Energy Efficient Skylight Installation

Every glazing quote in the country will tell you a U-value. Very few will tell you a g-value, and on a roof facing the Thames Estuary the g-value is the number that decides whether you actually use the room between June and September. Energy efficient skylight installation in Southend is not a matter of buying the thickest glass available and hoping. It is a sequence: work out where the sun comes from and at what angle it strikes your particular roof, decide how much of that energy you want to admit in December and reject in July, then choose the coating, the cavity, the frame and the ventilation that deliver it. This page sets out how we do that, with the physics written down rather than glossed over, so you can hold any quote you receive up against it.

01What we mean by an energy efficient skylight installation

The phrase gets used loosely. Most of the trade takes it to mean a low U-value and leaves it there, which covers roughly half the problem and the less pressing half at that. A rooflight has to do four things at once, and only the first of them is about keeping winter heat indoors.

• Hold heat in when it is cold. That is the U-value, and modern units are good at it.
• Keep solar energy out when it is hot. That is the g-value, also called the solar factor or total solar energy transmittance, and it is where most specifications fall apart.
• Deliver useful daylight without glare. Light transmission and diffusion, which pull in opposite directions to the point above.
• Let hot air leave the room. Ventilation, which is a design decision about where the openings sit, not an accessory ticked on an order form.

Those four pull against each other. Cut solar gain hard and you dim the room. Chase maximum light and you build a heat trap. Add a big opening vent for purge ventilation and you have introduced a moving joint and a motor into a roof. The work of specifying is finding the point where all four are acceptable for your room, your orientation and the way you actually live in it, and that point is different for a north-facing loft bedroom in Hadleigh and a south-facing kitchen extension a few hundred metres from the seafront.

So the survey starts outside, with a compass and a look at what stands between your roof and the southern sky. Only then do we talk about products. An installer who opens with a brochure has skipped the part that matters. What we install is not a hole with glass in it; it is a controlled amount of solar energy and daylight arriving in a specific room at specific times of day.

Two practical boundaries before going further. We install and replace, so if you have a sound unit sitting in a roof that is letting water past, a roofer is the trade you want. If the unit itself has misted, lost its gas fill, or is a single-glazed relic from the 1980s bleeding heat into the sky, taking it out and installing a properly specified replacement is squarely our work, and it is one of the highest-return changes you can make to an older extension.

02The estuary sun: orientation, tilt and angle of incidence

Southend-on-Sea sits at roughly 51.5 degrees north and looks south across open water. Both halves of that sentence change the sums.

Take the latitude first. At midsummer the sun reaches about 62 degrees above the horizon at solar noon here. At midwinter it manages about 15 degrees. Those two figures are the whole argument, because the amount of energy a surface collects depends on the angle at which the beam meets it. Hit a surface square on and you collect nearly all of it. Hit it at a glancing angle and you collect a fraction.

Now run the numbers for two pieces of glass, one vertical and one lying flat in a roof.

• Midsummer noon, sun at 62 degrees. A flat rooflight receives the beam at 28 degrees off its normal, collecting about 88 per cent of it. A south-facing vertical window receives it at 62 degrees off normal, collecting about 47 per cent, and in practice less once the reflectance of glass at steep angles is counted.
• Midwinter noon, sun at 15 degrees. The flat rooflight is now 75 degrees off normal and collects around 26 per cent. The vertical window is nearly square to the beam and collects about 97 per cent.

Read that twice. Horizontal glass roughly triples its solar collection between December and June, while vertical glass halves its own. Overhead glazing is, by geometry, tuned to the season you least want it tuned to. Tilt it a little, as a pitched rooflight or a lantern is tilted, and you nudge it further towards summer collection rather than away from it, because you are aiming it at the high sun.

Then the second half. A south-facing seaside city with the estuary in front of it has two things a typical inland suburb does not: an unobstructed southern sky and a large body of water reflecting light back up. There is no ridge line of houses on the sun side of a seafront property, and mature tree cover thins out as you get closer to the water. The result is that the shading factor most designers quietly assume, some obstruction taking the edge off the middle of the day, simply is not there on a large number of properties across this stretch of coast.

What we look at on the survey

Orientation is measured, not guessed, because a roof that a client describes as south-facing is frequently south-west, and south-west is the harder case. Late-afternoon sun is lower in the sky, so it comes in under the eaves and across the room rather than straight down, it arrives at the hour a kitchen is busiest, and it lands on a building that has already been warming since morning. West and south-west orientations produce more complaints about overheating than due south does.

