Heat loss through a rooflight: the four places warmth escapes

Warm air rises, and a rooflight sits at the very top of the room, exactly where the warmest air collects. That single fact explains why people ask whether skylights lose heat at all.

A wall has insulation, a cavity and a masonry leaf. A rooflight is a thin sandwich of glass and a slim frame set into the deepest insulation layer in the house. Glass passes heat far more readily than an insulated roof build-up, so any area of glazing in a roof will lose more per square metre than the roof around it. That is physics, and it applies to every rooflight from every maker. The useful question is how much, and how to keep it small.

Heat moves in three ways, and a rooflight uses all three:

• Conduction carries warmth through solid material: glass, aluminium, timber, the spacer bar between the panes.
• Convection moves warm air, both inside the sealed unit and in the room, and pushes it up against the glass where it cools and slides back down.
• Radiation sends warmth out as infrared, straight through ordinary clear glass towards the cold night sky.

The sky matters more than people expect. On a clear night the glass faces open sky rather than a neighbouring wall, so it gives up warmth by radiation faster than a window does. That is part of why a loft bedroom can feel cooler near the rooflight at dawn, long after the radiators have done their work. Our answer on why a skylight makes a room feel cold in winter covers what you feel standing under it.

The glass pack is the path everyone names first, and it is also the one that has improved the furthest. A modern sealed unit has two or three layers of glass with gas and a coating between them, and each of those does a separate job.

A single pane has nothing to slow heat down. Two panes with a gap trap a layer of still gas, which resists conduction and convection. A low emissivity coating, a thin transparent metallic layer on one glass surface, reflects the infrared back into the room instead of letting it escape. Argon in the gap moves heat a little more slowly than air. Our pages on low-E coatings on rooflight glass and what argon does inside a rooflight explain each layer separately.

The heat loss figure for the glazed part is called the centre-pane U-value. It is measured at the middle of the glass, away from the edges, so it flatters the unit. A rooflight as a whole always does a little worse than its centre pane, because the edges and the frame are added in. The difference between the two numbers is the subject of our answer on whole-unit and centre-pane U-values, and it is worth reading before you compare two quotes.

Polycarbonate sheet and old single glazed units sit at the poor end. Tired double glazing sits in the middle, because seals age and the gas slowly escapes. Current sealed units with a coating and a warm spacer sit at the good end. Triple glazing goes further still, but it adds weight and a little cost, so it needs a reason. See when the third pane earns its place.

The edge of a glass unit loses more heat than its centre, because of the spacer bar that holds the panes apart. The bar has to be strong, sealed and dry, and in older units it is made of aluminium, which conducts heat very well.

An aluminium spacer forms a thin metal ring round the perimeter of the pane. Heat runs along it from the warm inner glass to the cold outer glass. You can often see the result: a band of condensation, or a cold strip, round the edge of the glass while the middle stays clear. A warm edge spacer, made of a stiffer low conductivity material, cuts that path down and keeps the inner edge of the glass nearer to room temperature. That is why the edge strip dries out first in a good unit and clouds first in a poor one.

The same edge is where a failing seal shows itself. Once the outer seal lets in moist air, the gas fill drifts out and the insulating value of the whole pane falls. A misted unit is a unit that has already lost part of its performance. Our hub on misted and failed rooflight replacement deals with renewing those units, and the page on warm edge spacers and argon fill explains the newer build.

A frame is a solid lump of material in a line of glass, and solid material conducts. Aluminium conducts heat far better than still air, so an ordinary aluminium section passes warmth straight from the room to the outside.

That is the reason for the thermal break. A thermally broken frame has two metal sections, one inside and one outside, joined by a strip of low conductivity plastic. The metal keeps its strength and slim profile, and the break stops the cold outer section chilling the inner one. Where there is no break, the inside face of the frame runs cold, collects condensation and drips onto the sill or the ceiling reveal. Timber and uPVC frames conduct far less to begin with, but they bring their own limits on size and on how slim they can look.

