- U-VALUE:
- whole unit
- GLASS:
- double or triple
- RULES:
- Part L Wales
Energy-efficient rooflights in Swansea
Less heat out through the glass, less damp on it.
Free survey / fixed written quote / Building Control notification handled / 10-year workmanship guarantee
Run-off
Efficient glazing changes the heat, so the fall and kerb still do the draining.
Condensation
A warmer inner pane gives steam less to settle on; ventilation does the rest.
Rain noise
Extra panes add weight and cost; the room below shapes what you hear.
- MEASURE:
- W/m²K
- LOWER:
- slower heat loss
- UNIT:
- glass and frame
- AREA:
- developed surface
Where the heat goes, and the figure that measures it
A U-value says how quickly heat passes through a unit, and for a rooflight the figure that counts covers more than the glass.
Glass is thinner than the insulated roof around it, so it lets more heat out. That is the cost of daylight from above, and an energy-efficient rooflight is one built to keep the cost low. Warm air gathers under the ceiling, which puts a rooflight at the spot where a room is working hardest to hold its heat.
The measure is the U-value, written in watts per square metre per kelvin (W/m²K). It tells you how quickly heat passes through a built-up unit when the two sides sit at different temperatures, and a smaller figure means slower loss. Window makers also publish a Window Energy Rating as a lettered band, and Approved Document L (Wales) accepts Band B for a roof window in place of a U-value.
Centre-pane figures and whole-unit figures
A single rooflight can carry two different numbers. One describes the middle of the glass, where no frame or edge seal sits close by. The other describes the complete unit, frame and edge included, and it is the one that tells you about the rooflight you will have over your head. When two quotes give two figures, check which of the two each one is before comparing them.
Approved Document L takes the whole-unit view. A rooflight's U-value is worked out over the unit's full developed surface, which can be bigger than the hole it covers in the roof. Where the kerb arrives as part of the same assembly as the rooflight, the limit covers the kerb too. Our guide to rooflight U-values explained takes the two figures apart in more detail.
Two products, two limits
Wales gives a roof window and a rooflight different figures, because Approved Document L treats them as different things. A roof window sits in the same plane as the roof around it. A rooflight stands out of plane on a kerb or upstand, and the document says it is also sometimes called a skylight. For a new or replacement unit in an existing home, a roof window should reach 1.4 W/m²K or Window Energy Rating Band B, and a rooflight should reach 2.2 W/m²K. The document is guidance, which is why it speaks in "should".
It does not mention roof lanterns by name. For a lantern over a new extension, we confirm with the council which assessment applies. The page on what U-value a rooflight should have in Wales sets the two limits side by side.
Natural light, brightness and heat
More glass area lets in more natural light and sends out more heat, and the two move together. A deep room with a small window gets the most from an opening overhead, because light from above reaches the middle of the floor. A room that is already bright from its walls gains less. Weighing that for each room is the first job of a survey, and the glazing is chosen after it.
Brightness on a grey day is the other half of the sum. An efficient unit that keeps the heat in and still lets a low, cloudy sky into the room earns its place twice, and that is the combination to look for. Over a landing, a bathroom or a loft bedroom, a few square feet of well-glazed roof can change which lights are on at three in the afternoon.
The Welsh figures, existing homes
- 1.4W/m²K for a roof window, new or replacement
- BWindow Energy Rating band that also satisfies a roof window
- 2.2W/m²K for a rooflight on a kerb
Words on a spec sheet
- U-value
- How fast heat passes through a unit. Lower means slower.
- Centre-pane
- The figure for the middle of the glass alone, away from the frame and edge.
- Whole unit
- The figure for glass, frame and edge together, as fitted.
- Kerb
- The raised upstand a rooflight sits on.
- PANES:
- two or three
- FRAME:
- aluminium or uPVC
- OUTER:
- self-cleaning option
- INNER:
- laminated option
Panes, frames and coatings, one choice at a time
Efficiency comes from the whole build-up, so the glass, the frame and the coatings are settled together for the room.
| Option | What it is | Ask about |
|---|---|---|
| Double glazing | two panes, sealed gap | weight, price |
| Triple glazing | three panes, two gaps | weight, price, the room |
| Laminated inner pane | bonded layers | breakage, security |
| Self-cleaning outer pane | coated glass | reach, tree cover |
| Solar-control glass | sun-managing glazing | aspect, room use |
Glass and materials
Double glazing is two panes with a sealed gap between them, and triple glazing is three. VELUX offers its flat and curved glass rooflights in either form and says that glazing is meant to enhance both energy efficiency and sound reduction, which names the two things worth weighing. A third pane adds weight for the frame and the roof to carry, and it adds to the price. A room you sit in on winter evenings makes a better case for it than a cupboard-sized landing.
