Every rooflight sold in Britain comes with a number attached to it, and most people nod at it and move on. The number is the U-value. It is worth a few minutes of your attention, because it tells you how well a piece of glass in your roof will keep the warmth of the room below where it belongs.
This guide explains the rooflight u value in plain terms: what the unit means, why the figure on a leaflet is not always the figure for the whole unit, which limits the Building Regulations set when you replace or add one, and how to compare two products on a level footing. For the wider picture of glass, frames and comfort, start at the energy efficient rooflights hub.
The same ideas apply whether you call it a skylight or a rooflight. Homeowners say skylight, the trade says rooflight, and the maths underneath is identical.
What a U-value actually measures
A U-value measures heat flow. It tells you how much heat passes through one square metre of a material when the air on one side is one degree warmer than the air on the other. The unit is watts per square metre per kelvin, written W/m²K. You do not need to remember that. You only need to remember the direction: a lower number is better.
Think of it as a leak rate. A wall with a low U-value is a tight wall. A single sheet of old glass has a high U-value, so it leaks. Modern sealed units have a low one, so they leak far less. The skylight u value is the same kind of figure, applied to the whole opening in your roof.
The number covers three routes for heat to travel. It moves by conduction through the glass and frame, by convection in the air gap between the panes, and by radiation across that gap. Good design attacks all three, which is why a rooflight is a small stack of clever parts rather than a plain sheet of glass.
Why the figure on a leaflet can mislead you
There are two ways to quote a U-value, and the difference matters. The first is the centre-pane figure. It describes the glass alone, measured away from the edges. The second is the whole-unit figure, sometimes written Uw for window or Ur for rooflight. It describes the glass, the spacer bar that holds the panes apart, the frame and the way they all meet.
The centre-pane figure is always the more flattering of the two, because the edges and the frame are where heat escapes most easily. A product with excellent glass in a poor frame can still give a disappointing whole-unit number. When two quotes show different figures, the first question is always: is this the glass or the whole unit?
Ask for the whole-unit figure, in writing, for the exact size you are buying. A U-value changes slightly with size, because a small rooflight has proportionally more frame and edge than a large one.
What Part L asks for in an existing home
When you add or replace a rooflight in an existing dwelling, Approved Document L of the Building Regulations sets the standard. Table 4.2 gives the limits, and they are different for the two product families:
- Roof windows, the kind set into a sloping tiled or slated roof, are held to 1.4 W/m²K, or to WER band B on the window energy rating scale.
- Rooflights, the kind that sit on an upstand over a flat roof, are held to 2.2 W/m²K.
The two limits differ because the products differ. A roof window is a finished window set into the slope, with its own sash and frame. A rooflight on a flat roof is a different construction, with the glass carried on an upstand. Roof lanterns and flat glass fall into the rooflight group.
These are the limits for the work to be accepted, not targets to scrape under. A good installation sits comfortably inside them. Each installation follows the regulations in force, and we notify Building Control whenever the job needs it.
The limits in numbers
- 1.4 W/m²K
- Table 4.2 cap on roof windows in older homes; WER band B also passes
- 2.2 W/m²K
- Table 4.2 cap on rooflights in older homes
Roof window or rooflight: which number applies to you
The word on the spec sheet tells you which limit your job falls under. A pitched-roof product such as a VELUX window or another roof window takes the 1.4 limit or the WER band B route. For a flat rooflight, a lantern, or a walk-on unit over a flat roof, the limit is 2.2.
You will sometimes see a product described both ways in a catalogue, because one frame type can be sold as either. What decides it is how it is installed and what it is built into, not the name on the box. At the survey we confirm which family your opening belongs to and quote a product that clears the limit with room to spare.
The WER route is worth a word. WER stands for window energy rating, a banded scale from A down to G that also credits useful solar gain. Band B is acceptable for roof windows. It is an alternative way of showing compliance, so a product that lists a U-value and a band gives you two ways to check it.
What sits behind the number
A rooflight u value is the sum of its parts. Each part can be improved, and each improvement shows up in the final figure.
The glass
Double glazing traps a gap between two panes. Triple glazing adds a third pane and a second gap. A thin, nearly invisible coating on one surface reflects heat back into the room, and the gap can be filled with a gas that conducts heat more slowly than air. Our guide to double or triple glazed rooflights goes into how those choices play out.
The spacer
The strip that keeps the panes apart runs round the whole edge of the unit. A metal spacer carries heat across the edge more easily. A so-called warm-edge spacer, made of a less conductive material, narrows that bridge and improves the whole-unit figure without changing the glass at all.
