Guide

Aluminium U-Values Explained

By Paul Fradley, Founder and Technical Lead · Updated June 2026

A U-value measures how much heat passes through a unit: the lower the number, the better the insulation. For aluminium glazing the figure you specify and the figure you build to are not always the same, so this guide sets out what U-values mean, the numbers to expect by system, and how the whole-unit value is confirmed at quote.

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What is a U-value?

A U-value is the rate of heat transfer through a building element, measured in W/m2K. The lower the number, the less heat escapes and the better the thermal performance.

For glazing you will see three notations. Ug is the glass alone. Uf is the frame. Uw is the whole window or door, combining glass, frame, spacer and the way they meet. Uw is the figure that matters for Building Regulations and for real performance, because a strong glass unit in a weak frame still loses heat at the edges.

Aluminium conducts heat, so thermal performance comes from the polyamide thermal break that separates the inside and outside of the frame. The depth of that break, the glazing unit and the spacer bar together set the Uw you achieve.

Ug, Uf or Uw: which figure should you specify?

Specify to Uw. A glass supplier quoting a low Ug is describing the centre-pane glass, not the finished unit, so two products with the same Ug can perform very differently once framed.

Quote Uw against the actual sizes. Larger sashes carry proportionally more glass to frame, which usually improves the whole-unit figure; smaller units sit closer to the frame value. That is why a single headline number rarely fits every opening on a project.

Confirm the whole-unit Uw at quote for your sizes, glazing and configuration. Tell us what you need and we quote the right system.

Aluminium U-values by system

The figures below are the starting points we build from, quoted as from or up to where specified. The exact value varies by system, size and glazing, and is confirmed at quote.

Windows and casements start from a Uw of 0.85. The high-insulation tilt-turn route reaches from 0.66, which suits low-energy and quieter-running schemes.

Bifold doors start from a Uw of 0.9, with leaves up to 1.2 x 3.0m and 130kg, up to 14 leaves, and around 90% clear opening. Sliding doors start from a Uw of 1.0, with sashes up to 2.2 x 2.87m and 420kg, a 35mm interlock, and around 50 to 65% clear opening depending on configuration.

Entrance doors start from a Uw of 0.77 and heritage ranges from 1.0. For facades, curtain walling reaches a Ucw from 0.6, with units up to 1.5 x 3.6m and 350kg and a 66mm sightline. Commercial systems start from 1.2.

Casement and window: Uw from 0.85
Tilt-turn (high-insulation route): Uw from 0.66
Bifold: Uw from 0.9; leaves to 1.2 x 3.0m / 130kg; up to 14 leaves; ~90% clear opening
Sliding: Uw from 1.0; sashes to 2.2 x 2.87m / 420kg; 35mm interlock; ~50-65% clear opening
Entrance: Uw from 0.77
Heritage: Uw from 1.0
Curtain walling: Ucw from 0.6; units to 1.5 x 3.6m / 350kg; 66mm sightline
Commercial: Uw from 1.2

How U-values relate to weather and acoustic performance

Thermal performance sits alongside weathertightness and acoustics, and the same systems carry independent ratings for those. We supply to weather and air to Class 4, watertightness to Class 9A, and wind resistance to Class C5 where the system and install are specified for it.

Acoustic reduction reaches up to Rw 46 dB where specified, which is the route for roadside, trackside and city-centre projects. Security follows the same logic, with PAS 24 routes available including PAS 24:2025. See security and energy efficiency for how these are specified together.

These ratings are independent of the U-value, so a low Uw does not automatically mean a high weather or acoustic class. State the demands of the opening and we match the system to all of them.

How to specify and quote

Give us the opening sizes, the configuration and the target Uw, plus any weather, acoustic or security class the project calls for. We confirm the whole-unit Uw and the matching ratings against your drawings.

We fabricate and supply the trade: installers, builders, architects, joiners and commercial clients. Homeowners are routed to a vetted installer via find an installer.

Browse the range at products, see how we work with the trade at trade, or send your requirement to quote and we will quote the right system.

Common questions

What is a good U-value for aluminium windows?

Specify to whole-unit Uw rather than glass alone. VitrineAlu aluminium windows and casements start from a Uw of 0.85 W/m2K, and the high-insulation tilt-turn route reaches from 0.66. The exact figure depends on the system, the sizes and the glazing, and is confirmed at quote.

What is the difference between Ug, Uf and Uw?

Ug is the centre-pane glass value, Uf is the frame value, and Uw is the whole window or door combining glass, frame and spacer. Uw is the figure that counts for Building Regulations and real performance, because it accounts for heat loss at the frame and edges, not just the middle of the glass.

What U-values do VitrineAlu aluminium doors achieve?

Bifold doors start from a Uw of 0.9, sliding doors from 1.0, and entrance doors from 0.77. Heritage ranges from 1.0. These are starting points; the whole-unit Uw is confirmed for your sizes, glazing and configuration at quote.

Does a low U-value mean good weather and acoustic performance too?

No. The U-value measures heat loss only. Weathertightness, air, wind and acoustic performance are rated separately. Our systems supply to weather and air permeability Class 4, watertightness Class 9A, wind Class C5 and acoustic reduction up to Rw 46 dB where specified, independent of the Uw.

Can I get a single U-value for a whole project?

Not usually. Larger units carry more glass to frame and tend to give a better whole-unit figure than small ones, so the Uw varies opening to opening. Send your sizes and configurations and we confirm the Uw for each at quote.

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Paul Fradley, Founder and Technical Lead, has worked almost every role in windows and doors across more than twenty years: surveyed, fabricated on the floor, fitted on site, then ran the technical and operations side. He leads the spec on every project, so what we quote is what we fabricate.

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