What does a U-value mean?
A U-value measures how readily heat passes through a building element. It is expressed in W/(m²K). In simple terms, the lower the U-value, the better the thermal insulation.
For windows and doors the figure that matters is the performance of the complete product. The frame, thermal break, glass, spacer, dimensions and opening configuration all contribute to the final result. A centre-pane glass U-value should not be confused with the U-value of the complete window or door.
Sheerline Thermlock® technology
At the heart of Sheerline aluminium systems is its proprietary Thermlock® multi-chamber thermal-break technology. The closed insulating chambers separate the internal and external aluminium sections, reducing the path through which heat can travel.
Sheerline states that Thermlock® is wider, multi-chambered and less conductive than legacy polyamide thermal-break systems. It works together with the glazing to help keep the inside of the property insulated from cold external conditions.
Prestige U-values: double and triple glazing
Sheerline publishes clear thermal figures for its Prestige range. Depending on the product and the tested specification, Prestige windows, doors, bi-fold doors and Lift & Slide patio doors are available with published U-values of:
- 1.4 W/(m²K) with the relevant double-glazed specification.
- 1.0 W/(m²K) with the relevant triple-glazed specification.
Prestige windows can also achieve up to an A+ Window Energy Rating. Exact performance always depends on the product, size, configuration and glazing specification, so these figures should not be assumed for every possible design.
Independent evidence — not just a headline figure
Thermal claims are much more useful when the evidence behind them can be checked. Sheerline says its U-values are calculated to EN 14351 and BFRC standards and publishes independently verified reports for its aluminium systems.
The published evidence includes reports for Prestige casement and tilt & turn windows, French and residential doors, Prestige bi-fold doors and multiple Lift & Slide patio configurations. This makes it possible to compare the claimed performance with the specification that was actually assessed.
View Sheerline's independently verified U-value reports →
Why the exact specification matters
A U-value is not simply a property of the aluminium profile. Changing the overall size, opening arrangement, frame choice or glass specification can alter the result. This is particularly important on larger doors and glazing projects.
Sheerline itself recommends checking that the product supplied is made to the same specification as the evidence being relied upon. For that reason, Ali-Co will not apply one generic thermal figure to every aluminium window or door we manufacture.
Double glazing or triple glazing?
Both can provide excellent performance when used as part of the correct system. Triple glazing can reduce heat transfer further, but the right choice depends on the required U-value, product type, glass weight, dimensions, budget and the requirements of the project.
If you have an architect's specification or a target U-value, give it to us at quotation stage. We can then consider the frame and glazing specification together rather than treating the glass as a separate decision.
Thermal performance without bulky-looking frames
One of the attractions of modern thermally broken aluminium is the combination of insulation with the clean, slim appearance people choose aluminium for in the first place. Sheerline's Prestige range is designed around low U-values while retaining refined aluminium sightlines, making it particularly well suited to contemporary homes, extensions and large glazed openings.
Manufactured in Bristol by Ali-Co
Ali-Co manufactures Sheerline aluminium windows and doors in Bristol. The system design and independently verified thermal data come from Sheerline; we fabricate the finished windows and doors for the individual project and can also provide professional installation.
If thermal performance is important to your project, send us your approximate sizes, styles and required performance. We can help identify the appropriate product and glazing options and, where relevant, point you to the supporting Sheerline U-value evidence.