We also look at pitch, because the angle changes the answer; at whatever is on the sun side, including neighbouring gables, chimney stacks and anything deciduous that will be bare in March; at the floor finish, since a tiled or polished concrete floor in direct sun becomes a radiator by evening; and at the total glazed area of the room, because a rooflight sitting above three metres of sliding doors is part of a much larger heat budget. The same rooflight, in the same glass, is a good idea on one of those roofs and a mistake on another.

03Three numbers that decide how the room behaves

Any sealed unit can be described by three published figures. A quote that gives you one of them has told you a third of the story.

U-value, in W/m²K

This measures how fast heat escapes through the unit for each degree of difference between inside and outside, per square metre. Lower is better. Single glazing sits around 5.6. Old sealed units run about 2.8. A current argon-filled unit with a soft low-emissivity coating and a warm-edge spacer gets to roughly 1.1 to 1.4, and triple glazing reaches 0.8 or below.

One detail about rooflights that almost nobody mentions at the point of sale: the U-value of a sealed unit is worse lying down than standing up. The gas in the cavity convects more freely when the unit is tilted towards horizontal, so the same build-up that tests at 1.1 in the vertical plane might return something nearer 1.3 or 1.4 in the horizontal plane. Both figures are honest. They are just not the same figure. Ask which plane a quoted number refers to, and ask whether it is the centre-pane value or the whole-unit value including the frame, since centre-pane figures always flatter.

U-value also decides condensation. Take a January night, 21 degrees inside at 60 per cent relative humidity, which puts the dew point at about 12.9 degrees, and zero outside. With a unit at 1.2 W/m²K the inner glass surface sits around 18.5 degrees, comfortably dry. With old single glazing at 5.6 it drops to roughly 9 degrees, well under the dew point, and you get the running water down the reveal that people wrongly assume is a defect in the frame. It is not; it is physics, and the answer is the glass.

G-value, from 0 to 1

The g-value is the share of the solar energy striking the glass that ends up as heat in the room, counting both what passes straight through and what the glass absorbs and then re-radiates inwards. Clear double glazing is around 0.6. A neutral solar-control unit lands near 0.30 to 0.35. A high-performance one gets to about 0.25.

Halving the g-value halves the heat. It really is that direct. On a clear July afternoon a near-horizontal surface in south Essex can be receiving in the region of 700 to 800 watts of solar energy per square metre. Put four square metres of clear double glazing in that roof and you are admitting something close to 1,700 watts of heat with no control over it. Change nothing but the coating, at g 0.30, and the same aperture admits about 850 watts. That difference is the entire argument for solar-control glass, and it costs a few hundred pounds on a domestic job.

Light transmission, and the ratio that matters

Light transmission, written LT or Tv, is the percentage of visible light the unit passes. Clear double glazing is around 78 to 80 per cent. Solar-control units run 55 to 70 per cent depending on the coating generation.

The figure worth calculating yourself is selectivity, which is simply light transmission divided by g-value. It tells you how much daylight you are buying per unit of heat. Clear double glazing scores about 1.3. A decent modern solar-control coating scores 1.9 to 2.1. That is the honest measure of a good coating: not that it blocks the most heat, but that it blocks heat while keeping light. A cheap body-tinted or bronze glass will bring the g-value down and take the light with it, scoring barely better than clear. If a supplier will give you both numbers, you can spot that in five seconds.

04Comparing the glazing build-ups

Here is how the common specifications compare. These are typical industry figures for sealed units in the horizontal or near-horizontal plane rather than any one manufacturer’s product range, and the exact values move with cavity width, gas fill and coating generation. Use them to interrogate a quote, not as a purchase specification.

The coating

Low-emissivity coatings are microscopically thin metallic layers, usually silver-based, deposited on the cavity face of one pane. A basic low-E coating reflects long-wave heat back into the room and improves the U-value. A solar-control coating adds layers that reflect a large slice of the near-infrared part of sunlight before it gets in, which is what brings the g-value down. The good ones are described as triple-silver, and they are the reason a modern solar-control unit no longer looks like a pair of sunglasses. Older generations had a clear bronze or blue cast. Current neutral coatings read as the faintest grey when you stand under them, noticeable at the reveal edge for about a week and then not at all.

