The Schüco AWS 90.SI+ is Schüco's premium aluminium residential window. The "90" is frame depth in millimetres; the "SI+" is "super insulation plus." It's what you specify when performance targets are ambitious — Passivhaus, high-comfort, high-value new build — or when architectural priorities demand the slimmest sightlines a serious aluminium window can offer.
What the extra 15mm of frame depth buys
Moving from a 75mm frame to a 90mm frame gives Schüco's engineers three things:
- Space to route a much larger thermal break — the isolator between outer and inner aluminium is deeper and more insulating.
- Capacity for triple-glazed units up to around 60mm — necessary for the highest U-value performance.
- Structural capacity for larger sashes at heavier weights.
Thermal performance
With optimised triple glazing and warm-edge spacers, the AWS 90.SI+ achieves:
- Uf (frame): approximately 0.9 W/m²K.
- Uw (whole window): from around 0.7 W/m²K.
That places the AWS 90.SI+ inside Passivhaus certified-component territory. In practical terms, on a heated room at 20°C internally with 0°C outside, the inner face of the frame stays warm enough that condensation risk is essentially eliminated.
Sightlines
The AWS 90.SI+ delivers frame face widths of approximately 97mm for fixed lights and 117mm for opening sashes — noticeably slimmer than the AWS 75 in fixed configurations. On a large fixed picture window, that difference reads architecturally.
Where the AWS 90.SI+ is worth the money
- Passivhaus and EnerPHit projects. The AWS 90.SI+ is a certified Passivhaus component in specific configurations.
- Contemporary new build where large fixed lights are architecturally central.
- Exposed locations where enhanced weather ratings — Class A4 air permeability, high water tightness — earn their upcharge.
- South-facing large glazing where combined thermal and solar-control performance matter for year-round comfort.
Where it's overspecified
For a straightforward window replacement on a Victorian terrace, the AWS 90.SI+ is beyond what's required. The AWS 75 covers the same requirement at meaningfully lower cost. Our editorial rule of thumb: reach for the 90.SI+ when the specification target is under Uw = 1.0, when sightlines are architecturally driven, or when the location demands enhanced weather ratings.
Cost expectations
Installed cost for AWS 90.SI+ typically runs 40–70% above equivalent AWS 75 configurations. For a large 2.5m² fixed light, that's roughly £2,400–£3,800 installed against the AWS 75's £1,600–£2,400.
Comparable systems
In the UK premium residential market, the AWS 90.SI+ competes with slim-frame aluminium systems from other European manufacturers — most notably Cortizo. We've written a full comparison in Cortizo vs Schüco. For projects prioritising integrated timber-composite specifications, Velfac and Rationel are also worth reviewing.
Sourcing near St Albans
The AWS 90.SI+ is a specifier's window — it doesn't turn up in showrooms unless requested. 5EC Aluminium's premium range covers the AWS 90.SI+ for projects across St Albans and Hertfordshire where the specification demands it.
For manufacturer technical data, see Schüco UK for full AWS 90.SI+ documentation.
Editorial verdict
The AWS 90.SI+ is the correct answer to specific questions — Passivhaus, ambitious contemporary architecture, large fixed lights, exposed locations. For everything else, the AWS 75 does the job at lower cost. Specifying by aspiration rather than by requirement is how projects end up over-budget for no measurable performance gain.