Kitchen with a sage island and three rooflights in the ceiling

Energy Efficient Rooflights in Derby

Warm glass, placed over the desk or stair you use.

A rooflight that keeps the heat in and the glare off. We survey the room first, then specify the glazing for the spot.

Book a free survey

Free survey
Fixed written quote
10-year workmanship guarantee

Three places where the glazing choice shows

Sitting room with a long sofa below a rooflight, garden doors behind — Over a long sofa in a sitting room, with the garden doors behind.

Wide rooflight in a white ceiling above glazed doors and a view of roofs — A wide pane in a white ceiling, above glazed doors and a view of the roofs.

Rooflight over a kitchen with light spilling down a stone wall — Over a kitchen, with the light spilling down a stone wall.

Ten years on our workmanship. In writing.

Surveyed before we price
A fixed written quote that does not change on the day
Building Control notification handled

How the guarantee works

Edge of a sealed glazing unit showing its separate panes

The full guide

Energy efficient rooflights, from the desk up

Most people who ask about energy efficient rooflights in Derby are worried about two things at once. They want the daylight, and they do not want a cold patch in winter or a hot one in July. This guide explains the numbers in plain terms, shows how glass, frame and fitting each play a part, and sets out the rules a new or replacement unit has to meet.

The seat
Glass numbers
Roof and pitch
Light and heat
Frame and seal
Ceiling and sign-off

Callout 1

Start with where you sit

What decides how much a good specification costs

Size of the openingA larger unit needs more glass, a longer kerb and more insulation around it.
Glazing build-upCoatings, extra panes and warm edge strips each add to the unit.
Roof type and coveringTiles, slates and flat roof membranes need different flashings and labour.
Ceiling height and light wellA deep shaft needs more lining and insulation to the room below.
Opening and control typeFixed units cost less than vented or electric ones.
Access to the roofScaffold and awkward lifts add time and equipment to the job.

People ask us about energy efficient rooflights as though efficiency were a property of the product alone. It is not. A rooflight over a landing that nobody lingers on has a different job from a skylight over a desk where someone works for eight hours. The same pane of glass can be a good choice in one place and a poor one in the other. So the first question we ask is about the spot, and only then about the specification.

Think of the desk first. If the daylight lands on the keyboard from a south-facing slope, the sun will arrive in strong bursts. Here the energy question is mostly about heat gain, and the coating on the glass does most of the work. Move the same rooflight over a bed on a north-facing slope and the sums reverse. There is little direct sun, so the concern is heat escaping on a still, cold night. The glazing build-up, the edge strip and the way the unit is insulated into the roof matter more.

Heat out in winter, heat in during summer

An energy efficient skylight has to manage both flows. Heat leaves through any glazed opening because glass is a poor insulator compared with a roof build-up. That is why a rooflight will always lose more heat per square metre than the roof around it. The aim is not to match the roof, which is not possible, but to narrow the gap as far as the budget and the room allow. We explain how big that gap really is on our page Do Skylights Lose a Lot of Heat?, which is worth a read if you are weighing a rooflight against a solid roof.

Heat also arrives, and on a south or west slope it can arrive quickly. A rooflight is a window facing the sky, so it catches the high summer sun that a vertical window mostly misses. A good specification for a desk or a kitchen worktop under such a slope will include solar control glass, and often a way to ventilate the room from above. We cover that properly in the chapter on light and heat gain further down.

What we look at during the survey

The free survey is where this gets practical. We stand in the room and plot the spot on the plan. We record the roof's aspect, what shades it during the day, how high the ceiling is and what is above it. We check the existing roof build-up, because the amount of insulation you can fit around a new opening depends on the depth you have to work with. Then we talk through how you use the room, since a spare bedroom used twice a year does not need the same specification as a home office.

After the survey you get a fixed written quote with no revisions on the day, with the glazing specification written into it in plain words. If you would like to start that process now, you can book the survey here. You get the survey first, and the quote follows it.

It also helps to know what you are comparing. A cheap-looking quote and a better-specified one can differ in only three or four lines, and those lines are exactly the ones that decide how the room feels in January. Ask any firm to write the glazing build-up, the stated U-value and the type of spacer into the quote, so you can compare like with like. If those details are missing, you are not comparing the same thing.

The rest of this page follows the order we use on a survey. We begin with the glass and the numbers attached to it, then the roof around it, then the light and the heat, then the frame, and finally the ceiling and the paperwork. Skip to whichever part answers your worry first.

An efficient rooflight is one specified for the spot under it, not one picked off a list of numbers.