The frame
Frames of aluminium, timber and uPVC behave differently. Aluminium is slim and strong, which suits large flat rooflights and lanterns, and a good aluminium frame contains a thermal break so the cold of the outside does not run straight through to the inside. uPVC and timber insulate well on their own. The best choice depends on the look you want and the size of the opening.
The upstand
On a flat roof the glazing is carried on a raised kerb and tilts slightly so water runs away. A well-insulated upstand matters as much as the glass above it. If the kerb is poorly insulated, heat leaks round the edge of an otherwise excellent unit.
U-value is not the whole story
It is tempting to chase the lowest figure and stop there. A rooflight does more than hold in heat, and two other ideas deserve a place in your decision.
The first is solar control. In summer the sun is high, and the sky is exactly where it sits. A roof is the part of a house that takes the full force of it, so a unit that is excellent at holding warmth in winter may let a room get uncomfortably bright and warm on a hot afternoon. Glass can be specified with a coating that cuts the sun's heat while still letting daylight through. That is a separate property from the U-value, and it is worth asking about.
The second is condensation. The inside face of a well-made unit stays warmer, and a warmer pane is less likely to mist with moisture in a kitchen or bathroom. This is one practical reward of a lower whole-unit figure that you will notice day to day. For the room's point of view, our answer on whether rooflights lose heat applies, and the one on how a rooflight affects the temperature of a room covers summer.
Straight up is clear: a rooflight looks at open sky where a side window faces a fence or a wall, which is why its light falls so evenly. That is the reason to have one. The U-value is how you make sure the benefit does not cost you comfort.
How to compare two quotes fairly
Two quotes for rooflights can look similar and differ in ways that matter. Before you compare the totals, compare the products. Ask each supplier for the same short list of facts.
- Is the U-value given for the glass or for the whole unit?
- Is it for the actual size, or for a standard reference size?
- Which Part L limit does the product fall under, 1.4 or 2.2?
- What is the glass build-up: double or triple, coating, gas fill, spacer?
- What frame material, and does it have a thermal break?
- Is the upstand insulated, and is the internal finishing included?
If one quote answers all six and the other answers two, you are not comparing like with like. Our quotes are fixed and written, and they do not change on the day, so the specification you read is the specification that goes on the roof.
Ask every supplier these before you compare
- Is the U-value for the glass alone or the whole unit?
- Is the figure for my actual size?
- Which Part L limit applies to this product?
- What are the glass, spacer and frame details?
- Is the upstand insulated and the inside finished?
Words you will meet on a spec sheet
Spec sheets use shorthand. This short list decodes the terms you are most likely to meet when you read about a skylight u value, so you are not guessing when a salesperson says them aloud.
Words you will hear
U-value
How quickly heat passes through a material, measured in W/m²K, where lower is better.
Centre-pane
The U-value of the glass alone, measured away from the edges, so it looks better than the whole unit.
Uw or Ur
The U-value of the whole window or rooflight, including glass, spacer and frame.
Warm-edge spacer
A less conductive strip between the panes that cuts heat loss round the edge of the unit.
Thermal break
A layer of insulating material inside a metal frame that stops cold running straight through.
WER
Window energy rating, a banded scale from A to G that Part L accepts as a route for roof windows.
Upstand
The insulated kerb standing up from a flat roof, which carries the rooflight glass.
Getting the figure right in your own home
The best way to settle all of this is to see the product against your own opening. A free survey in Milton Keynes and the surrounding towns lets us measure the roof, check how it is built up, decide which Part L limit applies and talk through glass, frame and finish. The written quote fixes the price before work starts, so the figure on the day is the one you were given, and the work carries a 10-year workmanship guarantee in writing.
After you say yes, fitting takes a day or two, with the inside finished neatly so no raw plaster is left behind. If a planning question comes up, our guide to skylight planning permission sets out how to check. The energy side and the planning side are separate questions, and both are worth settling before you commit.
To book the survey, WhatsApp 07880 284133 or email info@vertexskylightsmiltonkeynes.co.uk. For the broader view of glass, frames and comfort, return to the energy efficient rooflights hub, or read what the downsides of a skylight can be in our answer on the drawbacks of skylights, so you go in with your eyes open.
Small questions about big numbers
Is a lower U-value always better?
For heat retention, yes. A lower figure means less heat escapes. Balance it against solar control and the look of the frame, and make sure you are comparing whole-unit figures rather than glass-only ones.
Can I fit a rooflight that does not meet the limit?
Approved Document L sets the standard that work to an existing dwelling has to meet. We specify products that clear it, and where the job needs Building Control involved, we make that call.
Does the U-value change with the size of the rooflight?
Slightly. A small unit has more frame and edge in proportion to its glass, so its whole-unit figure can differ from a large one of the same make. That is why we quote for your actual size.
