Double glazed steel windows: unit thickness, spacers and sightlines 

• 7 November 2025
• News

When planning a window replacement or specifying for a new build, the finer details of double glazing matter. For steel windows, understanding unit thickness, spacer choice and sightlines helps ensure a perfect balance between performance and design. 

Understanding unit thickness

Unit thickness refers to the overall depth of the double glazing unit, i.e. the two panes of glass and the sealed cavity between them. For steel frames, this dimension affects both thermal efficiency and how the glazing sits within the profile. 

Modern double glazed steel windows, such as Crittall’s W20 TE system, accommodate a range of unit thicknesses. This allows designers and homeowners to choose the right balance of thermal insulation and noise reduction. Our slim steel windows generally accommodate thinner units than uPVC or aluminium but achieve the same performance using inert gas filling, coated glass and warm edge spacers. This preserves the inherent elegance of steel windows while improving the energy efficiency of your home. 

The role of spacers

Between the panes of glass sits a spacer – the sealed strip that forms the insulating cavity. These advanced spacers improve U-values and help prevent condensation at the window edge. For heritage or conservation projects, Crittall offers discreet spacer finishes that complement traditional sightlines without reducing performance. The result is modern efficiency within an authentic frame. 

Slim sightlines, lasting appeal

Sightlines define the architectural character of steel windows. The strength of steel allows for exceptionally narrow profiles, creating light-filled interiors with a crisp, refined appearance. 

Crittall’s design expertise ensures that double glazing does not compromise the proportions that made its windows iconic. The W20 TE range delivers excellent thermal efficiency while retaining the elegance of original steel fenestration. This makes it ideal for listed buildings, conservation areas and design-led contemporary homes. 

The Crittall legacy

For 175 years, Crittall has set the standard for steel windows. Its name has become synonymous with the distinctive black steel frame; a design classic recognised across the world. From early twentieth-century modernism to today’s architectural developments, Crittall windows have defined entire eras of building design. 

Every window is designed and manufactured in Britain, combining craftsmanship with advanced technology to meet modern standards for energy efficiency, strength and sustainability. 

Built for performance and beauty

Double glazed steel windows provide real, long-term advantages. Improved thermal insulation means reduced heat loss and lower heating bills, while enhanced sound insulation offers protection from noise pollution. And with steel’s exceptional strength, these windows deliver security and durability alongside elegant proportions. 

Available in a wide range of RAL colours and finishes, including the classic black steel window, Crittall systems suit both period and modern properties. 

To explore glazing options, spacer details and sightline drawings, click here. Clear technical guidance and expert advice make it simple to specify with confidence and enjoy the unique advantages of Crittall steel windows.