Steel Windows and Crittall Windows London 2025: Original, Replacement, and Heritage Options

**The Crittall story and the dominance of steel windows in early-to-mid 20th-century London**:

• Crittall Manufacturing Company was founded in Braintree, Essex in 1884 and became the world's dominant manufacturer of steel window frames through the early-to-mid 20th century. Steel-framed windows were the modernist material of choice — they allowed sight lines far narrower than timber or cast iron alternatives, enabling larger pane areas with the same structural performance. By the 1920s and 1930s, Crittall windows were standard in:
• •Local authority housing estates across London (many surviving LCC estates still have original steel casements — now frequently listed)
• •Arts and Crafts and Modern Movement private houses
• •Commercial and civic buildings across London
• •Factory and industrial buildings (hence the 'industrial glazing' aesthetic that drives the contemporary revival)

The National Heritage List for England (NHLE) includes hundreds of London buildings specifically noted for their original steel casement windows — these are protected and must not be altered without Listed Building Consent.

**What makes Crittall-style windows visually distinctive**:

• •**Slimline section**: the key characteristic — the steel section (the frame profile around each pane) is very narrow compared to timber, aluminium, or UPVC alternatives. Original Crittall sections (the W20 range — still made by Crittall Windows Ltd today) have a face width as narrow as 20–35mm. Modern aluminium 'Crittall-style' profiles typically have a face width of 35–65mm — noticeably wider, though still narrower than standard casements
• •**Putty-glazed appearance**: original Crittall windows are direct putty-glazed into the rebated steel frame — the glass sits in a bed of linseed oil putty directly in the steel, creating minimal sight lines. Contemporary steel and aluminium systems typically use a gasket or structural silicone glazed unit in a purpose-made rebate — a slightly different appearance
• •**Gridded or geometric pattern**: Crittall windows are associated with geometric subdivisions — equal panes in a grid; horizontal emphasised grids (the 'landscape' look); or diamond and Gothic shapes in the interwar period
• •**Black or dark grey colour**: the dominant colour for both historic Crittall (original factory finish was mid-grey or bronze; painted black became the dominant colour for modern Crittall-style specification) and modern steel/aluminium profiles. The 'Crittall look' is almost always interpreted in black or anthracite
• •**Reveals and shadow**: steel windows traditionally sit in a deep reveal (the wall thickness behind them), creating strong shadow lines around the frame — an important part of the aesthetic. Flat-fitted shallow-reveal installations lose much of the visual impact

**Original Crittall Windows Ltd — the only manufacturer of true Crittall product today**:

Crittall Windows Ltd (Crittall's direct descendant company; now based in Witham, Essex) continues to manufacture the W20 Heritage range — hot-rolled steel sections in exactly the same profile as original interwar production — used for listed building restoration, conservation area replacements, and new homes seeking authentic heritage specification. The W20 Heritage range is the only product that satisfies Listed Building Consent conditions requiring 'like-for-like' Crittall replacement in most London Boroughs.

• *Other steel window manufacturers for UK residential*:
• •**Clement Windows** (Surrey): high-quality steel windows for residential and conservation use; a long-established alternative to Crittall for bespoke and residential projects
• •**Vierendeel** (Belgium — available in UK): slimline systems including thermally broken profiles
• •**Mondrian** (UK): specialist in steel frame glazing systems for extensions and residential
• •The **Steel Window Association** (SWA) is the UK trade body — membership indicates quality and technical competence; lists accredited suppliers

**Modern aluminium 'Crittall-style' systems — and how they differ from genuine steel**:

• The explosion in 'Crittall look' aluminium windows over the last 10 years has produced a wide range of aluminium systems with a slim profile and black coating designed to evoke the Crittall aesthetic without the thermal performance limitations of traditional steel:
• •Examples: Origin Aluminium; IQ Glass; Velfac; Smart Architectural Aluminium; Aluminium Trade Supply (ATS); Fineline Aluminium
• •Aluminium systems are thermally broken (a polyamide thermal break isolates the inner and outer aluminium sections, dramatically reducing thermal conductivity) and can typically achieve U-values of 1.2–1.6 W/m²K for the frame alone, and whole-window U-values of 1.0–1.4 W/m²K with triple glazing
• •The visual difference between a true Crittall steel window and an aluminium 'Crittall-style' window is visible to an expert eye — the section widths are slightly larger in aluminium; the depth profile is different; and the thermal expansion characteristics differ (steel expands and contracts more predictably along longer spans). For non-listed residential in a conservation area, many conservation officers accept high-quality aluminium 'heritage-style' systems as an appropriate modern interpretation

