Is a lower U-value better?

Yes. The U-value measures how much heat passes through one square metre of the window for every degree of temperature difference between inside and outside. A lower number means less heat escapes.

What U-value do Building Regulations require for windows?

In England, Approved Document L (2021) asks for replacement windows in existing homes to reach a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B. Listed buildings and many historic buildings are allowed to depart from this where meeting it would harm their character, which is why secondary glazing is widely used instead.

Does secondary glazing reach Building Regulations standard?

On its own, a standard secondary unit over single glazing lands at around 1.8 W/m²K, and low-E glass brings it to about 1.6. That is a little short of the 1.4 target for new replacement windows, but secondary glazing is an addition to an existing window, not a replacement, so the 1.4 requirement does not normally apply.

What is the difference between a U-value and a Window Energy Rating?

The U-value only measures heat loss. A Window Energy Rating (A++ to E) also accounts for solar heat gain and air leakage, giving an overall energy balance for the whole window.

Reference Guide

Window U-Values Explained

What the number on a window specification actually means, how your windows compare, and how to work out how much heat they lose.

Last reviewed October 2026 · Written by London Secondary Glazing

What it measures

Watts of heat lost through one square metre of window for each 1°C difference between inside and outside. Written as W/m²K.

Lower is better

Single glazing sits near 5.0. Halving the U-value halves the heat that escapes through the glass and frame.

Easy to calculate

Heat loss = U-value x window area x temperature difference. You can estimate any window in seconds.

Typical U-values and heat loss

The heat loss column is for a typical 1.5m² sash window on a cold evening, 5°C outside and 20°C inside (a 15°C difference).

Glazing	U-value (W/m²K)	Heat lost per window
Single glazing in a timber frame	4.8 - 5.0	112 - 118 W
Early double glazing (1980s - 90s)	2.8 - 3.0	63 - 68 W
Single glazing + secondary glazing	1.8 - 2.0	41 - 45 W
Single glazing + low-E secondary glazing	1.6 - 1.8	36 - 41 W
Modern A-rated double glazing	1.2 - 1.4	27 - 32 W
Triple glazing	0.8 - 1.0	18 - 23 W

Working out your own heat loss

Multiply three numbers: the U-value, the window area in square metres, and the temperature difference in degrees.

Heat loss (W) = U-value x Area (m²) x Temperature difference (°C)

Single glazing: 5.0 x 1.5 x 15 = 112.5 W

With secondary: 1.8 x 1.5 x 15 = 40.5 W

In this example, adding a secondary pane cuts the heat escaping through that one window from about 113 watts to about 41. A Victorian terrace with ten sash windows is losing over a kilowatt through its glass on an ordinary winter evening, roughly the output of a small electric heater running constantly. The U-value improvement also shows up on your EPC rating.

Frequently asked questions

Find out what your windows are costing you

Our free survey includes a room-by-room assessment of your windows and the U-value improvement secondary glazing would deliver.

Book a Free Survey 0207 060 1573

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