The Window Energy Rating scale
The British Fenestration Rating Council scale runs from A++ at the top down to C, which is the minimum standard currently permitted for new replacement windows in England and Wales under Building Regulations Part L. Anything rated below C should not legally be fitted as a replacement window in most circumstances.
The rating looks at the whole window as a system, glass, frame, spacer bar and how they perform together, not the glass in isolation.
| Rating | Approx U-value | What it means |
|---|---|---|
| A++ | 0.8 or better | Best available thermal performance |
| A / A+ | 1.2 to 1.4 | Strong standard modern performance |
| B | 1.4 to 1.6 | Above minimum, reasonable performance |
| C | 1.6 to 2.0 | Current legal minimum for replacements |
What a U-value actually measures
The U-value is the rate of heat loss through the window, measured in watts per square metre per degree Celsius. A lower number means less heat escapes, so a window with a U-value of 1.2 is performing better than one at 1.8. This single figure drives most of the rating and is the number worth asking for directly if a supplier only quotes you a letter.
The three things that build a good rating
Low-E coating on the glass reflects heat back into the room while still letting light through. Argon gas filling the gap between panes is a better insulator than plain air and slows heat transfer. A warm edge spacer bar, usually a composite rather than plain aluminium, reduces the cold bridge that used to form around the edge of every unit in older windows.
- Low-E coating: reflects room heat back inside
- Argon gas fill: slows heat transfer between panes
- Warm edge spacer: cuts cold bridging at the glass edge
- Frame chamber design: adds still air insulation within the profile itself
Part L and why the minimum exists
Building Regulations Part L sets minimum energy efficiency standards for replacement windows precisely so that homeowners are not sold anything below C rated as new. A FENSA style certificate, or a building control sign off, confirms the installation meets this. Always ask what certification route your installer will use for the paperwork.
Want the specification explained for your quote?
Request a quote and we will talk you through the exact rating and U-value for the windows we recommend.
Is it worth paying extra for A++ over A?
The step from A to A++ usually comes from a combination of triple glazing, an even better spacer and sometimes a thicker frame chamber, and it does bring a real, measurable reduction in heat loss. Whether it is worth the extra cost depends on how long you plan to stay in the property, how exposed the elevation is, and what the rest of the house's insulation looks like. For a typical semi with reasonable loft and wall insulation already in place, a solidly specified A rated window often represents the more sensible balance of cost against benefit.
How the whole window rating is actually calculated
The BFRC calculation does not simply average the glass performance and the frame performance. It models a standard sized window using the specific glass unit, spacer bar, frame profile and even the way the sashes and beading are put together, then works out the combined heat loss, solar gain and air leakage for that exact combination. This is why swapping just one component, such as upgrading the spacer bar without changing anything else, can shift the letter rating even though the glass itself is unchanged.
It also means two windows using the same glass can carry different ratings if one has a chunkier, better insulated frame profile than the other, which is a detail easy to miss when a quote only quotes the glass specification.
Window Energy Ratings versus a home's EPC
It is worth not confusing a window's own WER with your property's Energy Performance Certificate. The EPC scores the whole house, including the boiler, insulation, loft and walls, with windows contributing only one line among many. Fitting A rated windows will generally help an EPC score move in the right direction, but it will not transform a poorly insulated house on its own, and a surveyor assessing a property for an EPC looks at far more than glazing alone.
What the rating does not tell you
A high WER rating tells you how well the window itself resists heat loss, but it says nothing about your actual heating bills, which depend on the rest of the house, your heating system, occupancy patterns and how many windows you have. It also does not measure noise reduction or security, both of which are separate specifications entirely.