Frame material and glass do not cancel each other out. A very good pane in a poor frame gives a disappointing whole-unit figure, and the reverse is also true. Our page on thermally broken rooflight frames sets out what to ask for and how to recognise it on a data sheet.

The upstand, also called the kerb, is the short box of timber or insulated material that lifts a flat roof rooflight clear of the roof surface. It is a wall in its own right, and on older roofs it is often the weakest wall in the house.

Older kerbs were often built from bare timber with a felt or asphalt covering and no insulation at all. Others were a course of brick or block with the membrane dressed up the outside. In both cases the rooflight frame sits on a slab of material that joins the cold outside directly to the warm ceiling below. Heat crosses that kerb like water through a pipe. The same is true of the deep splayed lining of a pitched roof window, where a thin plasterboard reveal may have little or no insulation behind it.

A well-built upstand is insulated on the outside face, the inside face or both, sized so the insulation meets the roof insulation without a gap. The aim is a continuous layer from the roof build-up up and over the kerb to the frame. Our page on insulating around a rooflight goes through upstands and reveals in detail.

Where this layer breaks, the junction becomes a cold bridge: a stripe of the structure that runs colder than everything next to it. You find cold bridges by their effects, a line of damp, a patch of mould on the reveal, a streak of dirt on the ceiling where warm air has been cooled on the same strip. The answer on cold bridging around a rooflight shows how to read those marks.

The fourth path is not conduction at all. It is air moving through gaps, and it can carry more warmth out of a room than the glass does.

A rooflight has several places where two things meet: the sash and its frame, the frame and the kerb, the kerb and the ceiling lining, and the flashing or trim round the outside. Every one of them is a potential gap. Seals perish with age, sunlight and temperature swings. Mastic dries and shrinks away from the surface it was sealing. Timber kerbs move with the seasons. An opening rooflight adds a moving joint, which needs a compressible seal that stays springy.

The stack effect makes these gaps work harder than their size suggests. Warm air is lighter, so it pushes upwards against the ceiling and finds any route out. A rooflight at the top of a stairwell or an open-plan extension is the natural exit. As the warm air leaves, cold air is drawn in at low level elsewhere, and the heating works to replace it. You may feel that as a draught around the feet in winter, though the leak is overhead.

A loose seal also works against quiet. The gap that lets warm air out lets rain noise, wind and aircraft sound in. That is one reason we treat the seals and the upstand as part of the written quiet spec, rather than as an afterthought. Our page on one glazing build for warmth and quiet explains how the two aims line up.

The balance between the four paths changes with the age and type of the rooflight. The table gives a rough guide to where to look first. It is a pattern for orientation, not a measurement of your roof.

The pattern matters because the right fix follows the path. Fitting better glass in a frame that loses heat, or on a kerb with no insulation, buys far less than the glass figure promises. A new pane in an old frame inherits the old frame's heat loss.

Approved Document L, the energy part of the Building Regulations, sets limiting values for rooflights and the areas they may take up. It treats rooflights as a separate element with its own standard, because they lose more than a wall.

The standard is written as a U-value, the rate at which heat passes through a square metre for each degree of difference in temperature. Lower is better. Approved Document L also limits how much of an extension can be glazing, which is why a large lantern can push a design over the line. Our pages on Part L and rooflights and the glazing area limit under Part L go through these limits, and rooflight U-values explained starts from the beginning.

Replacement work is treated differently from new work, and the rules depend on what is being replaced and whether the work is notifiable. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, so the paperwork follows the job. The position for your roof is checked at survey.

You do not need instruments to find out which path is the problem. A few minutes on a cold morning tells you a great deal.

Start with the glass. Put the back of your hand near the middle pane, then near the edge. If the edge feels much colder, or the edge strip carries condensation while the centre is clear, the spacer and the seal are the issue. If the whole pane feels cold and wet, the glass pack has probably failed or is a single layer.

