From Georgian cottages to glass-box extensions, frame choice shapes everything.
TL;DR (Key Points)
- uPVC (A-rated) → cheaper, hits 1.2 U-value easily, widest wood-grain foils.
- Thermally-broken aluminium → slimmer sight-lines, spans > 3 m, lives 40 yrs+.
- In conservation zones (Old Hatfield, Ware), timber-look uPVC often passes; aluminium needs ovolo or dual-colour detail.
- Mix-and-match works: aluminium bifolds at the rear, uPVC casements elsewhere.
1 · Quick comparison
| Feature | Modern uPVC | Thermally-broken aluminium |
|---|---|---|
| Whole-window U-value | 1.2 W/m²K | 1.4 → 1.0 W/m²K (high-spec) |
| Installed cost (casement) | £330 | £490 |
| Frame depth | 70–85 mm | 55–75 mm (slimmer) |
| Expected lifespan | 25–30 yrs | 40+ yrs |
| Recyclability | Medium | 95 % |
2 · Energy & the Future Homes Standard
Draft rules for 2025 push new-build windows to 0.8 W/m²K. uPVC hits that cheapest; aluminium needs aero-gel breaks or triple glazing but is catching up fast.
3 · Heritage & planning hurdles
- Grade II listings / conservation areas → planners like flush-sash uPVC that mirrors original glazing bars.
- Aluminium passes if colour-matched (RAL 7032 Sage Green is popular) and shows putty-line detailing.
4 · Strength & design freedom
Spans over 3 m? Aluminium’s rigidity beats uPVC, which needs steel reinforcing, adding weight and cost.
5 · Cost-of-ownership (10 windows, Stevenage)
| Metric | uPVC flush-sash | Aluminium slimline |
|---|---|---|
| Supply & fit | £3,300 | £4,900 |
| Re-painting (year 27) | n/a | £750 |
| 30-year NPV (4 % disc.) | £3,300 | £4,570 |
6 · Quick FAQs
Will aluminium frames feel cold?
No—modern polyamide breaks stop cold-bridging.
Can I mix materials?
Yes—common combo: aluminium sliders + uPVC upstairs.
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