What goes into the letter
The rating combines three things: heat lost through the frame and glass, heat gained from sunlight passing through, and air leakage around the opening sash. A window can score well by keeping heat in, by letting sunshine in, or by a mixture of both.
That is why a rating alone can mislead on a north facing elevation, where the solar gain component is largely theoretical. On those openings the U-value and the air leakage figure describe reality better than the letter does.
| Term | What it tells you |
|---|---|
| Rating letter | Whole window score, heat lost against heat gained |
| U-value | Heat loss only, lower is better |
| g-value | Solar heat gain, higher means more free warmth and more summer heat |
| Air leakage | How much air passes the closed sash |
| Warm edge spacer | Reduces cold bridging at the pane edge |
Why the frame is half the answer
The label is for the window, not the glass, so a good sealed unit in a thin, poorly chambered frame will not deliver the number on the sticker. Chamber count, reinforcement and the quality of the gasket all sit inside that figure.
This is also why glass only replacement into old frames improves things without matching a new A-rated window. It is often still the right decision on cost, and we will say so, but it is not equivalent.
What we fit and why
A-rated as standard across the range, with a warm edge spacer bar and argon fill, because the manufacturing cost difference over a lower spec unit is small and the difference at the edge of the glass in February is not.
Where a room has a specific problem we change the glass rather than chase a higher letter: solar control on a south facing room that overheats, acoustic laminate on a road, obscure where privacy matters.
Ask for the numbers, not the letter
Our quotes state the whole window U-value and the glass build, so you can compare like with like against anyone else.