One extra pane, and what it changes
A double glazed rooflight has two panes of glass with a sealed gap between them. A triple glazed one has three panes and two gaps. The extra pane and gap slow the flow of heat through the glass, so a triple glazed unit usually has a lower U-value than a double glazed unit of the same size and frame. In return it is heavier, it usually costs more, and it lets a little less daylight through.
For a skylight over a hall or a bathroom, double glazing is often the sensible choice. For a large glass roof over a kitchen and dining space that is heated for long hours, or a room that feels cold under the glass in winter, triple glazing earns its place more easily. Most rooms sit somewhere between, which is why a double and a triple version both get a price on the quote.
Neither option is the clever one or the wasteful one. They are two answers to the same question, and the rest of this guide sets them next to each other on the same lines so you can see where each gives ground.
Double and triple glazing side by side
The table below compares the two on the points that homeowners raise most when we visit. The words are relative because the exact figures depend on the maker, the size and the frame, and we put the real ones on your written quote.
Double glazed and triple glazed rooflights on the same rows
Double glazed
- Heat kept in
- Good
- Weight
- Lighter
- Light let through
- More
- Lead time
- Often shorter
- Price
- Usually lower
- Best for
- Most rooms
Meets the Part L limits in the usual builds.
Triple glazed
- Heat kept in
- Better
- Weight
- Heavier
- Light let through
- Slightly less
- Lead time
- Often longer
- Price
- Usually higher
- Best for
- Large glass, long-heated rooms
A strong choice where the glass area is generous.
A pair of cards like this hides detail, so the next sections take the rows in turn.
Where a quote carries only one of the two, ask for the other. Our written quote puts the glazing options next to each other with the frame, the lead time and the price, and draws on more than one manufacturer if the roof can take them.
Heat kept in, and what the regulations ask
For a rooflight going into an existing dwelling, the figure in Table 4.2 of Part L 2021 is 2.2 W/m²K. Roof windows are held to the window standard instead, which they meet at 1.4 W/m²K or at Band B in the Window Energy Rating system. Many double glazed units from the main makers meet those limits when they carry a low-emissivity coating, a gas-filled cavity and a warm-edge spacer. A triple glazed unit goes further under the limit, which gives you more margin.
It is worth being clear about what that margin buys. The regulations set a ceiling and a unit under it is allowed, so a double glazed unit that meets the limit is a compliant choice. Triple glazing is a decision about comfort and running costs beyond the minimum, not about passing. Our guide to rooflight U-values and Part L explains how to read the figures on a data sheet and why the whole-unit figure is the one to compare.
Comfort is where the extra pane is easiest to feel. The inner pane of a triple glazed unit stays closer to room temperature in cold weather, so you feel less of that cool, falling air under a big expanse of glass, and the glass is less likely to get cold enough to attract condensation on the inside. Our answer on why rooflights get condensation covers the other causes, which include how the room is used and ventilated.
Frame first
A triple glazed pane in a frame with no thermal break loses much of its advantage at the edges. Frame and glass get judged together at the survey, because a better pane in a poor frame is the wrong place to spend.
Weight, frames and the roof that carries them
Each extra pane adds weight, and on a rooflight that weight is carried by the frame, the kerb and the structure of the roof beneath. A large triple glazed flat rooflight or roof lantern is noticeably heavier than its double glazed twin, and the difference shows in three places.
- The frame: bars and sections may need to be deeper or stronger to carry bigger, heavier panes, which can make a slimline look less slim.
- The opening and kerb: the structure around the opening must take the extra load, which we check with the roof's joists in mind.
- Installation: heavier units need more people and equipment on the day, and sometimes a hoist or crane on a large lantern.
Some makers set maximum sizes for each glass build, so the very largest rooflights may only be offered with certain glazing. Where you have a size in mind, it is the first thing we check with the maker. Our page on roof lantern sizes explains how size and glass weight interact on a lantern.
Opening rooflights and roof windows add another limit: the hinge or motor has to lift the glass. A heavier pane means a stronger mechanism, and some opening designs are simply not offered in triple glazing. If you want fresh air as well as daylight, we say so on the quote when only one glazing build is available for the opening you choose.
Daylight, tint and summer heat
Every pane of glass takes a small share of the light that arrives. A triple glazed unit passes a little less daylight than a double glazed one, and the glass can look slightly greener or greyer when viewed edge-on. In a room with a large rooflight the difference is small, and in a room that depends on one small skylight it may be more noticeable.