The cavity

Argon at around 90 per cent fill is standard and buys roughly 0.2 W/m²K over air. Krypton performs better in narrow cavities and is used where the sightline of the frame limits unit thickness, at a real cost premium. Cavity width has an optimum: about 16mm for argon, about 12mm for krypton, and going wider than the optimum makes performance slightly worse rather than better because convection sets in. A 28mm overall unit with a 16mm argon cavity is the sensible domestic default.

The spacer

The bar around the perimeter separating the panes used to be aluminium, which is an excellent conductor and therefore a cold bridge running right around the edge of every unit. A warm-edge spacer in stainless steel, composite or structural foam typically improves the whole-unit U-value by about 0.1 and lifts the edge-of-glass surface temperature by several degrees, which is exactly where condensation appears first. It is a small line on a specification and it is worth insisting on.

The panes themselves

Overhead glazing should have a laminated inner pane. Toughened glass breaks into small blunt granules, which is the right behaviour at waist height and the wrong behaviour above a dining table, since the whole pane releases at once. Laminated glass holds the fragments on a plastic interlayer. Part K of the Building Regulations covers safety glazing, and toughened outer with laminated inner is the standard answer. The interlayer has a side benefit: it absorbs most of the ultraviolet, so floors and furniture below fade far more slowly, and it takes an audible edge off rain noise.

05Part L, Part K and who notifies Building Control

New and enlarged rooflights are notifiable under the Building Regulations. That is not conditional on size and it is not something a competent installer treats as optional.

Part L, thermal performance

Approved Document L sets limiting standards for new and replacement elements. For rooflights the limiting standard is generally taken as 2.2 W/m²K, and there is a footnote of real practical importance: rooflight U-values may be quoted in either the vertical or the horizontal plane, and the comparison has to be like for like. Any competent modern unit clears 2.2 with room to spare, so treat the regulation as a floor to step over rather than a target to aim at. On a north-facing roof there is little reason to accept worse than 1.3. On a south-facing one the g-value should be doing at least as much work as the U-value, because Part L exists to control energy use across the whole year, and a room that has to run air conditioning in August is not an efficient room whatever the U-value says.

Part K, safety

Part K deals with protection from falling, collision and impact. For overhead glazing that means safety glass, laminated on the inner face, and a sensible answer to the question of how the unit will ever be cleaned or replaced without somebody standing on something they should not be standing on.

Part F, and the overheating rules

Part F covers ventilation, including the purge ventilation an opening rooflight can provide, which is covered in detail further down. Approved Document O, which deals specifically with overheating, applies to new residential buildings rather than to extensions or replacement work, so it will not usually be the rule your job is assessed against. Its logic is still the best free sanity check available: it limits glazed area by orientation, it requires a minimum openable area for getting heat out, and it treats shading as part of the design rather than an afterthought. If a proposed rooflight would fail that test on a new build, it is worth asking why it is a good idea on your extension.

Planning

Rooflights on a house are generally permitted development where they project no more than 150mm beyond the plane of the existing roof and sit below the ridge, with obscure glazing and restricted opening required below 1.7 metres on side elevations. Flats and maisonettes have no such rights. Listed buildings need consent. Conservation areas are looked at case by case, and the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction covering changes to roofing materials and windows, which removes those permitted development rights altogether; both appraisals were updated in March 2022. Treat all of that as general guidance and confirm your own address with the relevant authority before anything is ordered.

We make the Building Control notification to Southend-on-Sea City Council, or to whichever authority covers your address, on your behalf. The completion paperwork comes to you at the end, and it is the document a solicitor will ask for when the property is sold. It is a small administrative job that causes an outsized problem when it has been skipped.

06Getting the heat back out: ventilation and purge

Glass controls how much energy enters. Ventilation controls what happens to the energy that gets in anyway, and on a south-facing room the two have to be designed together. A perfectly specified non-opening rooflight in a room with no high-level opening will still stack heat under the ceiling on a still August afternoon.

Why a rooflight is the best exhaust point in the house

Warm air is less dense and rises. In a room with an opening low down and another high up, that difference drives a flow without any fan: air leaves at the top, cooler air is drawn in at the bottom, and the strength of the effect grows with both the temperature difference and the vertical distance between the two openings. A rooflight is by definition the highest opening in the room, which makes it the most effective natural exhaust available. A door and a rooflight ventilate a kitchen far better than two doors do.

This is why the position of the opening unit matters as much as whether there is one. On an extension with sliding doors at one end, the opening rooflight wants to be at the far end from the doors, so the flow crosses the room instead of short-circuiting straight back out. On a loft conversion, an opening rooflight high on the slope plus a window at the gable end will clear the space in minutes on a summer evening.