**The thermal performance challenge with traditional steel windows — the core technical issue**:

• Steel is one of the most thermally conductive common structural materials (thermal conductivity of steel: ~50 W/m·K; compare timber: ~0.12 W/m·K; aluminium with thermal break: ~0.2 W/m·K effective after break; UPVC: ~0.17 W/m·K). A non-thermally-broken steel frame has extremely high thermal conductivity — heat transfers rapidly from inside to outside through the steel frame itself, leading to:
• •Very poor whole-window U-value (non-thermally-broken steel with single glazing can be 5–6 W/m²K — worse than a single-brick wall)
• •Cold frame surfaces on the interior in winter — condensation forms on the steel frame as warm interior air meets the cold frame surface (condensation occurs when the frame surface drops below the dew point temperature of the interior air)
• •Significant heat loss through the glazing and particularly through the frame

**Part L 2021 (Conservation of Fuel and Power) — the current standard for glazing in extensions**:

• Part L of the Building Regulations (2021 edition, in force from June 2022) requires that:
• •New windows installed as part of a new extension must achieve a whole-window U-value of ≤ 1.6 W/m²K (or a window energy rating (WER) of Band C or better)
• •As of the 2021 Part L, the previous 'area-weighted average' approach has been tightened — individual windows must comply rather than allowing non-compliant windows to be averaged against better-performing glazing elsewhere

*Can traditional non-thermally-broken steel windows comply with Part L 2021?*:

• Generally NO — traditional non-thermally-broken steel sections with standard insulating glass units (IGUs) rarely achieve a whole-window U-value below 2.0 W/m²K, making them non-compliant with Part L 2021 for use in a new extension. This creates a problem for:
• •New extensions seeking a Crittall aesthetic with genuine steel windows
• •Conservation area work where the conservation officer requires steel windows to match original brickwork and existing windows in the street scene

*Solutions for achieving Part L compliance with steel-frame aesthetics*:

1. **Thermally broken steel frames**: several manufacturers now produce thermally broken steel profiles — a polyamide insert bonded between the inner and outer steel sections, breaking the thermal path. Crittall Windows Ltd offers their thermally broken 'Thermalite' range; Vierendeel and Clement also offer thermally broken steel options. Thermally broken steel with triple-glazed or warm-edge IGU can typically achieve whole-window U-values of 1.2–1.8 W/m²K — borderline compliant or compliant with Part L 2021. However, the thermally broken section is slightly wider than the non-thermally-broken heritage section — the sight lines increase slightly 2. **Aluminium with thermal break and slim profile** (the most common solution for new London extensions): high-performance slim aluminium systems achieve 1.0–1.4 W/m²K easily; they are fully Part L compliant; and the Crittall aesthetic is approximated closely (though not exactly) 3. **Part L exception for listed buildings**: where a listed building's original windows must be retained or replaced like-for-like under LBC conditions, a Building Regulations application can seek a relaxation from the energy performance requirement under Regulation 9 (listed buildings and buildings in conservation areas where compliance would unacceptably alter the character). This relaxation must be confirmed with the Building Control Body in advance 4. **Secondary glazing** (for listed buildings where original steel windows must be retained): fitting secondary glazing (an inner-frame secondary pane in a separately mounted slim aluminium or steel frame) behind the original steel window adds insulation without altering the window externally — see `conservation-area-windows-guide`. Secondary glazing for Crittall windows using a steel or dark aluminium secondary frame can achieve whole-assembly thermal performance of 1.0–1.5 W/m²K without touching the original listed windows

**Conservation area rules for steel windows in London — what officers typically require**:

*Replacing existing steel windows in a conservation area*:

• •In most London conservation areas, the replacement of original steel casement windows with UPVC is NOT acceptable — UPVC section widths and profile shapes are noticeably different from steel, and conservation officers consistently object to UPVC in steel-window properties
• •Replacement with like-for-like steel (Crittall W20 Heritage or equivalent slim steel) is generally acceptable and in many cases required — this may need a householder planning application (if Article 4 Direction applies) or may be PD (check with the Borough)
• •Replacement with high-quality slim-profile aluminium in a matching black or dark grey colour is accepted in some (not all) London conservation areas — the conservation officer's view depends on the specific area; the importance of the building within it; and how convincingly the aluminium profile replicates the steel sight lines. Always seek pre-application advice before specifying
• •Replacement with modern thermally broken Crittall steel (Thermalite range) is almost always acceptable — it maintains the Crittall steel character while improving energy performance

*New steel or aluminium windows in a new London extension*:

• For a new rear extension in a conservation area, new windows in a Crittall or heritage-steel style are generally supported by conservation officers where:
• •The windows are in a contemporary extension that is clearly a high-quality modern addition (not a pastiche copy of the original Victorian style)
• •The windows are in black thermally broken aluminium or thermally broken steel
• •The sight lines and proportions are appropriate to the design
• Conservation officers in London increasingly accept well-designed modern extensions with Crittall-style glazing as a positive architectural addition rather than a harmful intervention — provided the overall design is high quality and the extension reads as a distinct but complementary addition to the original building

