What is a U-value and what level should I be targeting?

A U-value measures how quickly heat passes through the window — the rate of non-solar heat flow — in watts per square metre per kelvin (W/m²K). The lower the number, the better the insulation. For replacement windows in England under current Part L, the target is clear: a maximum U-value of 1.4 W/m²K, or a Window Energy Rating of band B or better. That’s the legal minimum. In practice most standard uPVC double-glazed units we supply comfortably beat it — typically around 1.2 W/m²K — and triple-glazed options can reach 0.8 W/m²K or lower for installers chasing higher performance or working to Passivhaus-style specs. If you’re quoting a job, 1.4 is the line you must not cross; A-rated/1.2 is the sensible everyday standard. Upvcwindows-brighton