We love roof lights because they change how a room feels. Daylight drops in from above, colours look truer, and a flat roof extension stops reading like a deep box. The quiet part of the story is heat. The U-value in rooflights tells you how quickly warmth escapes through the product on a cold day. Lower numbers mean less heat sneaking out, steadier indoor temperatures, and smaller energy bills. If you’re an architect sketching options, a builder pricing a job, or a homeowner planning a loft or kitchen extension, getting comfortable with U-values will help you choose a roof light that looks good and feels good all year.
A U-value is a measure of heat loss. The unit is watts per square metre per degree (W/m²·K). In plain language, it says: “for every degree of temperature difference between inside and outside, how much heat is slipping through each square metre of this assembly?” Lower is better.
There are two flavours you’ll see in brochures. Centre-pane (Ug) is the number for the glass alone, measured in the middle of the pane. Whole-window (Uw) includes the frame, the glass edge spacer and the bits where heat prefers to take a shortcut. Ug is always lower than Uw, which is why it can look impressive on a spec sheet. For real decisions, use Uw. That’s the performance you actually install.
One more detail matters for skylights and roof lights. Test methods treat roof windows (on pitched roofs) as vertical, and roof lights (on flat roofs) as horizontal. The orientation changes how heat moves, so make sure any figure you compare has been tested in the correct plane. It sounds fussy, but it keeps you from comparing apples with pears.
Modern Bespoke Glazing systems with double glazing, a Low-E coating, argon gas, and a high-performance warm-edge spacer typically achieve around 1.2–1.4 W/m²·K Uw across many roof light sizes and frame types. You can feel the difference compared with older units — the inner pane feels warmer to the touch on winter evenings, and there’s far less of that “cool fall” near the opening.
Triple glazing moves the target again. With two gas-filled cavities and an extra pane, many triple units reach ≈0.8–1.0 W/m²·K Uw. The benefit is comfort rather than bragging rights. Sitting under a triple on a frosty night simply feels calmer, because the inner glass runs closer to room temperature. Do remember that very small sizes with chunky frames can test a little worse than mid-sizes with slimmer frames, because the frame-to-glass ratio drives the whole-window result.
Read more: When Do You Need Solar Control glass and How Does It Work?
Improving the U-value of a roof light isn’t just about ticking an energy-efficiency box it’s about creating a comfortable interior that stays warm in winter and cool in summer. Every element of the build-up, from the glass layers to the frame detailing and even the placement of the unit, plays a part in how well heat is retained and distributed. Here’s how to get the most from your roof light system:
Start with the glazing specification, this is where most thermal gains are made.
Frames contribute significantly to overall performance.
Even the best glass can underperform if installation is poor.
Performance isn’t just about materials, it’s also about where you place the roof lights.
Two smaller, well-positioned units above key working areas often save more lighting energy than one large pane in the wrong spot.
Smart daylight placement improves comfort, reduces glare, and enhances energy efficiency without compromising design.
Part L of the Building Regulations sets backstop U-values (legal maximums) for new work. Treat them as a floor, not a goal. In practice, most homes aim well below the backstop to help the whole building meet its energy targets and to keep rooms comfortable. Different nations in the UK publish their own guidance, and assessors use notional targets that are usually tighter than the legal limit, so take local advice for the exact figures on your job.
A simple rule of thumb works across most projects. If you specify a good double with Uw around 1.2–1.3 W/m²·K, or a triple at ≈0.8–1.0, you’ll sit comfortably inside typical expectations, and you’ll notice the comfort difference day to day. Make sure your supplier confirms that the numbers are whole-window and tested in the correct orientation for roof lights.
Let the room and the orientation guide you. A north-facing bedroom or studio that is used at night earns the case for triple glazing: that warmer inner pane means less radiant chill when you sit beneath it. A south- or west-facing kitchen may be perfect with a high-quality double at around 1.2-1.3, provided you also think about solar control and ventilation to keep summer comfortable. On heritage roofs or tight details where very thick units would spoil the line, a balanced double with excellent air-tightness around the upstand can outperform a nominally “better” unit installed with gaps and cold bridges.
Budget decisions are easier if you spend in the right order. First, buy whole-window performance you can trust. Second, buy good installation airtight tapes, insulated reveals, and a kerb that is warm and well flashed. Third, add triple glazing where comfort or exposure truly demands it. That sequence gives you the feel of a high-performance building without paying for numbers you won’t notice.
These three numbers work together. U-value tells you how much heat you lose on cold days. g-value (also called Solar Heat Gain Coefficient) tells you how much sun heat gets in on bright days. Visible light transmission (VT) tells you how bright the room will look under daylight. Push one to an extreme and you can hurt the others.
An ultra-low U-value with a very low g-value might look fantastic on paper, but it can make a north-facing room feel a little dull in winter if the VT is poor. A high VT without any control of g-value on a shallow south-facing roof can make a kitchen too hot in July. The sweet spot for most UK homes is a low Uw, a mid-to-low g-value on sunny orientations, and a high VT so the room stays bright without needing the lights at noon. If you’re unsure, look at how the room will actually be used through the year. A home office wants glare control and calm winter comfort; a dining space wants sparkle by day but not a greenhouse effect by late afternoon.
If you remember only a few things about the U-value in rooflights, let it be these. Compare whole-window numbers, not just centre-pane figures, and make sure they were tested the right way for roof lights. Aim for better than the backstop, because comfort and compliance both get easier when you do. And never treat U-value in isolation. The best U-value for skylights is the one paired with the right g-value, generous visible light, a warm upstand, airtight junctions and a placement that lands daylight where you’ll actually use it.
Need a quick steer for your plan? Share the room layout, the roof build-up and which way the opening faces. We’ll suggest a buildable spec glass, frame, upstand detail and shading that keeps heat where you want it, keeps summer in check, and makes the most of the light you’re paying for.
Send us your contact details and a specialist will get in touch with you.