Windows for Extensions: Aluminium vs Timber vs uPVC — Which to Specify?

Aluminium windows have become the dominant choice for contemporary London extensions and refurbishments — partly for their slim sightlines and clean aesthetic, and partly because they offer superior strength-to-weight ratio and decades of low maintenance.

• **Key advantages**:
• •Slim sightlines (typically 30–45mm frame face width vs 65–80mm for uPVC)
• •Inherent rigidity — large format openings (e.g., 3m+ sliding or bifold doors) are only practical in aluminium or steel
• •Low maintenance — powder-coated finish in any RAL colour lasts 20–30 years without repainting
• •Recyclable — aluminium has strong sustainability credentials at end of life

**Thermal bridging — the critical issue**: Aluminium is highly thermally conductive (conductivity ~160 W/mK vs 0.13 for uPVC and 0.12 for timber). A solid aluminium frame is a severe thermal bridge — historically a weakness of aluminium windows. **Thermally broken aluminium** resolves this: the inner and outer aluminium profiles are separated by a polyamide thermal break strip, reducing conductivity across the frame dramatically.

**U-values — what to specify for Part L compliance**: Part L 2021 requires windows in extensions to achieve a whole-window U-value of 1.4 W/m²K or better (or a window energy rating of C or better). Thermally broken aluminium with double-glazed Argon-filled units typically achieves 1.2–1.4 W/m²K. Triple-glazed aluminium: 0.8–1.0 W/m²K.

• **Aluminium window costs (supply and install, London 2025)**:
• •Standard casement window 1200×1200mm (thermally broken, double-glazed): £900–£1,600
• •Large fixed picture window 2400×1200mm: £1,800–£3,200
• •Bifold door 3000mm wide 3-panel: £3,500–£6,500 — see bifold guide for full detail
• •Sliding door 3000mm wide: £3,500–£6,000
• •Premium slim-frame aluminium (e.g., Wicona, Reynaers): add 30–60%

Timber windows have been used in UK residential construction for centuries and remain the material of choice in Conservation Areas, on listed buildings, and for clients seeking a traditional or warm aesthetic that aluminium cannot replicate.

• **Key advantages**:
• •Naturally low thermal conductivity (~0.12 W/mK) — solid timber frames have excellent inherent thermal performance without needing a thermal break
• •Traditional aesthetic — the only material accepted in most Conservation Areas and on listed buildings
• •Can be repainted in any colour — repainting refreshes the window every 5–10 years
• •Carbon sequestration — sustainably sourced timber has strong whole-life sustainability credentials

• **Disadvantages**:
• •Maintenance — timber must be maintained (repainted or re-oiled every 5–10 years) to prevent rot, swelling, and deterioration
• •Movement — timber expands and contracts seasonally; draught-sealing degrades faster than in aluminium
• •Cost — good-quality engineered timber windows cost similar to or more than aluminium

• **Timber species and engineering**:
• •Solid softwood (Redwood, Scottish Pine): traditional, lower cost, requires more maintenance
• •Engineered hardwood (Accoya, Meranti, Oak): superior stability and durability; Accoya (acetylated radiata pine) offers exceptional stability with 50-year guarantee
• •Engineered timber windows from European manufacturers (e.g., Rationel, Velfac) use laminated timber cores with aluminium-clad exteriors — combining timber's thermal performance with aluminium's maintenance-free exterior

**U-values**: Solid timber double-glazed windows: 1.3–1.6 W/m²K. Timber-aluminium composite triple-glazed: 0.7–0.9 W/m²K.

• **Timber window costs (supply and install, London 2025)**:
• •Softwood painted casement 1200×1200mm: £700–£1,200
• •Engineered timber casement 1200×1200mm: £1,100–£1,800
• •Accoya timber casement 1200×1200mm: £1,400–£2,200
• •Timber-aluminium composite casement 1200×1200mm: £1,500–£2,500

uPVC (unplasticised polyvinyl chloride) windows have dominated UK volume house-building for the past 30 years because of their low cost, low maintenance, and acceptable thermal performance. For extensions on standard residential properties where cost is the primary driver, uPVC remains a viable choice — though it is rarely specified by design-aware clients on contemporary extensions.

• **Key advantages**:
• •Lowest cost of the three main frame materials
• •Low maintenance — paint-free, does not rot
• •Good thermal performance — uPVC itself is a poor thermal conductor, so uPVC frames achieve good U-values without a separate thermal break
• •Wide availability — large supply chain, fast lead times

• **Key disadvantages**:
• •Aesthetics — chunky frame profile (65–80mm face) looks heavy; cannot be specified in slim-line configurations for large openings
• •Colour limitation — white and woodgrain foil are standard; coloured uPVC (anthracite grey, black) is available but more expensive and less durable than powder-coated aluminium
• •Not accepted in Conservation Areas or on listed buildings
• •Not suitable for large format openings (sliding or bifold doors) — structural limitations of uPVC mean maximum door widths are typically 3m
• •Longevity — uPVC degrades over 20–30 years (yellowing, frame distortion)

**U-values**: uPVC double-glazed windows: 1.2–1.6 W/m²K. uPVC triple-glazed: 0.8–1.1 W/m²K.

• **uPVC window costs (supply and install, London 2025)**:
• •Standard casement 1200×1200mm white (double-glazed): £400–£750
• •Anthracite grey casement 1200×1200mm: £550–£950
• •uPVC triple-glazed casement 1200×1200mm: £650–£1,100

Regardless of frame material, the glazing unit itself is where most of the thermal performance comes from. A correct glazing specification for a Part L-compliant extension should include:

• **Double vs triple glazing**:
• •Double-glazed (28mm unit): two panes of glass, one gas-filled cavity — the standard for most London extensions
• •Triple-glazed (44mm unit): three panes, two cavities — significantly better U-value but heavier (structural implications for opening units) and higher cost (30–50% premium)

• **Gas fill**:
• •Argon: standard — better than air, cost-effective, widely available
• •Krypton: better performance than Argon in a thinner cavity, more expensive — used in slim triple-glazed units

• **Low-emissivity (low-E) coating**:
• •Essential on all modern windows — a microscopically thin metallic coating on the inner surface of the inner pane reflects radiant heat back into the room
• •Hard-coat low-E: more durable, slightly lower performance — used on external-facing surfaces
• •Soft-coat low-E: better performance, must be within the sealed unit — standard for modern double and triple glazing

• **Solar control glazing**:
• •A solar control coating reduces solar heat gain (lower SHGC value) — relevant for south-facing windows in extensions where Part O overheating may be a concern
• •Solar control glass reduces heat gain in summer but also reduces passive solar gain in winter — a trade-off requiring assessment

• **Glass safety**:
• •All glazing in critical locations (within 800mm of floor level, or in doors) must be safety glass (toughened or laminated) under BS 6206 and Building Regulations Part N
• •All large fixed glazing panels and overhead glazing must also be safety glass