Ask ten aluminium door suppliers for a U-value and you'll get eleven different answers. Here's a plain-English guide to what U-values mean, what they don't mean, and how to read the numbers when you're specifying a Schüco sliding door for a Hertfordshire home.
What a U-value actually measures
A U-value is the rate of heat transfer through a structure per square metre per degree of temperature difference, in watts. The unit is W/m²K. Lower is better: less heat leaks out.
For doors and windows, three different U-values get quoted, and this is where confusion starts:
- Ug — glass only. Just the sealed glazed unit.
- Uf — frame only. Just the aluminium profile.
- Uw (or Ud for doors) — the whole assembly. This is the number that matters.
If a supplier quotes 0.7 W/m²K on a Schüco sliding door, they are almost certainly quoting the glass alone. The whole-door U-value will be meaningfully higher.
Building Regulations thresholds
Approved Document L (2021 edition, still current) sets the following for England:
| Element | New build target (Uw) | Replacement threshold (Uw) |
|---|---|---|
| Windows | 1.2 W/m²K | 1.4 W/m²K |
| Doors (glazed) | 1.4 W/m²K | 1.4 W/m²K |
| Rooflights | 1.7 W/m²K | 2.2 W/m²K |
Every major Schüco sliding system — ASE 60, ASE 80, ASE 80 HI — clears these thresholds in standard specifications. The question isn't whether they pass Building Regs; it's how much better than the threshold you want to go and what that costs.
What the numbers translate to in real terms
For a 4m × 2.2m sliding door (8.8m² of glass and frame), heat loss over an average UK heating season, comparing three U-values:
| U-value (Uw) | Approx. seasonal heat loss | Approx. gas cost |
|---|---|---|
| 1.6 W/m²K (double, argon) | ~700 kWh/year | ~£45/year |
| 1.3 W/m²K (triple, warm edge) | ~570 kWh/year | ~£37/year |
| 1.0 W/m²K (triple, high-spec) | ~440 kWh/year | ~£29/year |
The step from double to triple glazing typically adds £400–£900 to the door cost and saves you £8–£15 per year in heat. That is a very long payback. Triple glazing is rarely specified for the heating bill alone — the real reason is comfort near the glass on cold nights, and reduced condensation.
Solar gain: the summer question
U-value is only half the thermal story. The other half is g-value — how much solar radiation the glass lets through. On a south-facing sliding door in a Hertfordshire garden, uncontrolled solar gain can overheat a room from March to October.
Solar control coatings reduce g-value to around 0.3–0.4 (from a standard 0.6), typically adding £200–£400 per panel. For south-facing large-format glass, this is often better money spent than a triple-glazing upgrade.
How to read a Schüco data sheet
Schüco publishes Declaration of Performance (DoP) documents for every system. Three things to look at:
- Uw quoted for a specific reference size — usually 3.0m × 2.18m for sliding doors. Your actual door may be different.
- Air permeability class — Class 4 is the highest domestic class and what you want.
- Water tightness class — Class 9A minimum for coastal or exposed positions.
Local specification advice
For extensions in Hertfordshire, our editorial preference is a well-specified double-glazed door with solar control on south-facing openings, and a warm-edge triple only where the door is exposed and heated heavily. 5EC Aluminium quotes the U-values transparently on their specification sheets — worth requesting alongside any quote you compare.
Full manufacturer data is on Schüco UK's technical library.