Technical guide
U-values and Part L for commercial glazing
Part L is assessed on the whole window or the whole facade, frame included — not on the centre-pane figure printed on the unit label. A 1.0 W/m²K unit in a weakly broken aluminium frame commonly gives a whole-window figure nearer 1.6.
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Three different numbers
Centre-pane U-value describes the glass alone. Whole-window U-value includes the frame and the edge effect. Whole-facade U-value covers everything including spandrels. They can differ by 0.5 W/m²K or more.
What moves the number
Polyamide thermal breaks in the frame, warm-edge composite spacers instead of aluminium, argon fill, soft-coat low-emissivity coatings, and a higher glass-to-frame ratio. Big panes perform better than lots of small ones.
Refurbishment triggers
Replacing windows is controlled work and the replacement must meet the current standard for thermal elements. Larger facade works can trigger wider consequential improvements to the building.
Do not over-specify
Beyond about 1.2 W/m²K on a commercial facade the payback stretches a long way out, and solar control usually delivers more comfort per pound than another increment of insulation.
Model before ordering
The only reliable route is to calculate the whole-window or whole-facade figure for the actual system before manufacture. Discovering it after installation is expensive to fix.
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Related services
Where this applies
Curtain walling
Curtain walling is a non-loadbearing aluminium and glass facade hung off the structural frame of a building.
Commercial aluminium windows and doors
Commercial aluminium windows and doors are thermally broken framed units built for heavy use — far higher cycle ratings, wider spans and stronger hardware than residential equivalents, with a polyamide break separating inner and outer frame.
Double glazed unit replacement
A double glazed unit, or IGU, is two panes bonded to a spacer bar with a sealed cavity between them.
Tell us what the building needs
Free measured survey, fixed written quotation, and a straight answer about what is and is not worth doing. Mainland UK, commercial work only.