Energy efficient rooflights for Liverpool homes

The figure that measures all this is the U-value, written in watts per square metre per degree of temperature difference, or W/m²K. It tells you how much heat passes through one square metre of the unit for every degree between inside and out. Lower is better. A single pane of old glass sits far above a modern sealed unit, and the gap between them is large enough to feel from a chair.

Makers publish two numbers. One is for the glass alone, measured in the middle of the pane. The other is for the whole unit, with its frame and spacer included. The frame edge always loses more heat than the middle of the glass, so the whole-unit number is higher and it is the honest one. When we compare quotes or datasheets, we compare whole-unit figures, because that is the figure Building Control looks at and the one your heating bill lives with.

Size changes the picture too. A long, slim rooflight has more frame edge for its glass area than a squarer one, so its whole-unit figure can be worse than a smaller unit from the same range. That is one of the reasons we price from a survey and not from a brochure.

Efficiency is also more than the U-value. A rooflight can let in summer sun, gather condensation on a cold morning or leave a draught at the edge of the kerb, and each of those is part of how a room feels. The chapters below take them in turn, with a guide for each one if you want the detail.

The old aluminium spacer bars conduct heat straight round the edge of the glass, and the edge of the pane is where condensation starts. A warm-edge spacer, made from a plastic or composite material, slows that path and keeps the inner pane warmer at its rim. For a rooflight this matters more than for a wall window, because the glass is tilted up to a cold sky and the edge sees the worst of it.

Glass choice has one more side to it. Clear glass lets the sun's warmth in as well as its light, which is welcome in January and less welcome in July. Some coatings cut the warmth that passes through, at the cost of a slightly cooler tint and a little less winter benefit. We come back to that trade-off in the summer chapter, because it belongs there.

Safety and noise are part of the same decision. The inner pane of an overhead unit is normally laminated, so that if it ever breaks the pieces stay in the interlayer and do not fall into the room. A laminated pane also cuts rain noise, which suits a bedroom under a slate roof in a westerly gale.

Glass will always lose more heat than a wall or a well-insulated roof. A modern unit that meets the limits loses far less than the single-pane or early double-glazed unit it replaces, but it still loses more than the plasterboard around it. So the honest answer is that a rooflight can cost some warmth, and the question is how much and whether you will notice it.

What you notice is rarely the average figure. It is the surface temperature of the inner pane. On a cold night, a poor unit has an inner pane so cold that the air next to it sinks, and you feel that as a draught even though no air is moving through the seal. A better unit holds its inner pane nearer to room temperature. That is why a well-made triple-glazed rooflight can feel kinder to sit under than a tired double-glazed one, even when the heating bill difference is small.

The roof is also where heat naturally collects. Warm air rises, and a rooflight is at the highest point of the room. The pane, the frame and the kerb are the first surfaces that warm air meets, so any weakness shows up straight away. A kitchen with a high ceiling and a large flat rooflight will pay most attention to this. Our guide on whether rooflights lose heat tells you what to look for, and how to read the comparison between a new unit and the one you have now.

Houses differ, and so do the rooms in them. In Crosby or Wallasey, a rear extension to a 1930s semi may have radiators sized for a solid-roofed room, and a large rooflight brings a fresh surface for the heating to cope with. A loft room in a terrace in Wavertree has a different problem: a small roof window in a space with thin walls and a slate roof, where the glass is not the weak link at all. We look at the house as a whole before saying which glazing is worth paying for.

Winter sun brings free heat. A clear, south or west facing rooflight lets low sun straight into the room, which can offset some of what the glass loses overnight. A bedroom facing north does not get that benefit, so the same glass behaves differently on the same house depending on which slope it sits in. That is part of why a single recommendation never fits a whole street.

Airtightness of the room also matters. In a house that has been draught-proofed, a leaky rooflight stands out sharply, because it becomes the one place cold air finds a way in. In a loose old terrace with open flues and gaps round the skirting, the same leak hides among many others. Either way, the sealing at the kerb is worth getting right the first time, since the plaster and decoration below it are the hardest part of the room to redo.

Everything above comes down to a choice made house by house. A single-storey rear extension on a Victorian terrace, a 1930s semi with a kitchen knocked through, a Wirral bungalow, and a new-build with a flat-roof dining area each need a slightly different unit. Our house types guide goes through the region's housing in more depth, and the questions it raises for each roof.

If you are planning a roof lantern over a new extension, the glass choices here apply to it directly, and our roof lantern page covers the build. If you are fitting into a pitched roof, roof windows carry the 1.4 limit and the Window Energy Rating route. Both pages show how the unit is chosen for the opening, not the other way round.

Older units deserve a word. Many roof windows and rooflights fitted a few decades ago are single glazed or have early sealed units with plain air gaps and no coating. If yours is one of them, a modern replacement makes a clear difference, and you can feel it in the room before you see it on a bill. If the unit is only a few years old, the case is weaker, and we will say so.

Bring up the room as well as the roof when we talk. A rooflight over a kitchen island, a landing, a nursery or a home office asks for different things from the glass, and knowing how the space is used lets us weigh winter comfort against summer shade for that room in particular.

We survey before we price. You get a fixed written quote after the survey, so the figure does not move once the order is placed. Where the work is notifiable, we handle the Building Control notification and tell you which certificate to expect. On every job the workmanship is guaranteed in writing for 10 years, and the guarantee page lists what is included. For an idea of what moves the figure before you call, what rooflights cost explains the factors without quoting a number that would not fit your roof.

The route from first message to a finished roof is short, and none of it needs you to be at home all day.

Get a free quote and we will look at what your house needs.