Flagship guide · 7 min read
U-Values and Thermal Performance Explained
The single figure most quoted in glazing brochures, and the one most often misunderstood. Here is what a U-value actually measures, and what to do with it.
Window Frame Materials Compared: Thermal and Structural Performance
Frame material shapes thermal performance, sightline width and structural strength together. Here is how the main options compare on the numbers.
3 min read
Frame material choice affects far more than appearance. It determines how narrow the sightlines can be for a given structural span, how the frame achieves its U-value, and how the product ages over decades of Cumbrian weather.
This guide sets out the thermal and structural characteristics of the main frame materials used in premium residential glazing, as a technical companion to the broader aluminium vs uPVC comparison elsewhere in the Knowledge Centre.
Thermal comparison by material
| Material | Typical Uf (frame only) | Structural strength | Typical lifespan |
|---|---|---|---|
| Aluminium, thermally broken | 1.8 – 2.2 W/m²K | Very high — narrow sightlines possible | 35 – 45 years |
| Residence Collection (foam-filled aluminium) | similar to standard aluminium, improved chamber design | Very high | 35 – 45 years |
| Timber-look uPVC | 1.2 – 1.5 W/m²K | Moderate — needs steel reinforcement for larger spans | 25 – 35 years |
| Timber (engineered hardwood/softwood) | 1.0 – 1.4 W/m²K | Good, naturally insulating but needs maintenance | 30 – 60 years with upkeep |
| Steel (slim heritage sections) | 2.5 – 3.5 W/m²K uncoated | Very high — slimmest sightlines available | 60+ years |
Note that frame-only Uf figures are not directly comparable to whole-window Uw figures elsewhere in this section — uPVC's naturally lower conductivity frame can post a strong Uf figure while still landing at a similar whole-window U-value to a well thermally broken aluminium frame, once the glazing unit and frame factor are accounted for.
Why aluminium achieves narrow sightlines without a thermal penalty
Aluminium is inherently a poor insulator, but its structural strength allows much slimmer frame sections than timber or uPVC for the same span, meaning a smaller proportion of the window area is frame rather than glass. Combined with a well-designed polyamide thermal break, this is how systems like the Residence Collection achieve both narrow, elegant sightlines and whole-window U-values that compete with, or beat, bulkier materials.
Durability in exposed Cumbrian conditions
Aluminium's corrosion resistance and dimensional stability make it well suited to the exposed, wet conditions typical of the Lake District and Cumbrian coast, with powder-coated finishes generally holding colour and texture for decades with minimal maintenance. Timber requires more regular maintenance in this climate, though it remains the preferred choice on some listed buildings and conservation projects where planning conditions require it.
- Aluminium: minimal maintenance, excellent corrosion resistance, stable in temperature swings
- Timber-look uPVC: low maintenance, good value, can discolour slightly over decades in strong UV exposure
- Timber: highest maintenance burden, repaintable/refinishable, often required in conservation areas
- Steel: very durable structurally, needs a good factory coating to resist corrosion long-term
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