Window U-values: Hitting Part L with steel windows

• 26 September 2025
• News

As architects and homeowners already know, choosing a window is about more than appearance. Heat loss, insulation and energy efficiency define comfort inside a home or commercial space, while Part L of the UK building regulations sets precise standards that cannot be overlooked. Designers often face a difficult question: how to retain the slender profiles and expansive glazing that make steel windows so desirable as well as meeting modern U-value targets. The W20 TE system from Crittall shows that compliance and elegance can exist together.

Understanding U-values and Part L

U-values measure the rate of heat transfer through a window expressed in watts per square metre kelvin. Lower figures mean less heat escapes, helping to reduce energy bills and the carbon footprint of a building. Part L specifies that replacement windows in dwellings must achieve 1.4 W/m²K, while new non-domestic projects should meet 1.6 W/m²K. Compliance is assessed by building control and influences everything from design approval to energy efficiency ratings, making early consideration of U-values essential.

Steel engineered for performance

Steel has a reputation for lightness and strength but in its classic form it is highly conductive. W20 TE overcomes this with a refined profile that interrupts thermal transfer. When paired with high-performance insulating glass units, low emissivity coatings and argon fill, heat loss is reduced, draughts are minimised and overall thermal efficiency improves. Spacer bars and precision seals further enhance insulation maintaining comfortable indoor temperatures throughout the year.

Consider a compact city flat with small windows facing north. With conventional single glazing internal temperatures can fluctuate rapidly, increasing energy bills and leaving occupants reaching for extra heating. Installing W20 TE windows stabilises the interior temperature, limiting the rate of energy transfer and making the flat warmer and more comfortable. Similarly, in a large period home with tall sash windows, slim steel frames allow expansive panes of glass without compromising thermal performance. Heat loss is reduced, draughts are controlled and the interior remains bright and inviting.

Air permeability and airtightness testing confirms that W20 TE performs to the highest standards for both residential and commercial applications. Specifiers and homeowners can achieve Part L compliance without compromising the rhythm, light and proportion that make steel windows so distinctive.

Harmony of design and efficiency

Energy efficiency need not compromise design. W20 TE preserves the slender frames and large panes of glass that define both contemporary and period architecture. Reduced heat transfer through the frame combined with carefully engineered glazing limits condensation and supports the fabric performance of the building. The result is a window that respects both regulatory demands and visual ambition.

Durability contributes to long-term energy performance. Constructed from 100 percent recycled steel, galvanised and powder-coated, W20 TE frames are built to last beyond sixty years with minimal maintenance. Steel maintains its structural integrity far longer than timber or aluminium ensuring thermal performance remains consistent and energy bills stay lower over the lifetime of the building.

Looking ahead

The Future Homes Standard will raise the bar for energy performance in new dwellings. Selecting steel windows that already exceed current U-value targets ensures projects remain resilient to these changes. The W20 TE range is adaptable enough for period renovations, contemporary new builds and commercial settings offering a balance of style and technical sophistication.

Crittall provides project-level U-value assessments and technical consultations to help specifiers and homeowners match Part L targets with the right windows. By combining regulatory compliance with elegant proportions and superior thermal efficiency, W20 TE proves that steel can deliver comfort, durability and lasting value while maintaining the beauty that has made it a first choice for architects across the UK.

• For more information contact Crittall Windows here.