Schüco AWS 75 vs AWS 90: Which Window Depth to Choose

If you have received quotations for aluminium windows from a Schüco-approved fabricator, you will probably have seen AWS 75.SI+ and AWS 90.SI+ mentioned. On paper the difference is 15 mm of frame depth. In practice the choice affects thermal performance, cost, glazing options and even the visual proportions of the finished window. This article covers the practical difference and how to decide which is right for your project.

The technical short version

Both AWS 75.SI+ and AWS 90.SI+ are current-generation, super-insulated aluminium window systems from Schüco. The main technical differences:

• Frame depth: 75 mm vs 90 mm.
• Thermal break: both use a foam-filled thermal break; the AWS 90 has more depth of insulation.
• Uf value: AWS 75.SI+ around 1.3 W/m²K on the frame, AWS 90.SI+ down to around 0.95 W/m²K.
• Whole-window Uw: depending on glazing, AWS 75.SI+ typically hits 1.1-1.3 Uw with triple glazing; AWS 90.SI+ can go under 0.9 Uw with the same glass.
• Glazing capacity: AWS 90.SI+ takes thicker glass units, up to 68 mm — which matters if you are specifying warm-edge triple glazing.

Where AWS 75.SI+ is the right answer

For a straightforward domestic window replacement in an existing house — swapping tired timber or uPVC casements for aluminium — the AWS 75.SI+ is usually the correct choice. It hits Part L targets, has excellent long-term durability, and the sightlines are compact enough that windows do not look chunky from outside. Cost per window is meaningfully lower than the 90.SI+ counterpart, and for most homeowners the incremental thermal saving of moving to 90.SI+ does not repay the cost difference in reasonable time.

Where AWS 90.SI+ starts to make sense

Move to AWS 90.SI+ when the specification demands it, which typically means one of the following:

1. You are building to Passivhaus or an equivalent low-energy standard where whole-window U-values need to be under 1.0.
2. You have a large area of glazing on a heat-loss critical elevation and want to minimise thermal loss without moving to triple-glazed AWS 75.
3. You are combining large fixed lights with heavy triple glazing and need the frame depth to carry the glass weight.
4. You are specifying to match adjacent commercial-standard glazing — 90 depth reads more architectural.

Alternatives worth being aware of

Aluminium is not the only path to high-performance windows. Composite windows such as Rationel's alu-clad window range or Velfac achieve similar U-values with a different aesthetic — aluminium externally, timber internally. If you value warmer internal reveals, composite is worth costing. If you value clean modern lines and matching finishes across doors and windows, staying within the AWS family is generally simpler.

How the visual reads

On a wall, the 75 and 90 versions look nearly identical from outside. The difference is more apparent on the inside — the deeper AWS 90 profile shows more frame return at the reveal and reads slightly more substantial. On modern extensions where clean flush details matter, some architects prefer the 90 for this reason; on retrofit work where the window is being fitted into an existing reveal, the 75 is often the more graceful visual fit.

How much difference in cost?

Rule of thumb: expect a 15-25% supply-price increase moving from AWS 75.SI+ to AWS 90.SI+ on a like-for-like opening and glazing spec. The installation cost is broadly the same. On a whole-house window replacement of, say, twelve windows, this is a meaningful step in the total — worth deciding on the basis of thermal target, not aesthetics alone.

Getting a comparable quotation

When comparing quotations, insist on identical glazing spec across both — same manufacturer, same Ug value, same warm-edge spacer, same total unit thickness. This is the only way to see the frame's contribution to the whole-window U-value cleanly. Approved fabricators such as 5EC Aluminium can produce quotations on both AWS 75.SI+ and AWS 90.SI+ on request; their aluminium window sourcing pages cover the range they specify from.

Both systems are excellent. The 75 is the right answer more often than not on typical UK residential work. The 90 exists for the projects where the extra thermal performance genuinely earns its keep. Neither is a mistake — but choosing the more expensive spec without a reason for it is a common one.