Can an old rooflight be upgraded to save energy?

An older rooflight is a pane, a frame and a base. Upgrading means changing one of them or all three, and they do not contribute equally.

The first route is a new sealed unit. The old glass comes out of its frame and a new double or triple unit goes in, with a low-E coating, a warm-edge spacer and a gas fill. It is the quicker job and leaves the roof finish mostly undisturbed. It works when the frame can take a unit of the new thickness and is in good order.

The second route is a full replacement. The frame comes out with the glass, and a new rooflight goes onto an upgraded upstand or kerb. It costs more and takes longer, because the roof around the opening is involved, but it changes every part of the heat path at once.

It is easy to assume the glass is the whole story, because glass is what you see. On an old unit it often is the biggest single loss, since single glazing, early double glazing with a wide metal spacer and tired seals all perform badly by today's measure. A new unit fixes that. What it cannot fix is a frame that carries the cold straight through, a point we explain in cold bridging around a rooflight.

The survey answers it. We look at five things, and none needs more than a ladder and a torch.

• The glass. Is it single, early double or a newer unit that has misted? Single glass and misted units are the clearest candidates for renewal.
• The frame material and build. Timber, aluminium, PVC or steel, and whether there is a thermal break between the inner and outer faces.
• The rebate depth. A frame cut for thin glass cannot always hold a thicker modern unit. If it cannot, a unit swap is not possible without a new frame.
• The upstand or kerb. Its height, what it is made of, and whether it is insulated inside.
• The condition of the surround. Stained plaster or black spotting along the reveal points to cold bridging, which a unit swap alone will not cure.

Put together, those tell us whether the frame can carry on. A sound, well-fitted aluminium or timber frame with a decent rebate is a good candidate for a new unit. A frame that is bowed, corroded or made of a single piece of metal is not worth keeping, because the new glass would sit in a cold surround and the gain would be smaller than it looks on paper.

If the frame stays, the unit is where the gain is made, and it is worth choosing with care.

A double unit with a low-E coating, a warm-edge spacer and argon fill is the usual starting point for a replacement. It copes with the rebate of a typical older frame and offers a large step up from single or early double glazing. The page on low-E coatings explains what the coating does, and argon gas in rooflights covers the fill.

A triple unit gives a still better number, but it is thicker and heavier, and an older frame may not take it. It is a choice for a full replacement, where the frame is made for it. See triple glazed rooflights for when the third pane earns its place.

Noise can be handled in the same swap. Where the room hears aircraft, traffic or rain, the inner pane of the new unit can be laminated with an acoustic interlayer, and a thicker outer pane can be paired with it. It is easiest to do now, while the glass is being changed anyway, and the choice goes into the quiet spec so you can see how it was made. The wider point is in our page on one glazing build for warmth and quiet.

Replacing glass or a whole rooflight is work the Building Regulations care about, and we build that into the job.

A replacement window or rooflight in an existing home is covered by Part L, which sets a limit on heat loss for the new unit, and the notification to Building Control is our job where the work is notifiable. The details of the thresholds are in whether a replacement has to meet current U-value rules. The short statement is that we install to current Building Regulations, so a replacement unit is specified to meet them, not merely to beat the old one.

Some homes carry extra conditions. A listed building or a conservation area can limit what changes are allowed to the roof, and there the choice of profile and material comes first. Our conservation rooflight installation page covers those cases.

The honest measure is comfort first and bills second, with the gain depending on how poor the old unit was.

A room under single glazing is cold on the surface directly above you and prone to condensation on the coldest mornings. A new low-E unit changes that at once. The heating bill may come down as well, though how far depends on the size of the opening, how the house is heated and how it is used, so we do not promise a figure. Our page on whether energy efficient rooflights lower heating bills gives the reasoning, and costs shows what shapes the price of each route.

If the old unit has misted, it has already failed, and renewing it is simply sensible. Our page on misted and failed rooflight replacement covers that case. To have ours looked at, get a quote, and tell us the room, the roof type and, if you can, a photograph of the frame from inside.

Can I just replace the glass in my existing frame?

Often, yes, where the frame is sound and the rebate can hold a modern sealed unit. We check both at the survey. If either fails, the quote moves to a full replacement and we explain why.

Will a new unit stop the room feeling cold?

It reduces the cold you feel from the glass itself. If the chill comes from the frame or a bare kerb, the room will stay cooler around the edge until that is dealt with as well.

Can I add a low-E film to old glass instead?

Stick-on films are a different thing from a factory coating inside a sealed unit, and they do not change the spacer, the gas or the frame. A new unit is the lasting answer.

Does the whole rooflight have to match the rest of the roof?

The new one is chosen for the roof type, so a flat roof gets a flat unit on a kerb and a pitched roof gets a roof window. See our energy efficient rooflights hub for the range.