The short answer
For buildings other than dwellings in England, the 2021 edition of Approved Document L sets a limiting U-value of 1.6 W/m²K for new or replacement curtain walling. That is a backstop, so most designs need a better whole wall figure, frames and spandrels included, to pass the building's energy calculation.
On this page
A U-value measures how much heat passes through one square metre of a building element for each degree of temperature difference between inside and outside. It is given in watts per square metre kelvin (W/m²K). The lower the number, the better the element keeps heat in.
The Part L limit
In England, energy performance is governed by Part L of the Building Regulations. Its guidance comes in two volumes: Volume 1 for dwellings and Volume 2 for buildings other than dwellings. Both are published on GOV.UK.
For buildings other than dwellings, the 2021 edition of Volume 2 sets limiting U-values for new and replacement elements. The figure for curtain walling is 1.6 W/m²K. A 2026 edition of Approved Document L has also been published for buildings that fall under the new standards, so check which edition applies to your project with your designer or building control body.
| Situation (England) | Limiting U-value, 2021 edition |
|---|---|
| New curtain walling, building other than a dwelling | 1.6 W/m²K |
| Replacement curtain walling, building other than a dwelling | 1.6 W/m²K |
| Replacement windows in an existing dwelling | 1.4 W/m²K |
Scotland and Wales have their own building standards and their own figures. Check the current guidance for the nation where the building is.
A limit, not a target
The limiting value is a minimum standard for each element. A new building also has to pass a whole-building calculation of energy and carbon, using SBEM for non-domestic buildings. Heavily glazed buildings often find that 1.6 W/m²K is not enough to pass, so the energy assessor asks for a lower figure for the facade.
Whole wall, frame or centre of glass
Three different U-values often get mixed up in tenders and product literature.
Centre of glass
The figure for the middle of the glass unit only, away from the edges. A good double glazed unit with a low-emissivity coating and argon fill might reach around 1.0 to 1.1 W/m²K; triple glazing goes lower. This is the best-looking number and the least useful one on its own.
Frame
The aluminium mullions and transoms lose more heat than the glass, even with a thermal break. The narrower and better insulated the frame, the less it drags the overall figure down.
Whole wall
The figure that counts for the Building Regulations is for the curtain wall as built: glass, frames, spacers at the glass edges and spandrel panels, weighted by the area of each. It is calculated for the actual elevation using the method in BS EN ISO 12631. Always ask a tenderer for this figure for your design, not the centre of glass number.
What improves the U-value
- Better glass. Triple glazing, warm edge spacers and better coatings all help, at a cost and a weight penalty.
- Better frames. Wider thermal breaks and insulated inserts in the mullions and transoms.
- Fewer, larger panes. Less frame per square metre. Pane size is limited by weight, handling and cost.
- More insulated spandrel. Opaque, insulated zones in front of slab edges and ceiling voids perform far better than glass.
- Careful junctions. Heat leaks at the brackets, at the slab edge and where the wall meets other cladding. These are thermal bridges and need detailing.
Solar gain and overheating
U-value is only half the story for a glass wall. The g-value measures how much solar energy the glass lets through. A low g-value cuts overheating in summer but can increase heating and lighting needs. Solar control coatings, frit and external shading all affect it.
Solar control glass usually costs more. Published cost data from March 2025 put a high performance solar control coating at about £65 to £75 per m² extra over standard low-e glass on both stick and unitised systems. The curtain walling cost calculator lets you add it.
Replacing old curtain walling
Older curtain walls from the 1970s to the 1990s often have U-values far above today’s limits, with single glazing or early double glazing and frames without effective thermal breaks. Replacing them is a chance to cut heating costs significantly. When a whole facade is replaced, the new wall must meet the limiting U-value for replacement elements.
Where the frames are in good condition, reglazing with modern sealed units may improve the figure without replacing the whole wall, though the frame then limits how far it can go. See repair and refurbishment.
What to ask for in a tender
- The whole wall U-value for your elevation, calculated to BS EN ISO 12631.
- The glass make-up, including coatings, gas fill and spacer type.
- The g-value and light transmittance of each glass type.
- Thermal modelling of key junctions where the energy assessor needs it.
- Condensation risk analysis for cold-climate exposures or high-humidity buildings.
A curtain walling specialist will normally provide these as part of the design. If you are still at budget stage, use the form below and we will look for one that covers the project.