How much opening you need

Approved Document F asks for purge ventilation capable of rapidly clearing a room, and the usual rule of thumb for openable areas is around one twentieth of the floor area. For a 20 square metre kitchen extension that is roughly one square metre of clear openable area, which a pair of opening rooflights can provide comfortably where a single small vent cannot. Watch the difference between the frame size of an opening unit and its actual clear free area: a rooflight that opens to 30 degrees delivers considerably less free area than its overall dimensions suggest, and the manufacturer’s figure is the one to use.

Manual, electric, and night purge

A manual opener on a pole is reliable, cheap and gets used less than people expect, because nobody fetches the pole at seven in the morning. Motorised units with a wall switch or a remote get used, and that behavioural difference matters more than the specification difference. A rain sensor closes the unit automatically when the weather turns, which is what makes it practical to leave a rooflight open while the house is empty. Options and controls are set out on our electric skylight installation page.

The strategy that does most for summer overheating is night purge. Open the high-level vents in the evening, let the accumulated heat out and cooler air in through the night, and start the following day several degrees down. It works properly where the room has some thermal mass to cool, so a tiled or screeded floor over a concrete slab, which is the usual build-up in a modern extension, is an ally rather than an enemy provided the heat can be flushed out overnight. Being close to the water helps here too: the estuary keeps night-time temperatures moving even when inland Essex stays warm.

07Glare, contrast and shading

Overheating is what people complain about in August. Glare is what they complain about all year, and it is a separate problem with separate solutions.

Glare is about contrast, not brightness. A pool of direct sun on a worktop next to a shaded wall creates a luminance ratio the eye cannot handle comfortably, so everything outside the bright pool reads as gloom. Overhead glazing produces this more readily than a window does, because the bright source is high in the field of view and often falls on the very surfaces you are working at. If somebody sits at a laptop under a south-facing rooflight, they will end up drawing a blind across it in June, and a blind drawn across it is a rooflight that has stopped doing its job.

Shading options, in order of how well they work

1. External shading. An external awning blind, mesh screen or brise soleil intercepts the sun before it reaches the glass, so the absorbed heat is dumped outside the building. An external screen over solar-control glass can bring the combined effective g-value down towards 0.10, which is more than any glass can do on its own.
2. Solar-control glass. Cuts the beam intensity by roughly half and never has to be operated, which is its main advantage over anything with a mechanism.
3. Diffusing glass. An acid-etched or translucent inner pane scatters the beam into even light, which removes the hot spot and the hard shadow entirely. It admits essentially the same amount of heat, so it is a comfort measure rather than a thermal one, and it suits bathrooms, stairwells, studies and anywhere you want light without a spotlight.
4. Internal blinds. Useful for privacy and blackout, and by far the weakest option thermally, because by the time the blind absorbs the energy that energy is already inside the room. A reflective-backed internal blind takes perhaps a third off the effective g-value; an external one takes off two thirds or more.
5. Placing the glass somewhere else. Moving a rooflight two metres north on the same roof, or splitting one large aperture into two smaller ones over the parts of the room that need light, is free at design stage and impossible afterwards.

On loft conversions the calculation shifts, because most of the glazing in a loft is in the roof slope and pitched at whatever the roof happens to be. A south-facing slope at 40 degrees is close to square-on to the March and September sun, which is a genuine winter benefit and a shoulder-season nuisance. External roller shutters or awning blinds are more common on pitched installations for that reason, and they are covered further on the roof window installation page.

08Overheating: sizing the glass to the room

The single most common cause of an unusable extension is not bad glass. It is too much glass, chosen from a photograph rather than a calculation. Solar-control glazing will halve a heat load; it will not rescue a room with six square metres of rooflight over eighteen square metres of floor facing due south.

A workable rule of thumb

For daylight alone, overhead glazing at around 10 per cent of floor area gives a bright, evenly lit room, since a rooflight delivers roughly two to three times the daylight of the same area of vertical window and spreads it much deeper into the plan. Push past about 15 per cent on a south-facing roof and you are buying heat rather than light; past 20 per cent and you need external shading and a serious ventilation strategy to make the room work. On a north-facing roof you can be more generous, because the glazing sees diffuse sky rather than direct beam for most of the year.

Doing the sum properly

The rough calculation is not hard, and it is worth asking whoever is quoting to show it to you.

