice. Technical information on this page is general product-selection guidance, not a certification, compliance statement, design calculation or order confirmation. Before relying on a figure or standard, request current evidence for the selected product route, size, glazing, hardware and project. See our technical information.
Guide
How an aluminium thermal break works
A thermal break is a low-conductivity polyamide bar bonded between the inner and outer aluminium profiles, splitting the frame into two skins so heat cannot conduct straight through the metal. Aluminium conducts heat readily, so the break is what lets an aluminium window reach a whole-unit Uw from 0.85 W/m2K. The break sets the frame's thermal performance; the glazing, warm-edge spacer and gas fill set the rest. Both combine into the whole-window figure that Approved Document L is assessed against.
Windows from Uw 0.85Whole-unit, where specified; confirmed at quote
Bifold from Uw 0.9Whole-unit, where specified; confirmed at quote
Polyamide breakSplits inner and outer aluminium skins
What is a thermal break in an aluminium frame?
A thermal break is a strip of glass-reinforced polyamide mechanically locked into the inner and outer aluminium extrusions, so the two halves of the frame are joined structurally but separated thermally. Aluminium has high thermal conductivity, so without a break the frame forms a direct metal bridge from the warm inside face to the cold outside face, pulling heat out and risking condensation on the internal frame.
Polyamide conducts heat far more slowly than aluminium, so the break interrupts that bridge. The deeper the break and the more chambers within it, the lower the conduction. The inner skin stays close to room temperature, which keeps the internal surface warm and lifts the whole frame's thermal performance.
Request current product evidence for the selected route, size, glazing, hardware and project before relying on the specification.
| Inner aluminium skin | warm side, carries the internal surface temperature |
|---|---|
| Polyamide break | low-conductivity barrier between the two skins |
| Outer aluminium skin | cold side, exposed to outdoor conditions |
| Break depth and chambering | deeper, multi-chamber breaks conduct less |
How does heat conduct through an aluminium frame?
Heat moves through a closed window frame mainly by conduction through the solid material, plus radiation and convection across the air chambers inside the profile. In an unbroken aluminium frame, conduction dominates because the metal carries heat quickly from face to face. The thermal break attacks that conduction path directly.
The frame's heat loss is expressed as Uf, the frame U-value in W/m2K. A lower Uf means less heat lost per square metre of frame at a given temperature difference. Break geometry, the number of air chambers, and any foam infill all push Uf down. The frame is only part of the unit, so Uf alone does not tell you what the finished window achieves.
Frame Uf versus whole-window Uw: what is the difference?
Uf is the frame on its own; Uw is the whole window, combining the frame, the glass, and the spacer where they meet. The whole-unit Uw is the figure that matters for Building Regulations and for comparing real products, because it is what the installed window actually loses.
Uw is an area-weighted blend of three parts: the centre-pane glass U-value (Ug), the frame (Uf), and a linear thermal transmittance at the glass edge and frame junction (the psi-value, driven by the spacer). A frame with a strong thermal break can still be let down by a poor spacer or a thin glazing build, which is why the whole-unit number is the one we confirm at quote.
| Ug | centre-pane glass U-value, set by the glazing build |
|---|---|
| Uf | frame U-value, set by the aluminium profile and its thermal break |
| Psi-value | linear loss at the glass edge, set by the warm-edge spacer |
| Uw | the area-weighted whole-window result the regulations assess |
What role do glazing, warm-edge spacers and gas fill play?
The glazing usually covers most of the window area, so its build often moves Uw more than the frame does. A double or triple sealed unit with low-emissivity coatings, an inert gas fill such as argon, and the right cavity widths brings Ug down sharply. Triple glazing adds a second cavity and a third pane to cut centre-pane loss further.
The spacer at the edge of the glass closes the loop. A warm-edge spacer replaces conductive aluminium spacer bar with a lower-conductivity material, cutting the edge psi-value, keeping the glass edge warmer and reducing condensation risk there. Glazing depth runs 22 to 58mm depending on system, which sets how thick a sealed unit, and therefore which glass build, the frame can take.
| Low-emissivity coatings | reflect heat back into the room |
|---|---|
| Argon or other inert gas fill | slows convection in the cavity |
| Cavity width and pane count | double or triple builds tune Ug |
| Warm-edge spacer | lowers the edge psi-value and frame-junction loss |
| Glazing depth 22 to 58mm by system | governs the sealed unit build available |
What whole-unit U-values are achievable?
With a thermal-broken frame, the right glazing and a warm-edge spacer, aluminium windows reach a whole-unit Uw from 0.85 W/m2K and bifold doors from 0.9 W/m2K, where specified. These are starting points for the strongest builds, not every configuration, so the whole-unit figure is confirmed against your size, glazing and opening type at quote. The figures below are whole-unit Uw starting points across our product range, stated as from values.
Larger units, more opening leaves and slimmer sightlines shift the area weighting and can move the result, which is why performance is stated per finished unit. Tell us what you need and we quote the right system. Start a quote with your sizes and opening types.
| Casement windows | Uw from 0.85 where specified |
|---|---|
| Tilt-turn windows | from 0.71 on the high-insulation route |
| Bifold doors | Uw from 0.9 where specified |
| Sliding sashes | Uw from 1.0 where specified |
| Entrance doors | from 1.1 where specified |
| Curtain walling | Ucw from 0.6 where specified |
How does Approved Document L apply to aluminium windows?
Approved Document L is the part of the Building Regulations covering conservation of fuel and power, and it sets the U-value standards new and replacement windows and doors are assessed against in England. Compliance is judged on the whole-unit U-value, not on the frame or glass in isolation, so the Uw is the number that must be evidenced.
Thermal-broken aluminium with a suitable glazing build meets current standards and leaves headroom for lower-energy targets. We confirm the as-specified whole-unit Uw for each opening so it can be documented against the standard that applies to your project. For the wider performance picture, see our energy efficiency page and the detail in our guide to aluminium U-values. We supply the trade; homeowners can reach a vetted fitter via find an installer.
Common questions
What does a thermal break do in an aluminium window?
A thermal break is a low-conductivity polyamide bar bonded between the inner and outer aluminium profiles. It interrupts the metal path that would otherwise conduct heat straight through the frame, keeping the inside face warmer, cutting heat loss and reducing condensation risk on the internal frame.
What is the difference between Uf and Uw?
Uf is the U-value of the frame alone. Uw is the whole-window U-value, an area-weighted blend of the frame (Uf), the centre-pane glass (Ug) and the linear loss at the glass edge set by the spacer. Uw is the figure Building Regulations assess and the one to compare between products.
What U-value can an aluminium window achieve?
With a thermal-broken frame, low-emissivity glazing, gas fill and a warm-edge spacer, aluminium windows reach a whole-unit Uw from 0.85 W/m2K and bifold doors from 0.9 W/m2K, where specified. These are starting points for the strongest builds, confirmed per finished unit at quote.
Does the glass or the frame matter more for U-value?
Both matter, but glazing usually covers most of the window area, so the sealed-unit build often moves the whole-unit Uw more than the frame does. The thermal break sets the frame's contribution; the glazing, gas fill and warm-edge spacer set the rest. The two combine into Uw.
Does an aluminium window meet Approved Document L?
Thermal-broken aluminium with a suitable glazing build meets current Approved Document L standards, which are assessed on the whole-unit U-value rather than the frame or glass alone. We confirm the as-specified whole-unit Uw for each opening so it can be documented against the standard for your project.
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