What does a window energy rating or U-value actually mean?
The three numbers on a window specification, which of them you should actually compare, and why the rating on the label may not describe the window you are being sold.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
The U-value measures heat loss — lower is better. The g-value measures solar heat gained through the glass — higher gains more warmth and more summer overheating. The letter rating combines them into one grade. For comparing quotes, ask for the whole-window U-value, not the glass U-value.
Window specifications carry a letter grade, a U-value and sometimes a g-value, and quotes routinely quote whichever number flatters the product. Understanding what each measures makes three quotes comparable, and it is the only way to tell whether the more expensive option is actually better.
Step by step
- Understand the U-value.It measures how fast heat passes through, in watts per square metre per degree of temperature difference. Lower is better. Building regulations set a maximum U-value for replacement windows, and it has tightened over time, so an older window that was compliant when fitted may be well short of today's standard.
- Insist on the whole-window U-value.There is a U-value for the glass alone and a U-value for the whole window including the frame and the spacer. The glass figure is always better. If a quote states a U-value without saying which it is, ask — it is the most common way two quotes are made to look different when they are not.
- Understand the g-value.Also called solar factor: the proportion of the sun's heat that gets through the glass. Higher means more free warmth in winter and more overheating in summer. On a north-facing window it barely matters; on a large south-facing one it matters a great deal.
- Understand the letter rating.The window energy rating combines heat lost through the window, heat gained from the sun, and air leakage into a single letter grade. It is a useful shorthand but it hides the trade-off between U-value and g-value, which is why a window rated well for a cold room may overheat a sunny one.
- Look at what actually drives the numbers.The number of panes, the gas fill between them, whether there is a low-emissivity coating, the type of spacer bar at the edge, and the frame material and its thermal breaks. Warm-edge spacers and a low-emissivity coating make a large difference for modest money.
- Check what the regulations require now.Replacement windows have to meet a stated performance standard in the building regulations for your nation, and the standard has tightened. Check the current requirement rather than a figure from a few years ago, and make sure the quote meets it.
- Ask about trickle ventilation.Modern requirements normally mean replacement windows include trickle vents, particularly where the existing windows had them or the room's ventilation depends on them. A quote that omits them may be omitting a requirement, and blocking them afterwards causes condensation and mould.
- Do not expect the numbers to pay for themselves quickly.Upgrading from single glazing to modern double glazing makes a large difference to comfort and to condensation. Upgrading from reasonable double glazing to a slightly better grade rarely pays back in energy alone; it is bought for comfort, noise and condition instead.
- Compare quotes on the full specification.Frame system and material, glass make-up, gas fill, coating, spacer type, whole-window U-value, trickle vents, and the hardware. Get all of it in writing on every quote before comparing prices.
Questions people ask
Is triple glazing worth it?
It improves the U-value and helps with noise, but the gain over good double glazing is much smaller than the gain from single to double, the units are heavier and the hardware has to suit. Worth considering on an exposed or very cold elevation; rarely transformative elsewhere.
What is argon fill and does it matter?
Argon between the panes conducts heat less well than air, and it is standard on most modern units. It is cheap and it makes a measurable difference, so there is no reason not to have it.
Why do my new windows have more condensation on the outside?
External condensation on a very well-insulated window is a sign it is working — the outer pane stays cold because so little heat is escaping through it. It is a curiosity, not a fault.
Will new windows cure my condensation problem?
Not on their own, and they can move it. Sealing a draughty house without adding ventilation moves the moisture from the glass to the walls, where it grows mould instead.