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Why a headline U-value no longer tells the whole story
For years, one figure has dominated conversations about window performance: the U-value. It remains important, but it has never told the whole story.
BY TIM JAMES, NATIONAL SPECIFICATION MANAGER, VEKA PLC
That is becoming even more relevant as the 2026 edition of Approved Document L comes into effect in England.
For new dwellings, the guidance allows window U-values to be calculated using the actual size and configuration of the proposed window, including elements such as frame sections, sills, extenders, dummy sashes, mullions, transoms, glazing bars and certain couplers. The new guidance takes effect from 24 March 2027 for work not connected with higher-risk buildings, with separate timing for higher-risk building work.
That reinforces a simple point: The performance of the window being installed matters more than the headline figure attached to a product range.
Ask most people to describe a high-performance window and the first figure they are likely to quote is the U-value.
It remains one of the most important measures of thermal performance, but it has never told the whole story. As building regulations continue to evolve, understanding how a window performs in its actual installed configuration is becoming increasingly important.
The 2026 edition of Approved Document L for England reinforces this shift. For new dwellings, window U-values can be calculated using the actual size and configuration of the proposed window, including elements such as frame sections, sills, extenders, dummy sashes, mullions, transoms, glazing bars and certain couplers.
The implication is straightforward: the performance of the window being installed matters more than the headline figure attached to a product range.
It also means historic specification assumptions may need revisiting. A window configuration that satisfied previous project requirements is not necessarily the optimum solution when assessed against current thermal, solar gain and ventilation considerations.
Part L is only part of the picture
At the same time, Part O requires new residential buildings in England to limit unwanted solar gains and provide adequate means to remove excess heat.
That can pull design decisions in different directions.
A highly insulated building envelope helps reduce heat loss in winter. But glazing area, solar gain, orientation and the effectiveness of openable windows also influence summer comfort.
The right answer is therefore unlikely to come from pursuing one performance value in isolation.
A window specification needs to work as part of the building.
Configuration matters
Consider two windows using the same system. One may be a large fixed light with a high proportion of glazing. Another may be smaller, with multiple opening lights and additional frame sections.
They may belong to the same product family, but they are not necessarily the same thermally. The same principle applies more widely. Opening style, glazing, frame arrangement and additional components can all affect the final solution.
This is why I think our industry needs to become more comfortable moving away from the question:“ What U-value does this system achieve?” and towards:“ What does this proposed configuration achieve?”
Overheating adds another layer
Part O makes that conversation broader still. Glazing specification, glazed area, orientation, shading and ventilation all need to be considered together.
54 NOVEMBER 2026 CLEARVIEW-UK. COM