Energy efficient rooflights in Cambridge, bright and comfortable

The U-value measures heat escaping. It is given in watts per square metre per degree of temperature difference (W/m²K), so a rooflight rated 1.4 loses 1.4 watts through each square metre for every degree the room is warmer than the air outside. Lower is better. A well-insulated flat roof around it might sit at 0.15 to 0.18, which puts the glass in perspective: even a very good rooflight loses several times more heat per square metre than the roof it replaces. That is not a reason to avoid roof glazing. It is a reason to size it with care and to pick the glass properly, which our guide to rooflight U-values explained sets out in full.

The g-value measures heat arriving. It is the share of the sun's energy striking the glass that ends up inside the room, written as a decimal between 0 and 1. Clear low-E double glazing typically sits around 0.6, so roughly 60 per cent of the solar energy gets through. A solar control coating can bring that down to between 0.25 and 0.4. Unlike the U-value, a lower g-value is not always better: on a north-facing slope you may want the free warmth and the full daylight, and a heavily coated pane there only makes the room dimmer.

A third figure travels with the g-value, and it is the one people forget to ask about: light transmission, or LT. It tells you the share of visible light that passes through. The best modern coatings split the sun's energy, holding back much of the infrared heat while passing most of the visible light, so a pane can have a g-value of 0.35 and still transmit 60 per cent or more of the daylight. The relationship between the two is explained in our guide to g-value and light transmission, and the question people search most, do energy efficient rooflights let in less light?, has its own answer.

These numbers only help if they are read for the right position. Glass manufacturers publish U-values for panes standing vertically, as in a wall window. Laid flat or at a shallow pitch, the gas in the cavity circulates more freely and the same unit loses more heat. A pane quoted at 1.1 upright can behave closer to 1.4 or 1.5 lying horizontal. The Building Regulations now assess rooflights in the horizontal plane for exactly this reason, and it is the figure we look for when we compare products.

Cambridge is flat, and that sharpens the effect. With almost no hills or high ground to cut off the low sun, a roof in Chesterton or Cherry Hinton sees the whole sky from sunrise to sunset. At about 52 degrees north, the midday sun stands roughly 61 degrees above the horizon in late June and only about 14 degrees in late December. A rooflight on a shallow roof faces almost straight up, so in summer it catches the sun close to head-on for hours, while in winter the sun barely clears the terrace opposite. The same pane therefore does very different work in each season, and the orientation of the slope tilts the balance further.

A flat rooflight on a flat roof does not really face anywhere. Its glass looks straight up, so it takes the summer sun for most of the day whatever the orientation of the house. That is why flat-roof skylights in Cambridge extensions almost always benefit from some solar control, even on a north-facing garden, unless the rooflight is small and the room is used mainly on winter mornings. A roof lantern is different again, because each hip looks at its own part of the sky; the south hip may need a stronger coating than the north one, and a good specification can reflect that. For the target numbers on a sunny slope, see what is a good g-value for a south-facing rooflight?

North slopes deserve their own mention because they are often misjudged. A north-facing rooflight gives the best working light in the house: steady, shadow-free and without glare, the light painters and architects have always wanted. It also receives almost no direct winter sun to offset its heat loss, so its U-value matters more than anywhere else. The answer on whether north-facing rooflights lose more heat explains why the loss is the same but the balance is worse. The fuller picture of how each orientation lights a room is on our light planning page.

Horizontal glass takes the summer sun almost square on. Around midday in June a square metre of roof glass receives far more solar energy than a square metre of south-facing wall glass, where the high sun strikes at a glancing angle and much of it reflects off. Once through the glass, that energy lands on the floor, the worktop and the furniture, warms them, and they then warm the air. By late afternoon a room with a large clear rooflight can be several degrees warmer than the rest of the house, and in a well-insulated extension that heat stays after sunset. The mechanics, and the rooms most at risk, are covered in rooflight overheating in summer.

Solar control glass is the first line. A coating on the outer pane reflects a large part of the near-infrared energy while letting most visible light through. Coated units come in neutral and tinted versions. A neutral coating keeps the colour of the room and the sky true, with a slight reflective sheen from outside; a blue or grey tint cuts heat and glare harder but changes the colour of the light a little. The choice depends on how the room is used and what you want to see when you look up. See solar control glass for rooflights, is blue or neutral solar control glass better? and does solar control glass make a room darker?

Shading is the second line, and where it sits matters a great deal. An external awning blind or shade stops the sun before it reaches the glass, so the heat never enters the room. An internal blind stops the light, but the energy has already passed through the glass; the blind itself warms up and gives much of that heat back to the room. Internal blinds still help, especially pleated blinds with a reflective backing, and they help most alongside low-g glass. Our guides on external shading for rooflights and whether internal blinds can stop a rooflight overheating set out the options for flat rooflights, lanterns and roof windows.

Ventilation is the third. Warm air rises and pools under the roof glass, so an opening rooflight near the high point of the room can let it escape. An electric unit with a rain sensor can be left open on warm days without anyone standing guard; our electric and opening rooflight installation page explains the controls. Fixed glass cannot do this job at all, so on a south or west-facing extension we weigh up whether at least one unit should open.

