Double or Triple Glazed Rooflights: Which Is Worth It?

A sealed glazing unit is a sandwich: panes of glass separated by a spacer around the edge, with a gap in between that is filled with gas. The gap is where the insulation happens. Still gas conducts heat poorly, so each gap slows the passage of heat from the warm side to the cold side. Double glazing has one gap. Triple glazing has two.

The second gap does not simply double the insulation. Each additional layer gives a smaller return than the one before, and the edge of the unit, where the spacer and frame sit, loses heat whatever happens in the middle. In practice the centre of a triple unit loses clearly less than the centre of a double, while the whole product, frame included, improves by a smaller margin than the glass figure suggests. For the difference between those two measurements, see rooflight U-values and Part L.

Think of a hat and a scarf. The first layer on a cold day makes the big difference. The second helps, but you notice it less. That curve is why triple glazing makes sense in some places and is wasted in others.

Pane count is the visible bit of the specification. The invisible bits matter just as much, and a well-specified double can beat a poor triple.

Low-emissivity coating. A thin, clear metallic layer on one face of a pane reflects heat back towards where it came from. It is on almost every modern sealed unit and you cannot see it, except as a faint tint when you look at the glass at an angle. Without it, a double unit loses noticeably more.

Gas fill. The gap is commonly filled with argon in place of air, because argon conducts a little less. Some makers offer krypton in narrow gaps. The fill slowly leaks over the years, which is one reason sealed units fail in the end, and one reason the quality of the edge seal counts.

Warm-edge spacer. The strip between the panes is the weak point. A plastic or composite spacer loses far less heat than the older aluminium type, and it also helps keep the inside edge of the glass warmer, which is where condensation first appears. If your concern is misting on the inside, the guide to condensation on a rooflight goes further.

When two suppliers quote you, a double with all three of those against a triple with only a basic coating is a fair fight. Always ask what is in the unit, not just how many panes. The datasheet should list the coating, the gas and the spacer by name. If a quote says only "high performance glass", that is a prompt to ask again before you sign.

A third pane is more glass, and glass is heavy. A triple unit is thicker as well as heavier, which affects the product in several ways.

A flat-roof rooflight rests on a kerb, and the kerb takes its whole weight. A large triple unit, especially a lantern with a lot of glass and an aluminium frame, puts more load on the upstand and the roof structure underneath. For most extensions this is not a problem, since the structure is designed for glazing, but it has to be checked at survey and not assumed. On an older roof, or where a rooflight is being added to joists that were never designed for it, the support may need strengthening.

A heavier unit also changes how it is handled. It takes more people to lift to roof level and more care to set on the kerb without knocking the seal. For an opening rooflight, the actuators and hinges carry that extra weight on every lift of the lid, so they are chosen for it, and a very large triple lid on a pair of small stays is not a good match.

For pitched roof windows, triple glazing is a glazing option chosen when the window is ordered, not something added to a window already in the roof. The triple version of a model can differ in weight and depth from the double one, so check both for the window you choose. A good survey picks that up in advance. If you are replacing an old window of one make with the new window of another, the point is covered in roof window installation.

Triple glazing is a decision about the roof as well as the glass, because every extra pane lands on the kerb.

Each pane of glass, and each coating, takes a little light on its way through. A triple unit therefore passes slightly less daylight than a double, and the glass often has a very faint green or grey cast when seen edge on. In a kitchen extension with a large flat rooflight the effect is invisible. On a narrow landing lit only by one small roof window in a deep terrace, where you are trying to get every bit of daylight down a stairwell, it is more worth thinking about.

The same coatings that hold heat in also affect the sun's heat getting in. A triple unit generally lets in less solar heat than a double. In winter that is a slight cost, since the sun is free warmth. In summer it is a benefit on a south-facing roof. Solar gain is a different matter from heat loss, and the guide to rooflight overheating in summer covers it.

Noise deserves a mention because people worry about it. Rain on glass in a roof is louder than rain on a wall window, since the glass faces the sky. More panes do not guarantee a quieter room. What cuts the drumming is a thicker outer pane or a laminated one, which damps vibration. A laminated pane is two layers of glass bonded round a plastic interlayer, and it is available in double and triple units alike. If you are fitting a window over a bedroom, ask for the laminated outer pane whatever the pane count. It is a better noise decision than the third pane.

Triple is worth considering when the heat lost through the glass is a real share of what a room loses, or when the room is used in a way that makes cold glass noticeable.

• Large flat rooflights and lanterns. A big area of glass multiplies any difference in performance. Over an open-plan kitchen and dining extension, with the glazed doors already doing their part, the roof glass is a large proportion of the room's total glazing.
• Rooms you sit in. Under a rooflight, you sit directly below the cold surface in winter. Glass at a higher inside temperature feels more comfortable, and a triple unit has a warmer inner surface than a double one, so it is the better choice over a dining table or a sofa.
• Exposed, north-facing roofs. A roof that gets little winter sun gains little from the glass, and exposure to wind off the Irish Sea or the Dee estuary takes heat away faster.
• Bedrooms in the roof. A roof window above a bed puts cold glass over a sleeper for hours, and a better glazing unit takes the edge off it.

A roof lantern deserves its own line. Its glass faces several directions at once and rises above the roof, so more of it is exposed to wind and open sky than a flat unit of the same footprint. That is a point in favour of the better specification, and the choice of frame material matters alongside it. The options are set out under roof lantern installation.

Triple is hard to justify on a very small roof window over a landing or a bathroom that is warm, used briefly and where a cheap alternative is available. The saving in heat is small in absolute terms, because the area of glass is small, and the weight and cost are real. In most of those cases the sensible pick is double glazing built with a low-emissivity coat, argon and a warm-edge spacer.

The regulations are a floor, and not the guide to this choice. Both a good double and a triple can meet the Part L limits; the question here is whether you want to go beyond them.

It also helps to be consistent within one house. If an extension gets a large triple lantern and the old pitched windows next door stay single glazed, the new roof is working against the old one. Where several units are going in at once, a survey can put them in order of how much each one matters, so the money goes where the glass is biggest and the room is used most. A small bathroom window rarely needs the same answer as the kitchen lantern.

Start with the room and the size of the glass, then the structure, then the budget. A survey is where those meet. Someone has to stand on the roof, look at the kerb or the rafters, measure the opening and confirm what the structure can carry. We do that before any price, and the fixed written quote then names the exact unit and its glazing specification.

• Number of panes, coating, gas fill and spacer type
• Whole-unit U-value for the exact product
• Weight, and what the kerb or frame is designed to carry
• Laminated outer pane for a bedroom or any room where rain noise would bother you
• Which glazing can be fitted to the roof window or kerb already on site

All of this sits within the wider range on the energy efficient rooflights page. We send the Building Control notification where the job needs one, and ten years of written guarantee cover the workmanship. Get a free quote or message us on WhatsApp.