Energy regulations for glazing have tightened steadily, and most homeowners now choose a specification well above the legal minimum rather than to it.
Windows and doors lose more heat, area for area, than almost any other part of a house, which is why they are subject to specific energy regulations distinct from the rest of the building fabric. In England, these sit within Building Regulations Part L, and they set a minimum standard for any replacement window or door in an existing dwelling.
This guide gives an overview of how that standard works and how it relates to the figures you will see on product literature; our companion guide, Part L Explained, goes into the regulation itself in more detail.
A lower U-value means better insulation, so 1.4 W/m²K is a maximum figure, not a target to beat by a small margin. Most well-specified modern aluminium windows achieve 1.2–1.4 W/m²K double glazed and 0.9–1.1 W/m²K triple glazed, comfortably ahead of the minimum.
The whole-window U-value combines the performance of the frame, the glass unit and the spacer bar that separates the panes. A low-e coating on the glass reflects heat back into the room, argon gas fill between the panes reduces convective heat loss, and a warm-edge spacer reduces heat loss at the edge of the glass unit, where performance is otherwise weakest.
| Component | What improves performance |
|---|---|
| Frame | Multi-chamber aluminium profile with a thermal break, or an insulated uPVC profile |
| Glass | Low-e coating, appropriate pane gap, and argon or krypton fill |
| Spacer bar | Warm-edge spacer rather than a traditional aluminium spacer |
| Glazing type | Triple glazing improves on double glazing, particularly on exposed elevations |
In an exposed Cumbrian location, it is often worth specifying above the minimum standard, particularly on north- and west-facing elevations that take the brunt of the region's weather. Triple glazing typically adds 10–20% over an equivalent high-performance double-glazed specification and can be worth prioritising selectively rather than across an entire house.
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