What is a U-value, and is triple glazing worth it?

A U-value measures how quickly heat passes through a building element, in watts per square metre per kelvin (W/m²K). Lower is better — it is the opposite convention to insulation R-values, which is a frequent source of confusion.

Typical figures give a sense of scale:

Single glazing — around 5.0 W/m²K. Effectively a hole in the wall thermally.

Old double glazing — around 3.0.

Modern double glazing with low-emissivity coating and argon fill — around 1.2 to 1.6.

Triple glazing — around 0.6 to 0.9.

A modern insulated wall — around 0.2 to 0.3, which shows how much heat windows lose even when good.

What makes glazing perform. The gas gap matters, with argon or krypton conducting less than air. Low-E coatings — microscopically thin metallic layers — reflect infrared back into the room while admitting light, and contribute more than most people realise. Warm-edge spacer bars reduce the thermal bridge at the panel edge, and the frame material matters as much as the glass.

Is triple glazing worth it? Honestly, it depends:

The marginal gain is smaller than the jump from single to double. Going from 5.0 to 1.4 eliminates most of the loss; going from 1.4 to 0.8 captures a fraction of what remains. Payback on energy savings alone is frequently measured in decades.

Where it genuinely pays: cold climates, very well insulated homes where windows have become the dominant loss, large glazed areas, and where you want to eliminate the cold downdraught near a window — a comfort benefit that does not show in energy calculations.

Acoustic performance is often the better reason, though asymmetric double glazing with differing pane thicknesses can outperform triple for noise.

Triple glazing is much heavier, requiring stronger frames and hinges.