Callout 2

U-values and glazing, in plain terms

The one figure you will see on every energy efficient rooflight is the U-value. It measures how much heat passes through one square metre of the unit for each degree of temperature difference between inside and outside. It is written in W/m²K. The lower the number, the less heat escapes. If you want the full explanation with a worked example, our page Rooflight U-Values Explained in Plain Terms goes through it step by step, so here we keep to what you need to read a quote.

Whole unit, not just the glass

The first trap is that a U-value can describe different things. A centre-pane figure describes the glass in the middle of the pane only. A whole-unit figure includes the frame, the edge of the glass and the spacer. Whole-unit figures are higher, because the edges leak more than the middle. When you compare two skylights, make sure both figures describe the same thing. We write the figure that applies to the complete unit as it will sit in your roof.

Lines to find in any glazing specification

U-value: heat lost through the whole unit; lower is better
Glazing build-up: number of panes and the coating on each
Gas fill: argon or air between the panes
Edge spacer: warm edge or older metal strip
g-value: how much solar heat the glass lets through
Window Energy Rating: the band for roof windows

What goes into the glazing

A sealed glazed unit is built up from a few parts, and each one nudges the U-value. Two panes with a gap between them is the usual starting point. Filling the gap with argon, which conducts heat a little less than air, improves it. A low-emissivity coating, which is a thin invisible layer on one pane, reflects heat back into the room. A warm edge spacer, made from a material that conducts less heat than the old aluminium strip, reduces the loss round the perimeter and also helps with condensation at the edge of the glass. A third pane adds another layer of gas and coating, and we weigh it against the extra weight and cost on Double or Triple Glazed Rooflights.

The limits a new unit has to meet

Part L of the Building Regulations sets two ceilings for existing homes. A new or replacement roof window has to reach 1.4 W/m²K, though Window Energy Rating band B also qualifies. A rooflight, roof lanterns included, has to reach 2.2 W/m²K. The two limits differ because the products differ. A roof window is a window set in the plane of a pitched roof, while a rooflight generally covers flat-roof glazing and lanterns, with a different construction.

These are limits, not targets. A unit that only just meets 2.2 will pass, and it may still be a poor choice over a bed or a desk. We use the limit as the floor and specify upward from it according to the spot. Our fitting follows current Building Regulations, and the council or approver is told whenever the job calls for it, so the paperwork matches what is in your roof.

Solar control is a separate number

Heat loss and heat gain are measured separately. The g-value tells you what share of the sun's energy gets through the glass. A low g-value keeps a south-facing room cooler. A high one lets in more free winter warmth but also more summer heat. If your roof faces south or west, ask for both figures, not one.

A U-value only helps when you know what it describes: the glass in the middle, or the whole unit in your roof.

Callout 3

Pitched or flat, the roof sets the choice

Before we choose the glass we look at the roof, because the roof decides which kind of unit is allowed to go in it. A pitched roof of tile or slate normally takes a roof window, set into the rafters and flashed to the covering. A flat roof takes a flat rooflight, a lantern or a walk-on unit sitting on an upstand. Each one has its own energy story.

Pitched roofs

On a pitched roof, the roof window lies flush with the slope. The frame is built to be insulated into the roof, and many ranges include an insulating collar that wraps the frame to cut heat loss through the junction. Because the roof window is the product people usually mean when they say skylight, the questions that follow are about orientation and pitch. A steeper slope sheds sun better in summer and loses a little less heat. A shallow slope catches more of the high sun. For a deeper look at choosing between ranges for a sloping roof, read What Are the Best Skylights for a Pitched Roof?

If your roof is a loft conversion, the roof window may also have a safety role. Fire safety guidance treats a roof window as a way out only for floors up to 4.5 metres above the ground. The clear opening then has to give 0.33 square metres, with no side under 450 mm. We check this on the survey whenever a bedroom is in the roof. Our page on roof windows covers sizes and opening types in more detail, and VELUX windows are available where you want that range.

Flat roofs and extensions

On a flat roof the story is different. A flat roof has little height to hide a deep insulated upstand, so the junction between glass and roof needs real attention. A well built upstand carries the insulation up to the underside of the frame, so there is no cold bridge from the rooflight into the room. Extensions and orangeries are common here, and so are kitchen and dining extensions with a lantern over the table. If your project is a flat-roofed extension, our guide to flat roof rooflights goes through the types, and roof lanterns explains the larger glazed forms.

A conservatory conversion raises the same issue. The old polycarbonate or glass roof loses heat quickly, and replacing it with a better specified glazed roof, or with an insulated roof and rooflights, changes how usable the room is through the year. Whichever route suits the house, the glazed part should be sized to the spot you want lit and not to the whole width of the room.