Frames in aluminium or uPVC
Aluminium frames allow slim bars and a crisp edge to the glass, and their colour can be chosen to sit with the windows below. uPVC frames are the alternative. Either can be made to measure for an opening, and a bespoke size starts from the dimensions of the kerb or the roof opening it has to fill, which are checked at the survey. Seen from inside, the frame is a pattern across the ceiling, so its colour deserves a look from below as well as from the garden.
Orangeries, extensions and loft rooms
The rooms that ask most of a rooflight are the ones with the most glass overhead. An orangery or conservatory-style extension may carry a lantern or a run of flat rooflights, and the glazing decides how comfortable the room is in January and in July. A loft room under a roof window has less glass but a smaller volume of air to warm, and the same logic applies on a smaller scale. In each case the efficient choice is the one matched to what the room does, rather than the highest specification on the sheet.
Coatings and laminated panes
Pilkington lists roof glazing among the places its Activ self-cleaning coating can go. The coating works with daylight and rainfall, one loosening the dirt and the other washing it off, which helps on glass that is out of reach from the ground. VELUX lists a dirt-repellent easy-clean coating, rain noise reduction and safety lamination among its own options, with the choice on any one window set by the model and glazing variant. A laminated inner pane is one that keeps the glass from dropping into the room if the unit is broken by accident.
Every extra layer alters how heavy the unit is, how it looks from underneath and what it costs. So the layers are specified as a set for the room, and a unit above a kitchen table gets different answers from one above a hallway.
Sun, security and the room below
A glass roof lets the sun in as well as the light. An extension that takes full sun through a summer afternoon can run warm, and solar-control glass is the option to raise for that. Whether it earns its place depends on the size of the rooflight and the way the room faces, which the survey puts in front of us together.
Security is part of the choice on any roof that someone could climb onto. Ask how the glass is retained in the frame and whether the inner pane is laminated. Residential rooflights come in many sizes and finishes, so those questions are best answered for the actual opening.
- INNER PANE:
- kept warmer
- MOISTURE:
- needs a way out
- MIST:
- inside or between
- SOUND:
- glass and room
Steam, noise and the air under the glass
A warmer inner pane gives moisture less reason to settle, and ventilation does the rest of the work.
Run a hot shower on a frosty morning and the roof window above it shows how the room is coping. Condensation is what happens when warm, damp air touches something cooler than itself, and a pane in a roof can be the coolest thing in the room.
Better glazing helps, because it keeps the inner pane warmer and gives the moisture less reason to settle there. It cannot work alone. The air has to change as well, and the steam from showers, cooking and drying clothes needs somewhere to go.
Ventilation when a unit is swapped
Approved Document F (Wales) asks that a home breathe no worse after a window is replaced than it did before. Where trickle vents were fitted before, the replacement's vents should be no smaller, and the occupant or an automatic control should be able to work them. Each habitable room should also be able to be aired rapidly, and an opening roof window can do that job. A fixed rooflight cannot, so a bathroom or kitchen under one relies on its extractor and the other windows.
Water on the glass, water inside it
Moisture on the room side of the glass comes from the air in the room, and better glazing and better ventilation reduce it. Moisture trapped between the panes points to a sealed unit whose seal has gone, and no amount of wiping will reach it. A failed unit can be replaced on its own, and our page on misted rooflights covers that job. Whole-unit efficiency and mist are separate questions with separate cures.
Rain noise
A downpour is crisper on glass than on tiles. VELUX lists rain noise reduction as a glazing option, again depending on the window and variant, and the extra pane in a triple unit is part of the same conversation. What you hear also depends on the floor and walls below. A bedroom with carpet and curtains softens a downpour that a tiled kitchen would carry.
Letting the room breathe by itself
VELUX builds a sensor into its solar and electric roof windows that shuts the window as rain starts. That suits a steamy room on a showery day, because the window can vent the moisture and shut itself when the weather turns. Our electric opening rooflights page goes through the options.
- PITCHED:
- roof window
- FLAT:
- rooflight or lantern
- OLDER HOMES:
- conservation glass
- DARK MIDDLES:
- sun tunnel
Types of rooflight, and efficient glazing in each
The same glazing question lands differently on a pitched slope, a flat roof and a house with a history.
Roof windows are the pitched-roof product, and they carry the 1.4 W/m²K figure. For pitches of 10° to 20°, VELUX points to its low-pitch roof window, so the pitch of the roof helps choose the model before the glazing is considered. Our roof windows page covers the range, and the choice of glass follows the room: a loft bedroom, a landing, a bathroom.
