A straight, spec-based comparison for installers and self-builders choosing between aluminium and UPVC — based on the actual Smart Alitherm aluminium and Liniar UPVC systems we stock, not generic marketing claims.
Most online comparisons of aluminium and UPVC windows are written for homeowners browsing on price and looks. This one is written for the trade: real U-values, real sightlines, and where each system actually earns its place on a job — using the Smart Alitherm aluminium range and Liniar UPVC profile we fabricate and stock at our Kinmel Bay counter.
Aluminium gives slimmer sightlines and greater structural strength for the same opening size, which is why large sliding doors and bifolds are almost always aluminium rather than UPVC. It costs more per window, and its thermal performance depends entirely on the quality of the polyamide thermal break — a bare aluminium frame without one performs poorly.
UPVC remains the most cost-effective option for standard house openings, is quick to turn around, and modern multi-chamber profiles like Liniar comfortably meet current Building Regulations. It's the default choice for most straightforward window replacement jobs.
Aluminium conducts heat far more readily than UPVC, so every aluminium system we supply uses a polyamide thermal break to separate the inner and outer faces of the frame — without one, thermal performance would be poor regardless of the glazing fitted. Here's how the two ranges compare on their own published figures:
| System | Typical U-value | Notes |
|---|---|---|
| Smart Alitherm 400 (aluminium) | 1.4 W/m²K | Thermally broken, WER Band A (windows), PAS 24:2022 |
| Smart Alitherm Heritage (aluminium) | 1.5 W/m²K* | Slim 59mm sightlines, WER Band B, for heritage/conservation projects |
| Liniar standard (UPVC) | as low as 1.2 W/m²K | WER up to A++ achievable with double glazing |
| Liniar EnergyPlus (UPVC) | as low as 0.8 W/m²K | 6-chamber profile — Liniar's top-tier thermal system |
*As published on our aluminium windows page. Both ranges are Document L compliant.
The glass matters as much as the frame. A thermally-broken aluminium frame paired with a good glazing unit can outperform a basic UPVC window with standard glass — and vice versa. Ask us for the full energy-performance figures for your exact spec.
Aluminium's strength lets frames be built narrower for the same structural performance — Alitherm Heritage runs to a 59mm sightline, which is why it's the standard choice for period and conservation properties wanting a slim, steel-window look. Standard UPVC casement profiles are typically bulkier by comparison, though Liniar's flush-sash Resurgence range (available on our UPVC windows) narrows that gap considerably for a more contemporary UPVC look.
Aluminium's strength-to-weight ratio is why almost every large sliding door and bifold set in the trade is built in aluminium, not UPVC — it supports bigger single panes and wider door leaves without extra reinforcement. That's the case for our Smart Visofold 1000 Slim bifolds and Visoglide sliding doors (see our bifold sizes & configurations guide). UPVC remains perfectly capable for standard window and door openings, but it isn't the material you'd specify for a 4-6 metre opening.
Both materials achieve strong, independently tested security ratings when specified correctly. Smart's Alitherm 400 aluminium is PAS 24:2022 approved for windows and doors; Liniar UPVC windows carry PAS 24 Enhanced Security accreditation and Secured by Design status, with multi-point locking and reinforced frames as standard. Security comes down to the specific system and hardware fitted, not the frame material on its own.
A quality powder-coated aluminium finish is very hard-wearing and typically needs no more than an occasional wash. UPVC is also low-maintenance and won't rot, though very dark foiled UPVC colours are generally more prone to heat-related expansion on strongly sun-facing elevations than an equivalent dark aluminium finish — worth factoring in if you're specifying a dark colour on a south-facing wall.
As a rule, aluminium framing costs more than UPVC for a like-for-like window, reflecting the material, the thermal break and the powder-coat finish. UPVC remains the most cost-effective option for standard replacement windows. The actual difference varies by size, colour and glazing spec — ask the trade counter for a specification-based quote rather than a rule-of-thumb percentage.
Generally yes, for a like-for-like window — the material, thermal break and powder-coat finish all cost more than a UPVC profile. The gap depends heavily on size, colour and glazing spec, so ask the trade counter for a specification-based comparison rather than relying on a rule of thumb.
It depends on the exact system, not just the material. Smart’s thermally-broken Alitherm 400 aluminium achieves a typical U-value of 1.4 W/m²K, while Liniar’s standard UPVC windows achieve U-values as low as 1.2 W/m²K, rising to as low as 0.8 W/m²K on Liniar’s EnergyPlus range. The glazing unit fitted matters as much as the frame material.
No. Many of our customers mix materials — aluminium bifold or sliding doors for a wide opening, alongside UPVC casement windows elsewhere in the same project.
Both can be specified to a high standard. Smart’s Alitherm 400 aluminium windows are PAS 24:2022 approved, and Liniar’s UPVC windows carry PAS 24 Enhanced Security accreditation and Secured by Design status — security depends on the specific system and locking hardware fitted, not the frame material alone.