Next, the frame and the kerb. Run a hand along the frame. A frame that feels icy and drips is conducting. Look at the reveal or the inside face of the upstand for a line of darker paint, a bloom of mould or a shadow of dust. Those marks follow the colder strips of the structure almost exactly.

Finally, the gaps. On a windy day hold a strip of tissue near the joint between the sash and the frame, and along the ceiling trim. If it stirs, air is moving. A tight, well-seated seal shows no movement at all.

A thermal imaging camera shows all of this at once, with the cold bridges lit up as bright lines around the opening, but the hand, the eye and the tissue tell a good part of the story. Write down what you find. It helps at survey, and it shapes the specification.

Swapping the rooflight is the moment to fix all four paths together. It is also the moment when it is easiest to fix only one, and leave the others behind.

A sensible replacement works through the list in the same order as this guide:

1. New glazing with a low-E coating, a gas fill and a warm edge spacer, in a thickness the frame can carry.
2. A thermally broken frame, sized to the glass and the load.
3. An upstand insulated to meet the roof insulation, with no gap at the join.
4. Fresh seals and a clean, sealed junction between frame, kerb and ceiling lining, finished so they stay tight as the roof moves.

Some of these can be upgraded without a full strip-out, and some cannot. A sound frame with a failed glass pack may take a new sealed unit. A rotted kerb or a frame with no thermal break usually calls for the whole rooflight to come out. Our page on upgrading an old rooflight explains which is which, and whether a replacement rooflight has to meet current U-value rules covers the regulatory side.

The glass choice is also the moment to think about sound. A laminated inner pane adds safety glazing, which good practice asks for over a place where people stand, and it helps with rain and traffic noise. We write the build down in the quiet spec that comes with every quote, along with the seals and the upstand finish, so the warmth and the quiet are specified together. Our page on whether energy efficient rooflights are quieter too has the detail.

Daylight stays the point. A better specification does not have to cost light, since low-E coatings and gas fills are clear, and the surest way to keep a room warm and bright is to fix the edge, the frame and the kerb rather than shrink the glass.

A blind helps at night and does little for the frame, the kerb or the gaps. It is a worthwhile addition, but it is an extra, not a substitute for the build.

A close-fitting blind traps a layer of still air between itself and the glass, which slows both convection and radiation on a cold night. The better designs run in side channels so the air cannot circulate round the edges. A loose blind hung from a headrail does much less, because warm air slips past the sides. Either way, the blind does nothing for the cold reveal above it, and it may make the glass colder on the room side, which can bring condensation if the room is damp. Our answer on whether a thermal blind helps keep a rooflight room warm goes through the trade-offs.

These are the points that come up when a homeowner is deciding between keeping an old rooflight and replacing it.

Do skylights lose more heat than windows?

Per square metre, yes, for the reasons above: they face the sky, they sit where warm air collects, and they are fitted into a thick roof build-up that is hard to seal perfectly. A well-specified modern rooflight closes much of the gap. The regulations allow a higher U-value for rooflights than for walls, but the difference shrinks every time the glass, frame and upstand improve.

Will a new rooflight cut my heating costs?

It can reduce them, and the amount depends on what you are replacing and how big the opening is. The benefit of a good specification is clearest in a room that is used daily. Comfort also improves, because the cold face above you warms up. Our page on whether energy efficient rooflights lower heating bills gives a fair picture without promising a figure.

Is the glass or the frame losing the heat?

Often both, and the upstand or the gaps too. The hand test above separates them. Cold at the edge points to the spacer, cold along the frame to a missing thermal break, a line of mould on the reveal to the upstand, and moving air to the seals.

Can I reduce the heat loss without replacing the rooflight?

Partly. A blind, a better lining and insulation round the opening all help. Where the glass pack has failed or the frame has no thermal break, the gain from smaller steps is limited, and renewing the unit is the route that addresses the cause. A survey shows which case you have. You can get a quote and the answer goes into the written spec. The wider picture sits in our energy efficient rooflights hub.