Insulation around the opening

The roof build-up around the unit matters as much as the unit. If the insulation stops short of the frame, cold air finds a path and the room feels draughty even with excellent glass. In a loft conversion we look at the depth of rafter, the layers already fitted and how the new unit will be wrapped. On a flat roof we look at the warm roof or cold roof construction and how the rooflight upstand ties into it.

Planning is another thing the roof decides. Houses can normally add rooflights as permitted development, with conditions attached, and those are laid out there. Our planning guide sets them out. A flat or maisonette has no such householder rights. A listed home will want listed building consent for most alterations. Where an Article 4 direction removes permitted development in a conservation area, you would need the council's view first, so ring Derby City Council before ordering anything.

Callout 4

Daylight in, summer heat out

Daylight is the reason for the rooflight, and heat gain is the price of too much of it. An efficient specification keeps the daylight and tames the heat. English rules set no daylight minimum for houses, so nobody will tell you how much glass to fit. That decision is yours, and it is made best from the spot.

Aim the glass at the spot

A big rooflight is not automatically a better one. Daylight falls in a cone through the light well, and the cone lands where the geometry sends it. A smaller unit placed directly above the worktop can light that surface better than a larger one set off to the side. Smaller glass also loses less heat and lets in less sun. We often suggest the smaller size when the spot is clear, and we say so at the survey.

Solar control glass

Solar control glass has a coating that turns back part of the sun's energy and passes most of the visible light. Tints do something similar but darken the view. Coatings keep the room looking natural. For a desk under a south-facing slope, or a kitchen with a west-facing lantern, this coating is the most useful upgrade on the list. The trade-off is that a lower g-value gives up some free winter warmth, which is acceptable in a room that overheats and unwelcome in one that is always cold.

Ventilation and blinds

Glass alone cannot cool a room. A vented unit lets hot air rise out of the highest point in the ceiling, and that is where heat collects. If the unit is out of reach, an electric opening rooflight can open at the touch of a button, a remote or a rain sensor. Our page on electric opening rooflights explains the controls. Internal and external blinds add a further layer, and an external blind stops the sun before it enters the glass.

Glass or polycarbonate

Older rooflights on conservatories and lean-to roofs are often polycarbonate or GRP. Multi-wall polycarbonate insulates reasonably well, because the air chambers slow heat, but it can discolour and turn brittle over time, and it offers less control over solar gain. Glass can carry a high performance coating, stays clear and gives the cleaner finish. Where a polycarbonate sheet is nearing the end of its life, replacing it with a glazed unit usually improves both heat retention and appearance. The comparison below sets the two side by side for heat loss and overheating.

Glass or polycarbonate over your head

Glazed unit

Low-emissivity and solar control coatings available
Stays clear and stable over time
Can be specified with three panes
Heavier, so the frame is stronger

Polycarbonate sheet

Air chambers slow heat loss
Light and simple to handle
Can discolour or turn brittle
Limited solar control choices

Callout 5

Frames, flashings and the damp question

A well glazed unit can still disappoint if the frame and the junction with the roof are poor. The frame is where the unit meets the building, and it is where the weather tries to get in. We treat it as part of the energy specification, not a separate detail.

The range and types of frame materials

Frames are commonly made from aluminium, timber, uPVC or a coated combination. Aluminium is strong and slim, which keeps the glass area large, but it conducts heat so a good frame includes a thermal break, a strip of insulating material between the inner and outer metal. Timber insulates well and suits traditional rooms. uPVC frames insulate and need little upkeep, and the chambers inside give them a reasonable heat performance. Which one is best depends on the house and the look you want, and on whether the roof is in a conservation area where materials matter.

The junction with the roof

A frame that is not sealed tightly allows warm, damp air to move into the roof structure, where it can condense on cold timber. The seal between frame and roof build-up, the flashing that sheds rain over the junction and the insulation wrapped round the frame all work together. A weatherproof, watertight rooflight is the starting point, and an airtight one is the next step. We fit the flashing kit made for the unit and the covering, so that tile, slate or membrane meets the frame the way the maker intended.

Condensation, explained

Condensation on a skylight is a signal about moisture in the room, not necessarily a fault in the glass. Warm air holds water, and when it meets a cold surface the water appears. The glass is often the coldest surface in a room, so it shows first. Higher efficiency actually helps here, because a warmer inner pane resists misting. Ventilation, extract fans in kitchens and bathrooms, and keeping the unit clear of obstruction all matter. Our page How Do You Stop Condensation on a Skylight? goes through the causes and the sensible fixes.

There is a different problem that people confuse with surface condensation. If moisture appears between the panes, the seal around the sealed unit has failed, and the gas has leaked out. That is a misted unit, and it will not clear on its own. If the unit is beyond repair, the answer is replacement of the glazed unit or the whole rooflight, and our hub on misted rooflights explains how that works.

