Energy Efficient Rooflights in York

An efficient rooflight is one that lets daylight in while keeping the heat you have paid for inside, and the rating on the box is only part of how well it does that.

Four things decide how a rooflight behaves once it is in the roof. The glass sets the base figure. The frame and the spacer bars carry heat around the edges of the glass. The insulation and membrane around the opening decide whether the product's rating survives contact with your actual roof. And the way the room is used, ventilated and heated decides whether you notice the result as comfort or as a damp patch on the lining.

Most people start by asking for the best number. It is a fair start, but a lower figure on paper does nothing if the opening is badly packed, and a modest product fitted with care can outperform a better one fitted carelessly. Our survey looks at the whole chain, then the written quote names each part of it so you can see what you are paying for. If you are still deciding which kind of unit suits the house, the full range sits on our services page.

A U-value tells you how quickly heat passes through one square metre of a material for each degree of difference either side, so lower means warmer.

The unit is watts per square metre per kelvin, written W/m²K. A single pane of old glass sits near the top of the scale. A modern sealed unit with a coating and a gas fill sits far lower. When you compare two rooflights, the thing to ask is which figure you are being shown. Some makers quote the centre of the glass alone, which flatters the product because the edges lose more heat than the middle. Others quote the whole window, glass and frame together, which is the figure that counts.

Roof windows in pitched roofs have a second rating, the Window Energy Rating, which scores the product from A to G and takes account of heat gained from the sun as well as heat lost. Part L accepts Band B or better for a roof window in an existing home. Flat rooflights and lanterns are judged on U-value alone.

Our guide to rooflight U-values walks through the figures with worked examples.

Each extra pane adds a layer of still gas and a surface that heat has to cross, which is why the third pane makes such a difference on paper and so little in a room that is hardly used.

A double unit has two panes with a sealed cavity between them, filled with argon, which conducts heat more slowly than air. A triple unit has three panes and two cavities. The result is a lower whole-window figure, a warmer inner surface and a little more protection from rain noise. The cost is weight, thickness and a slight dip in the light that gets through, since every pane absorbs and reflects a small amount.

Where we advise triple glazing, it is usually for a room that is occupied in the evening and early morning, such as a loft bedroom, a sitting room or a home office, where the inner glass is something you sit under. For a landing, a utility room or a pantry, a good double unit does the job and leaves more of the budget for the parts you cannot see. We bring both samples to the survey and let you hold them.

The longer comparison is in double or triple glazed rooflight, with the arguments on both sides.

Two details that you will never see once the rooflight is in do a surprising share of the work: a microscopic coating on one pane and the spacer bar that holds the panes apart.

Low-emissivity glass carries a thin metal-oxide layer on one inner surface. It lets short-wave sunlight pass through but reflects the long-wave heat that radiates from the warm room, sending it back in. The coating sits inside the sealed unit where it cannot be scratched, and the glass looks clear. On a roof glazing, which faces the cold sky at night, that reflected heat counts for a great deal. There is a separate short answer on what low-E glass is if you want the coating described step by step.

The spacer is the strip around the perimeter of the unit that keeps the panes apart and holds the gas in. A traditional aluminium spacer is an efficient conductor, so it makes a cold line around the edge of the glass, exactly where condensation likes to start. A warm-edge spacer is made of a less conductive material, which lifts the temperature of that edge. The difference shows in the whole-window figure and in how clear the glass stays on a cold morning. Read more in what a warm-edge spacer is.

Glass makes up most of a rooflight's area but the frame is a continuous path from the cold side to the warm side, and it needs to be designed to interrupt that path.

Aluminium conducts heat very well, so an aluminium rooflight frame needs a thermal break: a strip of insulating plastic that separates the outer section from the inner one. Without it, the inside face of the frame turns cold and collects condensation. With it, the frame stays close to room temperature. Frames in multi-chamber uPVC have the air pockets built in, and timber, as used in many roof windows, insulates by nature. Each material can reach the Part L limits, so the choice is mostly about the look, the span and the maintenance you want.

