The short answer
For an existing home in England, a new roof window should reach 1.4 W/m²K or Window Energy Rating band B. That comes from Part L 2021, Table 4.2. A rooflight in a flat roof is held to a different figure, 2.2 W/m²K, and we confirm the rating with the maker for the exact size we quote.
The figure to meet and where it comes from
People asking what a roof window U value needs to be are asking about Part L, the section of the Building Regulations that sets energy standards. For work to an existing dwelling, Table 4.2 gives the limits for new and replacement fittings. A roof window must achieve 1.4 W/m²K, or a Window Energy Rating of band B. That is the roof window Part L requirement in plain terms.
Any unit in a pitched slope is classed as a roof window, and that includes conservation rooflights fitted in a pitched slope. Rooflights in flat roofs and lanterns are held to the higher figure of 2.2 W/m²K. The difference reflects the way the products are built and used.
What U-value and WER mean
A U-value measures how quickly heat passes through a material. The lower the number, the slower the heat escapes. It is stated in watts per square metre per degree of temperature difference, written W/m²K. Glass, frame and spacer all contribute, so the figure is for the whole window, not just the pane.
The Window Energy Rating is a banded label, from A++ downward, that also counts the solar heat the window lets in. A window can reach band B by balancing heat loss, solar gain and air leakage, all rated under standard conditions. Either route meets the regulation, and the maker's data gives both.
When the rule applies
The standard applies when a whole roof window is replaced or a new one is fitted. Both are building work, and both are notifiable, so the new unit must comply. The rule does not apply where only the sealed glass unit is changed in a sound frame, because that is not a controlled fitting. If you are unsure which applies to you, the short answer on Building Regs for rooflights sets out the difference.
What affects the figure in practice
- The glazing: double or triple glazing, the gas fill and low-emissivity coatings all lower the figure.
- The frame: timber, PVC and coated materials each conduct heat differently.
- The spacer: a warm-edge spacer reduces heat loss at the rim of the glass.
- The size: the figure applies to the standard size tested, so larger or smaller units can differ.
- The installation: insulation around the frame and a well-sealed flashing keep the finished opening from losing heat.
That final point is the one people forget. A window with an excellent rating still leaks heat if the gap around it is poorly insulated. We insulate the frame surround and line the reveal, splaying it where the roof depth allows, then plaster and make good inside.
Reading a maker label
The data sheet for a roof window gives the U-value for the whole unit, usually for a stated size, and often a Window Energy Rating as well. Check that the figure is for the glazing and frame you are buying, not only for the centre of the glass, which always looks better. If a brochure quotes only a centre-pane figure, ask for the whole-window value. We do this for you at the quote stage, and we put the figure in writing so it can be shown to Building Control.
Some windows are sold in several glazing grades. A basic double-glazed version and a higher-performance one may look the same from outside, and the label is the only way to tell them apart. When we replace an old unit, we check the new one against the rating of what came out, so you know the change is an improvement.
Going beyond the minimum
The Part L figure is a minimum. Many roof windows reach better, and the benefit shows in a loft room that is warmer in winter and cooler in summer. Glass with a solar control coating helps on a south slope. Our page on roof windows for loft conversions talks about how to balance light, warmth and shading in a converted attic.
Why the rating matters in a loft
A loft room has a large share of its surface in the roof, so the glazing affects comfort more than it does in a downstairs room. A window with a poor rating loses heat on winter nights and can leave the glass cold to the touch, which brings condensation. A better-rated unit keeps the inner surface warmer, helps the room hold its temperature, and cuts the draught that cold glass can create. The figure on the label is the starting point, and the installation around the frame is what keeps it.
How we handle it
We check the rating of each unit we specify, give the figure in the fixed written quote, and notify Building Control where the work is notifiable, with the local authority or a private registered approver. You receive a completion certificate at the end. Our 10-year workmanship guarantee covers the fitting, and the fixed quote is in writing before we start. If you have an old unit and want to know how it compares, we can read the label at the survey and tell you.