• Take the glazed area in square metres.
• Multiply by the peak solar irradiance on that plane, which for a near-horizontal surface here on a clear summer day is in the region of 700 to 800 W/m².
• Multiply by the g-value of the unit.
• Multiply by any shading factor from an external blind or an obstruction.

Three square metres of clear double glazing at g 0.60 gives you roughly 1,300 watts. The same three square metres at g 0.30 gives roughly 650. Add an external blind at a shading factor of 0.35 and it falls to about 230. For scale, an adult at rest gives off around 100 watts and a typical oven on full is in the region of 2,000. Once you can see the numbers in those units, the specification decision stops being an argument about brands.

Where the heat goes afterwards

Two rooms taking identical solar gain can behave completely differently. A room with a heavy floor absorbs energy through the day and releases it slowly, so the peak is lower but the evening is warmer, which suits a household out at work and purging at night. A room with a timber floor over a ventilated void heats quickly and cools quickly. Neither is better in the abstract; they need different ventilation habits. What is always true is that a lightweight room with no exposed mass and no high-level opening is the worst combination available, and it is unfortunately the default on a lot of quickly built extensions.

The winter side, which is genuinely good news

Everything above is about summer, so it is worth stating the other half plainly. Overhead glazing is the most efficient daylight opening in a building. Through a south Essex winter, when useful daylight lasts about eight hours and the sky is more often grey than blue, a well-placed rooflight means the lights stay off in the middle of the day in a room that would otherwise need them, and the light it gives is even and comes from above, which is how people prefer to see. Solar-control coatings cut the summer beam without eliminating that winter benefit, because in December the beam is arriving at a glancing angle anyway and most of the light you are getting is diffuse sky. You lose a little. You keep almost all of what you actually wanted.

09What the performance costs, and what to ask before you sign

The uplift from a standard unit to a properly specified one is smaller than most people assume, and it is the part of the job that cannot be changed afterwards without taking the glass out. The figures below are general market guidance for supply and installation in south Essex, not a quotation. A survey is what turns a range into a price.

Replacing an old single-glazed or early sealed unit is where the arithmetic is at its most favourable. Going from a U-value near 5.0 to one near 1.2 over two square metres saves in the order of a couple of hundred kilowatt hours of heat a year across a heating season, and it removes the winter condensation at the same time. A fuller breakdown across all our work sits on the costs page.

Nine questions that separate a specification from a sales pitch

1. What is the g-value of the glass you have quoted? A quote that cannot answer has not considered summer at all.
2. What is the light transmission? Divide it by the g-value. Below about 1.5, the coating is taking your daylight as payment for the heat it blocks.
3. Is the quoted U-value in the vertical or the horizontal plane, and is it centre-pane or whole-unit? Four different numbers describe the same product.
4. Is the inner pane laminated? Overhead, it should be.
5. What spacer is in the unit? Warm edge, not aluminium.
6. What is the openable free area, and where does it sit relative to the other openings in the room? Free area, not frame size.
7. Which way does this roof face, and at what pitch? If nobody has stood outside with a compass, the specification is guesswork.
8. Who notifies Building Control, and when do I receive the certificate?
9. What covers the workmanship, separately from the manufacturer’s product warranty? Two different documents, and both should be named.

How we specify it

We survey the roof before recommending anything: orientation, pitch, obstructions, floor finish, the rest of the glazing in the room and how the room is used through the day. From that we set a target g-value and U-value, then choose the unit and the ventilation arrangement that hit them, and put the figures in writing on the quotation so you can compare it against anybody else’s. Installation follows the same discipline, with the opening measured rather than taken off a drawing. Fifteen years of installing skylights and rooflights along this stretch of the estuary is mostly fifteen years of watching which specifications people are still happy with in their fourth summer.

Every installation carries our 10-year workmanship guarantee, is installed to current Building Regulations including Part L and Part K, and comes with the Building Control notification made on your behalf. If you are weighing up a pitched glazed structure instead, the roof lantern page covers those, and flush units set into a flat roof are on the flat roof skylight page.

We work across Southend-on-Sea and south Essex, from the seafront and the 1930s semi belt out through Leigh, Rochford, Hadleigh and Shoeburyness. If your extension faces the estuary and you want it to be a room you use in July as well as January, phone 01702 898232 or send the details through the quote form. Tell us which way the roof faces and roughly how big the room is, because those two facts decide most of the rest.

Get a fixed quote