The sash or opening part matters too. An opening rooflight has a seal where the sash meets the frame, and a worn or poorly compressed gasket leaks both air and heat. That is one reason older openers feel colder than their glass would suggest. A new unit has fresh seals and a tighter fit, and the difference is easy to feel on a windy night.

A figure for the glass tells you about the middle of the window, and the cold you feel comes from the edge.

From a survey of a loft bedroom in Acomb

Every rooflight sits in a gap that was cut in a roof, and the way that gap is packed decides whether the product performs as its figure promises.

On a pitched roof the frame is wrapped in an insulation collar, a shaped surround that closes the space between the frame and the roof structure. The rafters are trimmed to take the opening, and the insulation in the roof is cut tight to the collar so that no gap is left for cold air to track around. A vapour control layer inside the roof is lapped to the frame and taped, so warm, damp air from the room cannot pass into the structure and condense in the cold. On a flat roof the same thinking applies to the upstand: it is insulated to match the roof, and the insulation overlaps the roof's own layers.

This is hidden work, and it is the first thing to be skimped when a job runs late. We do it carefully and photograph it before it is covered, and the photographs are yours at handover. If a rooflight is ever questioned, those pictures show what is behind the plaster.

A rooflight faces the night sky, and warm air rises towards it, so it is a more demanding place for glazing than a wall window.

Heat leaves a window in three ways: by conduction through the glass and frame, by radiation to colder surfaces, and by air moving through leaks. A rooflight is hit by all three. The glass looks up at a clear, cold sky on a frosty night, the warmest air in the room is gathered beneath it, and any gap in the seals lets that air out. So an old rooflight can feel noticeably cold even when the room is heated, and a draught from the frame line is a common sign of a failed seal or a gap in the packing.

The good news is that modern units cope with that harsh position far better than old ones, because each of the layers described above reduces one of the three losses. A coating handles radiation, the gas fill and extra panes reduce conduction, and fresh seals and a taped membrane stop the leaks. Our short answer on whether rooflights are cold in winter sets out what to expect from a new one.

Efficiency is not only about holding heat in, and a rooflight that is warm in winter can be uncomfortable in summer unless the glass has been chosen with the sun in mind.

Roof glazing takes the sun at its strongest, and a south or west-facing slope can turn a loft room stuffy by mid-afternoon. Solar control glass carries a coating that reflects part of the sun's heat while still letting daylight through. There is a small cost in light, and some glass looks very slightly tinted when you look at it from the right angle. External blinds and awnings stop the heat before it reaches the glass, which is far more effective than an internal blind, and an opening rooflight lets hot air escape from the highest point of the room.

The right answer depends on the direction the roof faces and on whether the room is somewhere you spend afternoons. A north-facing slope rarely needs special glass, and a south-facing loft office usually does. You can read more in solar control glass for rooflights and the answer on whether rooflights make rooms hot in summer.

Water on a rooflight tells you that the surface is colder than the air touching it, and the fix depends on whether the surface is too cold or the air too damp.

Warm air carries water vapour, and when it meets a cold surface the vapour turns to droplets. Cooking, showers, drying washing indoors and even breathing add moisture to the air, and a rooflight, being the coldest surface in the room, shows it first. A better inner pane, a warm-edge spacer and a thermally broken frame all raise the temperature of that surface, so condensation forms less often and clears sooner. Extraction in the kitchen or bathroom, and a trickle vent or a rooflight that opens, reduce the damp at its source.

Condensation inside the sealed unit is a different problem. Misting between the panes means the seal has failed and the gas has gone, and the only cure is a new unit, which we cover in our misted and failed rooflight service. Our guide to condensation on rooflights goes into the causes and cures in more depth.

When we add a rooflight or replace a whole one, the work has to meet current thermal standards, and some jobs also need a planning check.