Roof window
- In the plane of the roof
- 1.4 W/m²K or Band B
- Opens to vent the room if chosen
Rooflight on a kerb
- Stands out of plane on an upstand
- 2.2 W/m²K
- Fixed or opening, flat or curved glass
Flat roofs: rooflights and lanterns
A flat roof takes a rooflight on a kerb, and the kerb belongs to the efficiency question, because a cold kerb is another surface for condensation. Manufacturers want the water to run off. Rainclear's installation guide asks for the kerb to carry a minimum 5° slope to minimise water pooling, and VELUX gives 0° to 15° as the pitch range for its flat roof dome. The weathering where kerb meets roof covering has to be right for the unit to stay weatherproof. See flat roof rooflights for the full picture.
A roof lantern adds height and a pitched glass shape. Over a kitchen extension it makes a bright room, and in an orangery or conservatory-style space it gives the roof a centre of glass inside a solid edge. Lanterns are made to measure for their upstand, and the glazing is decided with the room below in mind. Our roof lanterns page goes into upstands, sizes and planning.
Opening units and the weather
An opening rooflight lets the room vent, which helps the damp side of efficiency. An electric one can be set to open and close without anyone reaching up to it, and the rain sensor option closes a VELUX electric roof window when rain arrives. Both the opening gear and the seals around the sash are part of how well the unit holds heat when it is shut, so they are looked at with the glass.
Older houses and conservation work
Where a roof window replaces an old one in a home of architectural or historic interest and cannot reach 1.4 W/m²K, Approved Document L (Wales) allows a fallback of 1.2 W/m²K at the centre of the pane, or low-emissivity secondary glazing. There is no such fallback for rooflights. A conservation rooflight is the product for older roofs, and the planning side is on the same page.
Projects: walk-on glass and sun tunnels
A walk-on rooflight is built to be stood on, and its glazing is its own specification, which we go through at survey. A sun tunnel carries daylight down a tube to a windowless room, and it suits a landing or a hall where a full rooflight cannot reach.
Cold kerbs make damp as surely as cold glass.
- ENERGY:
- Part L (Wales)
- NEW OPENING:
- approval generally needed
- PLANNING:
- 150 mm rule
- LISTED:
- consent as well
What Welsh rules ask of a new or swapped rooflight
Energy limits, building control and planning each have their say, and Wales has its own versions of all three.
Putting in a rooflight is building work in law, because windows, roof windows and rooflights are controlled fittings. For a brand new rooflight the Welsh Government expects Building Regulations approval to be needed in general, and gives four reasons: cutting the opening normally means changing the structure of the roof, the roof must carry the weight of the unit or be strengthened, the unit has to be energy efficient, and its fire performance comes into it near a boundary.
| Where the house is | New rooflight in the roof slope | Decided by |
|---|---|---|
| House, no designation | permitted development within the limits | the Class C limits |
| Conservation area | planning application | council planning team |
| Gower AONB | planning application | council planning team |
| Bannau Brycheiniog | planning application | National Park Authority |
| Listed building | listed building consent as well | local planning authority |
| Flat or maisonette | no permitted development rights | council planning team |
A new opening
- Roof structure altered
- Approval generally needed
- Planning rules apply to the slope
A swap on the same opening
- Opening already there
- Still building work
- Can be self-certified by a registered installer
Swapping an old unit
A like-for-like replacement of a window or roof window is still building work. A repair is not, and for the energy rules nor is a new pane fitted into a frame that is kept. Welsh Government guidance lets an installer on one of its authorised competent person schemes self-certify a replacement window, roof window or rooflight in an existing home. Where the work is notifiable, we deal with the notification through local authority building control or a registered building control approver, and you can choose which of the two oversees it.
Bigger openings and the 2027 edition
If an opening for a window is made bigger, or a fresh one is cut, the combined area of windows and doors should stay inside the share of floor area that the document sets. A new Welsh edition of Approved Document L takes effect on 4 March 2027. That edition counts roof windows and rooflights within the total and allows more than that share where the energy efficiency of the home is improved to compensate. The limits for roof windows and rooflights are the same figures in the 2027 edition.
Draughts, ventilation and glass safety
Approved Document L (Wales) also asks that installed windows, roof windows and rooflights be well fitted and reasonably draught-proof, which is a matter of workmanship at the frame as much as of the glass. Approved Document N is the Welsh document for glazing safety, kept apart from the energy rules, and the two sit side by side on any job. Approved Document F carries the ventilation points described above.
Planning
On a house, a rooflight can stay inside permitted development when it projects no more than 150 mm from the plane of the original roof slope and rises no higher than the highest part of the original roof. A rooflight on a side-facing slope within 10.5 m of the boundary opposite needs obscure glass, and it must be fixed shut unless the opening part sits at least 1.7 m above the floor or stair below.
Adding a rooflight to a roof slope is outside permitted development in a conservation area, a National Park such as Bannau Brycheiniog, an Area of Outstanding Natural Beauty such as the Gower AONB, or a World Heritage Site, and a planning application is needed there. Householder permitted development does not reach flats or maisonettes. Rights on a house can be withdrawn by an Article 4 direction from the council, or by a condition written into the house's original planning permission. On a listed house, an alteration that affects its character needs listed building consent, applied for through the planning authority; Cadw sets out the process.
