Steel window details: What architects need to resolve early 

• 15 July 2026
• News

Getting steel window details right at design stage can prevent costly revisions, reduce contractor queries and keep projects on programme. The key decisions should be resolved well before tender stage, particularly around profile selection, fixing methods, thermal performance, glazing requirements and regulatory compliance. 

For architects, these details are far more than technical footnotes. Steel windows sit at the intersection of structure, envelope design and aesthetics, making early coordination essential if performance targets and design intent are to be achieved. 

Why steel window details need early coordination

Steel windows offer exceptionally slim sightlines and a strength-to-weight ratio that few alternative materials can match,  creating elegant glazing arrangements that maximise daylight and preserve architectural character. 

That slim profile leaves little margin for error where the window frame meets the surrounding construction. Decisions deferred until construction stage often result in redesign work, programme delays and avoidable costs. 

Many of the technical issues that arise on site can be traced back to unresolved Stage 3 design details. Establishing a clear strategy early helps ensure that architects, contractors and specialist suppliers are working from the same assumptions throughout the project. 

Fixing methods and interface design

One of the first decisions to make is how the steel window will be fixed into the structural opening. 

Steel’s flexibility extends to installation. Frames can be fixed directly to the structure or fitted using subframes. Where subframes are used, the finished steel windows can be installed after wet trades, reducing the risk of damage. 

The steel frame is installed into the prepared subframe using substantial stainless steel fittings, typically with a continuous mastic seal, incorporating an EPDM if required  around the perimeter. 

Direct fixing remains an option for certain applications, using mechanically fixed lugs attached directly to the surrounding structure. The suitability of this approach depends on the construction type, opening tolerances and installation sequence. 

Whatever fixing method is selected, architects should allow an appropriate perimeter movement joint. This accommodates fabrication tolerances, construction variations and thermal movement. Where gaps exceed 3mm, suitable backing materials should be specified to support the sealant system and maintain weather performance. 

Thermal performance and Part L compliance

Thermal performance should be considered from the outset because the choice of steel window system directly influences the U-values that can be achieved. 

Steel is an excellent conductor of heat, with a conductivity of approximately 50 W/m²K. Without careful detailing, the frame can become a significant thermal bridge within an otherwise well-insulated envelope. 

Modern thermally broken systems address this by introducing an insulating barrier within the frame profile. Materials such as polyamide or high-density polyurethane interrupt the transfer of heat between the internal and external steel sections, significantly improving thermal performance. 

Crittall produces a true thermally broken steel window and door system. When combined with suitable glazing, whole-window U-values can be as low as 0.8 W/m²K, making it suitable for high-performance commercial and residential projects. 

For conservation and heritage applications, the W20 TE provides a different solution. Rather than incorporating a full thermal break, it uses a thermally enhanced profile design that achieves U-values of approximately 1.4 W/m²K while maintaining the slender appearance associated with traditional steel windows. This allows projects to meet Approved Document L requirements without compromising historic character. 

Thermal continuity extends beyond the frame itself. Architects should ensure that insulation returns fully to the window frame and that junction details avoid unnecessary thermal bridges. Window-to-wall interfaces play a major role in overall performance and should be carefully coordinated with the building envelope design. 

Glazing integration and specification

Glazing requirements should be confirmed at an early stage because they influence frame selection, rebate depths and thermal performance. 

Unlike many alternative systems, steel windows are generally glazed on site rather than delivered ready glazed. This improves handling and installation but places greater importance on coordination between frame design and glazing specification. 

Insulating  glass units ranging from 16 mm – 50 mm thick can be accommodated either  gasket glazed or  wet-glazed . The precise capacity depends on the system being specified. 

Architects should verify glazing build-ups against profile capabilities during Stage 3. Changes to glazing specification after tender can affect frame dimensions and trigger redesign work that impacts programme and cost. 

Access requirements should also be considered. Several Crittall systems can be manufactured for either internal or external glazing, which can be particularly valuable where site access is restricted. 

Choosing the right system early

Selecting the correct steel window system at concept and developed design stage helps avoid later compromises. 

The W20 TE is particularly suited to conservation projects, listed buildings and heritage-led developments where traditional proportions and sightlines are critical. It is often employed on new builds where a traditional or industrial aesthetic is preferred. 

Crittall Thermally Broken Systems is designed for projects requiring higher thermal performance and is often specified for contemporary commercial and residential buildings. 

Other systems within the Crittall range address specialist requirements, including bespoke commercial applications, residential security specifications and internal steel screening. 

Matching the system to the performance requirements of the project at an early stage allows technical detailing, glazing selection and compliance assessments to proceed with confidence. 

Compliance, security and specification support

Building Regulations, security requirements and planning considerations should all be reviewed before the specification is finalised. 

For residential applications, systems such as Homelight Plus are tested to PAS 24 standards,  incorporate multipoint locking and can help satisfy the requirements of Part Q for new-build dwellings. For commercial projects, compliance with Approved Document L remains a key consideration, particularly where energy performance forms part of the building approval process. 

Listed buildings and properties within conservation areas may require additional approvals. Early engagement with conservation officers, supported by accurate section drawings and technical information, can help streamline the process. 

Architects should also make use of technical resources available during specification. CAD drawings, NBS clauses, product data sheets and performance information provide valuable support when coordinating steel window details across the wider design team. 

Resolving these technical issues early creates a smoother path through procurement and construction. More importantly, it ensures that the finished steel windows deliver the appearance, performance and durability the project demands. 

• For more information about Crittall’s steel windows, contact us here.