Under Part L of the Building Regulations (the 2021 edition for existing homes), a new or replacement roof window should reach a U-value of 1.4 W/m²K or Window Energy Rating Band B, and a rooflight should reach 2.2 W/m²K. Forming a new opening is a separate matter, because the Building Regulations then look at the framing and at heat loss. If the whole rooflight comes out and a new one goes in, frame and glass together, it is a controlled fitting and therefore notifiable, and the notification is ours to send. Changing just the sealed glass unit does not trigger it. Where a private firm handles the checks rather than the council, it is a registered building control approver, and the paperwork you receive is the same.

On planning, rooflights on a house are usually permitted development under the roof alteration rules, provided they rise no more than 150mm from the roof plane and stay below the highest part of the roof. Where the rooflight faces a side slope, only obscure glass that does not open is allowed, except for an opening part more than 1.7m above the floor. A flat or maisonette has no permitted development rights for this, and a listed building asks for listed building consent. Conservation areas can carry an Article 4 direction that withdraws the rights from certain streets, so the council has the final word. Every condition is laid out in our planning and Building Regulations guide.

A rooflight improves comfort and daylight for certain, but its effect on an EPC and on a heating bill is smaller than people hope and depends on what it replaces.

An Energy Performance Certificate rates a whole home from its fabric and heating, and the assessor looks at windows and rooflights as part of that survey. Replacing a failed single-glazed unit with a modern one helps the fabric score, but the largest recommendations on most certificates concern loft insulation, walls and the heating system. Whether the change shifts a band depends on the home and on the assessor's inputs, so we would not promise a movement in rating. What we can promise is that the new unit is properly specified and its figures are on paper.

The same is true of running costs. A rooflight cannot reduce a bill on its own, but a cold, leaking one can add to it, and a new unit that stops the loss will help. If you want the fuller picture, our answer on whether a rooflight can improve an EPC rating goes through what the assessor looks at.

Treat the EPC as a by-product, and judge the rooflight by how the room feels on the coldest morning.

From a survey of a bungalow near Haxby

Many York homes have a rooflight from an earlier extension or conversion that was glazed to the standards of its day, and a few clear choices cover most of the upgrades.

If a roof window is sound but its glass is failing, a new sealed unit may be all it needs, and it is the lightest-touch option. If the frame is cold, warped or leaking air, replacing the whole window in the same opening brings the thermal performance up to current standards and gives a new sash and seals. Where the existing rooflight is a plastic dome or a flat panel from a 1980s extension, we usually recommend a modern flat rooflight on an insulated upstand, which suits a flat roof much better. In a pitched roof, a roof window in the same opening is the natural swap, and on a flat roof a flat rooflight does the job. In an older house where slim frames matter, a conservation rooflight offers a very low profile with good thermal figures.

On the written quote every option has a line to itself, which makes plain what the cheaper route would leave in place.

Swapping a rooflight is a short job on paper, and a plan for each day keeps it that way in a house where people are living.

First we cover the floors, stairs and furniture on the way in, and put up a screen around the working area if the layout lets us. A delivery, a child's nap or a day spent at your desk is worth mentioning at the survey, so the timings can bend around it. By evening the roof is sealed, either because the new unit is in or because a fixed temporary cover is. Waste goes away with us, and one team does the job from first dust sheet to handover.

Plaster needs a little time to dry before it is painted, and the plan says so in advance.

When the work is finished we walk you round it and leave you with every document you might want later.

A well-fitted rooflight needs very little attention. Wash the glass a few times a year, keep gutters and valleys clear, and wipe condensation from the frame on cold mornings. We show you how to operate any opening sash, how to adjust the blinds and what to avoid putting on the glass. If you ever sell the house, the handover pack answers the questions a buyer's surveyor is likely to ask.

The guarantee covers our workmanship, including the opening, the insulation and the weathering into your roof, and the details are on our guarantee page. To start with a free survey, use the form below.