Two things we would advise against relying on. Window film applied to a sealed rooflight unit can raise the temperature of the inner pane and may affect the manufacturer's warranty; the answer on does window film work on rooflights? explains why. And a very dark tint, chosen to fix the heat, usually leaves the room gloomy on the grey days that make up much of a Cambridge winter. Solar control coatings chosen for the slope avoid both problems. Sun through the roof also fades fabrics and timber over time; the answer on do rooflights fade furniture? covers the part laminated glass plays in reducing that.

Aluminium framed rooflights with a proper thermal break perform well and give slim sightlines. Frameless designs bond the glass to a hidden frame or directly over the kerb, so almost nothing but glass shows from inside. People sometimes assume frameless must be worse, because the glass edge sits right over the kerb, but a well-designed frameless unit with an insulated kerb and a warm edge spacer can match or beat a framed one. The answer on whether frameless rooflights are less energy efficient explains what to check. Our flat roof rooflight installation page covers the choice between the two in terms of looks and use.

Polycarbonate deserves a plain answer because many older Cambridge extensions still have it. Multiwall polycarbonate domes and sheets are light and tough, but they are noisy in rain, go cloudy and yellow with age, and their thermal performance depends heavily on thickness. Thin twin-wall sheet sits well above the Part L limit; thick multiwall sheet can come close to it, but still falls well short of modern sealed glass on both heat loss and clarity. See are polycarbonate rooflights energy efficient?

Around the frame, three details matter on a flat roof. The kerb or upstand should be insulated on its inner face and, where the build-up allows, on the outside under the roof covering. The roof insulation should run right up to the kerb with no gap. And the joint between the rooflight frame and the kerb should be sealed with a compressible gasket or tape so that warm, moist room air cannot reach the cold outer layers. On pitched roofs the principles are the same, but the parts differ: an insulating collar, a vapour-control lining, and a properly fitted flashing kit.

The finish inside matters too. We line and plaster the reveal, the short tunnel between the ceiling and the glass, and we splay it outward where the roof depth allows. A splayed reveal spreads more light into the room, and a lined, insulated reveal stops the cold spot that otherwise forms in the corner where the ceiling meets the opening. That corner is where mould tends to appear first above older rooflights.

Condensation is the everyday sign of how well all this has been done. Water forms on any surface colder than the dew point of the room air, so a cold pane or a cold reveal over a kitchen with pans boiling will run with water. A low U-value, a warm edge spacer and an insulated reveal all keep the surfaces warmer; extraction and ventilation keep the air drier. Our guide to rooflight condensation goes into both halves.

Old units give themselves away. Single glazing runs with condensation on winter mornings. Older double glazing with a failed edge seal mists between the panes, which means the gas has gone and moisture has entered the cavity. Polycarbonate turns cloudy and yellow. In each case, the glass can no longer do its job, and the fix is to replace the unit, not to clean it. Our misted and failed rooflight replacement service covers the replacement itself; the energy side is explained in upgrading single-glazed rooflights.

Whether it pays back in pounds depends on the size of the rooflight, the orientation, the way you heat the house and how many hours the room is warm. A single small rooflight in a hallway will save little. A large area of single glazing or old polycarbonate over a kitchen heated all day is a different matter. We would rather give you the reasoning than a payback figure that cannot be true for every house, so the answer on is it worth replacing an old rooflight to save energy? sets out how to judge it, and our costs page explains what drives the price of the work.

An upgrade is also a chance to look at the light again. The first rooflight in an extension was often chosen by size and budget, with no thought for orientation. When it is replaced, we check whether the glass suits the slope, whether a solar control coating would help, whether an opening unit would let heat escape in summer, and whether the reveal could be splayed to throw the light further into the room. Sometimes the opening stays exactly where it is and only the glass changes; sometimes a slightly different size makes more sense. The written light plan records which and why.

There is also a less obvious benefit. A modern rooflight can reduce how often you need the lights on, especially in a deep terrace plan where the back of the kitchen has always needed a lamp by mid-afternoon in winter. The answer on whether a rooflight can reduce lighting costs looks at that honestly: the electricity saving from LED lighting is small, but daylight is simply better to live in.

Is an energy efficient rooflight worth it on a north-facing roof?

Yes, and arguably more than anywhere else. A north slope gets almost no winter sun to offset heat loss, so a low U-value keeps the glass warmer and the room more comfortable. Choose clear low-E glass with high light transmission rather than solar control, because heat gain is not the problem there and you want every bit of the soft north light.

Will solar control glass make my kitchen dark?

A good neutral solar control coating cuts much of the sun's heat while passing most of the visible light, so the room stays bright. On a dull winter day the difference is small. Very dark tints are another matter, which is why we match the coating to the slope and the room rather than using the strongest one everywhere.

Do I need triple glazing?

Not to meet the regulations, which modern double glazing clears comfortably. Triple glazing makes sense for large rooflights, north-facing slopes, very well-insulated houses and rooms heated for long hours. It adds weight and takes a little light, so it is a choice for the room, not a default.

Can I improve an old rooflight without replacing it?

Blinds and shading can reduce summer heat, and better ventilation can reduce condensation. But the heat loss of single glazing, failed double glazing or old polycarbonate is fixed by the material. To change it, the unit has to be replaced with a modern one, which is often a one-day job on the existing opening.

If you would like to see what the right glass would do for your room, book a light survey. We will look at the slope, the sun and the way the room is used, and the written light plan with your quote will set out the glass, the shading and the reasons for each. For the wider picture of how we plan skylights and rooflights across Cambridge, start at our home page, or compare this with roof lantern installation.