Heat is the other side of the same trade. A unit that holds heat in well can also hold the sun's heat in, so a south or west facing rooflight over a living room may overheat in summer however many panes it has. A solar control coating tackles this directly, and it can be added to double or triple glazing. Coated and plain versions both appear on the quote for comparison, and our guide to solar control glass for rooflights covers how the coating works and the places where it can darken the view.
Noise is a smaller factor but real. Rain on glass is louder than rain on tiles, and an extra pane with a different thickness of glass can soften it. If the room below is a bedroom or a quiet study under a flat roof, tell us, and we can ask the maker about acoustic glass alongside the thermal options.
Self-cleaning coatings sit on the outer pane and work with either build, though they have their own limits on a low-pitched roof, covered in our answer about self-cleaning skylight glass.
A solar control coating can be specified on either double or triple glazing.
Cost and lead time
Triple glazing usually costs more, because it uses more glass, a deeper frame and often a heavier hardware set. It can also take longer to arrive, since not every maker holds triple glazed versions in stock and some build them to order. Each option on the quote shows the typical range its manufacturer gives, so you can weigh the wait against the benefit.
| Item | Double glazed | Triple glazed |
|---|---|---|
| Upfront price | Usually lower | Usually higher |
| Running cost of the room | Higher, if the glass is large | Lower, if the glass is large |
| Comfort under the glass | Good | Better in cold weather |
| Frame and structure | Standard | May need more |
| Lead time | Often shorter | Often longer |
Whether the extra spend pays back depends on how large the glass is and how many hours the room is heated. On a small skylight the difference is small and the saving is small too. On a large expanse of glass over a heated living space, the gap grows, and so does the case for the third pane.
We avoid putting a payback figure on it, because the answer turns on your heating and your habits. Our page on how much a roof lantern costs explains how the parts of a quote add up, and the same logic applies to any rooflight.
Which rooms suit which
These are the patterns we see at surveys. Each is a starting point and the roof, the aspect and the frame can change the answer.
| Room or roof | Double glazed | Triple glazed |
|---|---|---|
| Hall, landing, bathroom | Usually the sensible choice | Rarely needed |
| Kitchen with a flat rooflight | Suits a modest rooflight | Worth pricing for a large one |
| Open-plan kitchen and dining | Fine with a good frame | Strong choice for generous glass |
| Bedroom or loft room | Common and comfortable | Helpful if the room feels cold |
| Home office | Fine | Helpful for steady comfort all day |
| Roof lantern over a living space | Common | Often chosen for comfort |
If you are fitting a roof window in a loft conversion, remember that the Part L figure for the unit is the window limit, and that under Approved Document B a roof window counts as an escape only on a storey within 4.5m of ground. We cover the choices for pitched roofs in our guide to energy efficient rooflights.
Choosing, and what we do at the survey
This is how we help decide between the two on a Birmingham roof:
Look at the room
Its size, how long it is heated and which way the glass faces.
Check the structure
The kerb, the joists and the frame sizes the weight will need.
Price both builds
Double and triple glazed versions, with and without a solar control coating where the aspect asks for it.
Set them side by side
One page gathers the glazing, the frame, the opening method, the lead time and the price, with different manufacturers compared wherever the roof suits them.
You choose
With the Part L limit shown beside each figure.
Every quote states the declared figure for each option, fitting to current Building Regulations, notification to Building Control if the job is notifiable, our 10-year workmanship guarantee, and the warranty that the maker gives on the glass unit. To see both glazing builds priced for your roof, book a free survey.
Common questions
Is triple glazing overkill for a skylight?
On a small skylight over a hall or bathroom, often yes, because the area of glass is small and double glazing already meets the Part L limit. On a large rooflight over a heated living space, triple glazing can be a reasonable step up.
Will a triple glazed rooflight look different from a double glazed one?
Slightly. The glass can look a little greener or greyer from the side and passes a little less light. From inside a room with plenty of glazing the difference is hard to spot.
Can I have a triple glazed roof window that opens?
That depends on the maker and the design, because the extra weight needs a stronger mechanism. We check what each maker offers for the size you want and show it on the quote.
Does triple glazing stop condensation on the inside?
It helps, because the inner pane stays warmer, but condensation also depends on humidity and ventilation in the room. We look at both when we survey.
Do both builds meet the Part L limit?
Many double glazed units with a low-emissivity coating, gas fill and warm-edge spacer already satisfy the 2.2 W/m²K that applies to rooflights, and triple glazed units sit further under it. Each option on our quote shows the maker's figure beside the limit.