Will an energy efficient rooflight lower your heating bills?

Warm air rises, so the top of a room is where heat collects and where a poor rooflight gives most of it away.

A rooflight is a hole in the best-insulated part of the house. Around it sit rafters or joists packed with insulation. In the opening itself there is a single pane, or a sealed unit that has aged. Glass was never going to match insulated roof build-up, so even a good rooflight is the weak patch in that roof. An old one widens the gap.

Older units lose ground in particular ways. Single glazing has no cavity at all. Early double glazing has a wide gap, plain glass and an aluminium spacer that conducts cold around the edge. Sealed units fail over the years, and once the gas has gone and moisture has crept in, the insulation goes with it. You feel that as a cold patch above the bed or the dining table, and as a room that takes longer to warm up.

Modern units change each of those things together: a low-e coating, argon in the cavity, a warm edge spacer and a frame with a thermal break. Our page on how heat escapes through a rooflight takes the routes one at a time.

The same new rooflight can make a large difference in one house and a small one in another. Four things do most of the deciding.

1. What is being replaced. Moving from single glazing to a modern sealed unit is a bigger step than moving from a sound early double unit to a modern one.
2. How much glass there is. A single small roof window is a small part of a house's heat loss. A large lantern over an open-plan kitchen is a much bigger share.
3. The rest of the envelope. If the loft is thinly insulated and the walls are leaky, the rooflight is one of several leaks, and fixing only that one moves the total less.
4. How the room is used. A loft bedroom heated for a few hours a night behaves differently from a home office warmed all day.

Because those four vary from house to house, any single figure quoted for "the saving" would be a guess. We would rather tell you what drives it, so you can judge for your own home. The measure used on the data sheet is the U-value, which rooflight U-values explained sets out in plain language. A lower number means less heat passes through.

Bills are one reason to replace a rooflight. The others are just as real and easier to notice.

A cold inner pane is where moisture from breathing, cooking and washing condenses first. Streaming glass in a loft bedroom is a sign that the surface is colder than the room air. A modern unit with a warm edge keeps the inner face warmer, so there is less condensation on the glass and along the frame. That is also better for the plasterwork and paint around the opening.

Then comes how the room is heated. People with a chilly rooflight room often turn the radiator up, run a plug-in heater, or simply stop using the space in January. A warmer room needs less of that. The saving shows up as a room you can use, which matters as much as a line on an energy statement. Our guide to why a skylight makes the room feel cold in winter goes through the causes.

In summer the picture reverses. A rooflight that lets heat in can force you to spend on cooling or blinds. A solar control coating manages that side, and the g-value is the figure to read for it. Heating cost is only half of a rooflight's energy story.

A new unit works best when the surroundings do not undo it.

The gap between the frame and the roof structure is where much of the benefit can be lost. If the upstand is thin or the reveals are bare, warmth bypasses the glass altogether. Insulated upstands, sealed joints and a proper lining inside make the new rooflight perform as its data sheet says. The page on insulating around a rooflight explains what that involves.

Building Regulations also apply. Replacement glazing has to meet the thermal standard in Part L, and we install to current regulations and handle the Building Control notification where the work is notifiable. You can read what the energy rules ask of rooflights in Part L and rooflights.

The same glazing that keeps heat in can be built with a laminated acoustic inner pane, so the warmer room is a quieter one too. We set that out in your written quiet spec when we quote. The wider range of options is on our energy efficient rooflights in Crawley page.

Can you tell me how much I will save?

No honest installer can do it from a photo. Savings depend on the old unit, the rest of the house and how you heat the room. We can tell you which parts of your rooflight are losing heat and what the new unit changes.

Will a bigger rooflight raise my heating cost?

More glass means more surface for heat to cross, so it can. Part L also limits how much glazed area a building may have. A well-specified unit keeps the extra loss small, and the daylight often lets you switch lights on later.

Does a new rooflight help my EPC rating?

It can contribute, but an EPC looks at the whole building. Our page on whether a rooflight improves your EPC explains how it is treated.

Is triple glazing worth it for heating bills?

Sometimes, depending on the room and the rest of the build. The page on triple glazed rooflights covers when the third pane earns its place.

To find out what a replacement would change in your house, get a quote. The costs page gives a general idea of what the work involves.