Our skylight planning guide for Wales and the Building Regulations guide go through each case, and Part L in Wales covers the energy rules for rooflights on their own.
- PRICE:
- after survey
- QUOTE:
- fixed, in writing
- GUARANTEE:
- 10 years, workmanship
- RADIUS:
- 30 road miles
Survey, fitting and keeping the glass working
Nothing is priced before the roof has been seen, and later upkeep comes down to water and air.
A free survey comes before any figure. We look at the roof, the room and the opening, measure the dimensions that matter and talk through the glazing, and then give a fixed written quote. Better glazing adds to a unit's price, and the fitting job around it is the same work; does an energy-efficient rooflight cost more? goes through what moves the figure, and the costs page covers each of the other products.
A new opening, roof up
- Opening Formed in the roof, with the structure trimmed around it.
- Kerb or frame Set to the measured dimensions and insulated.
- Weathering Roof covering dressed up to the unit.
- Glass and lining Glazing fitted, then the inside finished.
The installation process
On a new opening the work goes from the roof structure upward. The opening is formed and the structure trimmed around it, the kerb or the frame is set, the roof covering is dressed to the unit so the junction stays watertight, and the glass goes in. Inside, the lining between ceiling and frame is finished. A replacement uses the existing opening where it is sound and the new unit is made to its dimensions.
What the survey looks at
The survey is where efficiency is settled. We look at the roof and what it is made of, at the room and what happens in it, and at the opening or the old unit. A measured opening gives the size of a bespoke or made-to-measure unit. The aspect of the room tells us whether solar control is worth raising. A bathroom or kitchen leads to the ventilation conversation. Where an old rooflight is being swapped, the existing kerb and frame are checked for soundness before anything new goes on.
Aftercare and maintenance
An efficient rooflight asks little. Wash the glass when it looks dull, and expect to do it sooner under overhanging trees. Keep the flat roof outlets and gutters clear so water leaves the kerb. Use the ventilation the room was given: the extractor, the trickle vents, the roof window set ajar in a shower room. If a sealed unit mists between its panes, the sealed unit can usually be replaced on its own.
Ten years of workmanship cover come in writing, and the guarantee page explains what that covers.
Areas covered
Installation and replacement work runs from Swansea out to about 30 road miles, taking in parts of Neath Port Talbot, Carmarthenshire and Bridgend. Towns on the list include Neath, Llanelli, Gorseinon, Bridgend, Ammanford and Mumbles, with the rest on the areas page. Ring 01792 929062 or book a survey online.
Cost readout
What moves the price of an efficient unit
- GlazingTriple units, coatings and laminated panes each add to it.
- SizeA larger opening takes more glass, frame and roof work.
- OpeningA new hole needs trimming; a replacement may keep the old one.
- AccessDifficult roof access adds time.
Guides and answers for this service
Asked about energy-efficient rooflights
What is the difference between the glass figure and the whole-unit figure?
The glass figure describes the middle of the pane. The whole-unit figure covers glass, frame and edge together, which is the unit as it will be fitted. Approved Document L (Wales) judges a rooflight by its whole developed surface, so that is the number to compare between quotes. The U-value guide goes through both.
Which Welsh figure applies to my roof window?
For a new or replacement roof window in an existing home, Approved Document L (Wales) says 1.4 W/m²K, or Window Energy Rating Band B. A rooflight on a kerb is judged against 2.2 W/m²K instead.
Is triple glazing worth having in a rooflight?
It depends on the room. A third pane adds weight and cost, and VELUX offers its flat glass rooflights double or triple glazed, and says the aim is better energy efficiency and sound reduction. A bedroom or a living room makes a stronger case than a hallway. We talk it through at the survey.
Will better glazing stop my rooflight misting up?
Mist inside the room is moisture from the air, and a warmer inner pane plus better ventilation reduce it. Mist between the panes comes from a sealed unit whose seal has gone, and that needs a new unit. See misted rooflights for that job.
Can an old rooflight be swapped for an efficient one on the same opening?
Usually, where the opening and kerb are sound. A like-for-like swap is still building work, and in Wales an installer registered with an authorised competent person scheme can self-certify it. If the job is notifiable, the notification is ours to handle. The survey checks the kerb first.
Does an energy-efficient rooflight cost more than a standard one?
The better glazing adds to the price of the unit. The fitting is the same job. Nobody gets a figure from a photo, because the roof and the opening change the answer, so a free survey comes first and a fixed written quote follows.
Bulletin Energy-efficient rooflights
Book your free survey
- Survey:
- before we price
- Quote:
- fixed, in writing
- Guarantee:
- 10 years on our workmanship, in writing
No obligation. Building Control notification handled where the work is notifiable.