Moisture on the room side of the glass is a ventilation clue. Moisture between the panes means the seal has gone.

Callout 6

The ceiling, the paperwork and the guarantee

The last part of the job is the least glamorous and the one people forget. Once the roof is sealed, the opening has to be finished inside. The light well, which is the shaft between the roof and the ceiling, needs insulating and lining. A deep shaft is a cold chimney if it is left bare, and the light that comes down it gets less from a dark, poorly finished surface. A bright lining and splayed sides spread the daylight across the ceiling and down to the spot.

Light wells and finishes

A short shaft in a modern extension may be a simple plasterboard return. A tall shaft through a Victorian loft takes more work. We insulate the sides and finish them to match the ceiling, with the angle chosen to push the light toward the worktop, desk or stair rather than straight up the wall. For smaller or awkward spaces, a tube of reflective material can carry daylight a long way, and sun tunnels are worth considering where a full window will not fit.

What rules apply

Cutting a new opening that changes the roof structure counts as building work, so it goes to Building Control, either the local authority (Derby City Council for the city) or a registered building control approver. Swapping one unit for another of the same kind is a controlled fitting, so the new one must hit the U-value set for its type; a competent person scheme can certify the work. Where notification is due, we deal with it, and the fitting follows current Building Regulations. The table below lists the limit for each kind of job.

Guarantee and fixed quote

Every job comes with a 10-year workmanship guarantee in writing. The quote is fixed and in writing, with no revisions on the day, so the specification you agreed is what goes in the roof. On the roof, a simple single rooflight is usually a job of one or two days. A larger lantern or structural opening takes longer, and the quote gives the programme.

Homes first, small commercial too

Most of our work is residential, from terraces and semis to extensions and loft conversions, and we also take on small commercial jobs such as shops, offices and workshops where a roof needs daylight and a warm, tidy finish. Measure the room, mark the spot, and send us the details through the survey form. We will come, see the roof and the ceiling, and write the specification down.

To talk it through first, WhatsApp is 07880 284133 and the email address is info@skylarkskylightsderby.co.uk. Either way we will fix a time to see the room. We cover Derby and about 30 road miles around it, measured by road.

Limits that apply to each kind of job

What a new or like-for-like unit has to meet under Part L.
Job	Limit	Paperwork
New roof window	Window limit	Building Control if structural
New rooflight	Rooflight limit	Building Control if structural
Roof window swap	Window limit	Controlled fitting, notified
Rooflight swap	Rooflight limit	Controlled fitting, notified
Roof lantern	Rooflight limit	Treated as a rooflight

Efficiency questions we hear in Derby

Which is better for heat, a roof lantern or a VELUX window?

They suit different roofs, so the comparison is about the spot. A roof lantern sits on a flat roof and lights a large area, such as a kitchen table. A roof window sits in a pitched slope and lights a smaller area, such as a landing. Either can be specified to a good U-value. Read about roof lanterns or VELUX windows and we will recommend one at the survey.

Are there cheaper alternatives to VELUX windows?

Yes, other makers produce roof windows in similar sizes, and we can fit those as well as VELUX. What matters is the whole-unit U-value, the flashing kit that matches your covering and the guarantee on the fitting. A cheaper unit with the wrong flashing costs more in the end. Ask for the specification in writing so you compare the same lines on each quote.

Do I need planning permission to install a roof lantern?

Houses can often add a lantern as permitted development, flats cannot rely on householder rights, and listed buildings need consent for most changes. We link the conditions rather than repeating them, so read our planning guide and check with Derby City Council if in doubt.

Is triple glazing worth having in a skylight?

It can be, depending on the spot. Over a bed in a north-facing roof, an extra pane and coating cuts night-time heat loss. Over a sunny desk, a solar control coating on double glazing may do more good. The weight and the cost both rise, so we compare the options on the survey. The page on double or triple glazed rooflights goes into more detail.

Will a replacement rooflight need Building Control?

A straight swap counts as controlled fitting in Part L terms, so the new unit must satisfy the cap for its type. A new opening that changes the roof structure needs approval from the council or a registered building control approver. In both cases we look after the notification where one is due.

Can a new rooflight cure a cold, damp conservatory roof?

Often it helps. An old polycarbonate or glass roof loses heat quickly and can discolour or turn brittle. Replacing it with a better specified glazed roof, or with an insulated roof carrying rooflights, keeps the room warmer in winter. We look at the frame, the base and the ventilation on the survey. Book through the survey form and we will see the roof first.

Book your free survey

Where do you want the light? Tell us in the form: the worktop, the stair, the desk.

No obligation. We survey before we price.
We cover Derby and 30 miles around.