**How steel windows are supplied and installed — key differences from standard casements**:

*Lead time*: steel windows are manufactured to bespoke order — every project has specific opening sizes, configurations (fixed; top-hung; side-hung; pivot; tilt-and-turn), subdivision patterns, and ironmongery specifications. Lead time for Crittall Windows Ltd W20 Heritage: typically 12–16 weeks from confirmed order. Modern thermally broken steel (Clement; Vierendeel): typically 8–14 weeks. High-quality aluminium slim-profile (Origin; IQ Glass etc.): typically 4–8 weeks. **Lead time must be factored into the extension programme** — ordering too late is the most common steel window project delay. Order should ideally be placed at the same time as planning permission is granted or building regulations are approved, so windows arrive when the extension structure is ready to receive them.

*Sub-frames and fixings*: most steel windows are installed into a timber or steel sub-frame fixed into the structural opening. The sub-frame dimensions must be specified with the window order — the window is then fixed into the sub-frame on site. For a London extension with a structural steel lintel (RSJ or hollow section) at head level, the window head detail (how the window frame meets the underside of the steel beam) must be detailed carefully — thermal bridging at the lintel is a common cold-bridge point.

• *Glazing unit specification for Crittall-style windows in London 2025*:
• •Single glazing (not compliant with Part L 2021 for new extensions): only appropriate for listed building consent replacement like-for-like
• •Slimline DGU (double-glazed unit, 20–24mm total thickness): the standard for most contemporary Crittall-style installations — 4mm glass | 12mm argon-filled cavity | 4mm glass (4/12/4) with warm-edge spacer bar; achieves glass centre-pane U-value of approximately 1.1 W/m²K. Most Crittall heritage putty-glazed applications use slimline units to maintain the visual integrity of the slim rebate
• •Triple-glazed unit (28–36mm total thickness): available for thermally broken steel and aluminium frames; achieves glass centre-pane U-value of approximately 0.6–0.7 W/m²K; preferred for north-facing or exposed elevations; requires slightly wider frame rebate — check with manufacturer
• •Vacuum glazing (Pilkington Spacia; LandVac): 8.3mm total thickness; U-value ~0.7 W/m²K; specifically designed for heritage slim-profile applications where the window rebate cannot accommodate a conventional DGU or TGU thickness. Highest cost option; used primarily for listed building heritage conservation

*Ironmongery and opening gear for steel windows*:

Steel casement windows use specific ironmongery — cockspur handles; monkey tail fasteners; casement stays with peg sets. Authentic Crittall heritage ironmongery (in bronze; chrome; or black) is available from specialist suppliers (Crittall Windows Ltd; W.H. Newson; Framing Now). Conservation officers may specify authentic ironmongery types for listed building work.

**Supply and installation costs — steel and Crittall-style windows in London 2025**:

| Specification | Supply cost (per m² of glazed area) | Installation cost (per m² glazed area) | Notes | |---|---|---|---| | Crittall W20 Heritage steel (non-thermally broken; single or slimline DGU glazed; putty-glazed) | £600–£1,200/m² | £150–£350/m² | Listed building / heritage restoration | | Crittall Thermalite (thermally broken steel; slimline DGU) | £800–£1,500/m² | £200–£400/m² | Part L borderline compliance; conservation area appropriate | | Clement steel (thermally broken; slimline DGU or TGU) | £700–£1,300/m² | £200–£400/m² | Alternative to Crittall; SWA member | | High-quality slim aluminium (Origin; IQ Glass; ATS; Fineline; thermally broken; TGU) | £400–£900/m² | £150–£250/m² | Most common for new London extensions; Part L compliant | | Aluminium 'Crittall-style' (mid-range; standard slim profile; thermally broken; DGU) | £250–£500/m² | £100–£200/m² | Most cost-effective Crittall aesthetic; Part L compliant |

*Typical total supply-and-install cost examples for a London rear extension*:

• **Rear extension rear wall full-width glazing: 4.5m wide × 2.4m high = 10.8m² glazed**:
• •Slim aluminium Crittall-style (mid-range; thermally broken; DGU): £3,500–£7,600
• •High-quality slim aluminium (Origin or IQ Glass; TGU): £5,900–£12,400
• •Crittall Thermalite steel: £10,800–£20,300

• **Side return extension link passage with Crittall-style glazing: 6m × 2.1m = 12.6m²**:
• •Slim aluminium (mid-range): £4,400–£8,800
• •High-quality aluminium: £6,900–£14,300