Warm edge spacers and argon in a rooflight, the quiet details

A sealed glazing unit is two or three sheets of glass held a fixed distance apart by a spacer bar, with a seal around the rim and a gas in the gap. Four parts, and each has a job.

The glass does the obvious work. It carries the coating that reflects heat back into the room, and on a rooflight it is usually toughened on the outside and laminated on the inside, for reasons of safety that we come to later. The spacer is a hollow bar, bent into a rectangle, that runs round the edge of the unit. It keeps the panes at an even distance and holds a drying agent that soaks up any trace of moisture trapped at the factory. The seal is a primary layer against the spacer and a secondary layer of sealant outside it, and between them they keep water vapour out and the gas in. The gas fill sits in the cavity. Dry air is the plain version. Argon is the common upgrade.

Of these, the spacer and the gas are the two that people skip past. The coating gets the headlines, and the frame gets the photographs. Yet the spacer sits at the coldest line in the whole unit, and the gas fills most of its volume. Our guide to low-E coatings on rooflight glass covers the coating. This page stays with the edge and the gap.

Heat leaves a room by the easiest path it can find. In a sealed unit the easiest path is often not through the middle of the pane but round the edge, where a conductive spacer bridges the two sheets of glass.

Think of a winter night over a kitchen lantern. The outer pane is close to the air temperature outside. The inner pane is close to room temperature. In the middle of the unit, the coating and the gas stand between the two. At the rim, the spacer touches both panes. If the spacer is a good conductor, it carries heat from the inner pane across to the outer one, and the inner pane goes cold in a band a few centimetres wide all the way round. That band is the coldest surface in the room.

A cold band has three effects you can notice. It pulls the warmth out of the room locally, which makes the area under the rooflight feel draughty even with no air moving. It drops the surface of the inner pane below the temperature at which water vapour in the room air turns to droplets, so a line of condensation appears along the edge. And over a long run, it can leave a dark line of mould on the sealant or the reveal if the moisture sits there for weeks. It is the same mechanism that we describe in cold bridging around a rooflight, only here the bridge is inside the glass unit instead of in the roof build-up.

This is why a good centre-pane figure on a data sheet can flatter a unit. The middle of the glass may be excellent while the rim is not. The edge matters more on a small rooflight than on a large one, because the rim is a bigger share of the total area. Our answer on the difference between a whole-unit and a centre-pane U-value explains how the two figures are measured and why the whole-unit one is the one to compare.

A warm edge spacer is a spacer bar made from a material that carries heat poorly, or built so the heat has a longer and narrower route to travel. The aim is a warmer rim and less condensation along the edge.

The older standard was a hollow aluminium bar. Aluminium is light, stiff and cheap, and it conducts heat very well, which is exactly the wrong quality at the coldest line of a window. Warm edge spacers replace it with one of a few alternatives:

• A bar of rigid plastic or composite, often with a thin metal foil on the gas side to keep the seal tight.
• A flexible foam bar, where the spacer and the drying agent are one continuous silicone-based strip.
• A thin stainless steel bar. Stainless conducts heat far less than aluminium, and a thin wall makes the path narrower still.

The technical measure of the difference is the linear thermal transmittance of the edge, written with the Greek letter psi. It is the extra heat loss per metre of edge, over and above what the glass and frame alone would lose. A lower psi means a warmer rim. You will meet the figure on manufacturer data, and it feeds into the whole-unit U-value calculation for a given rooflight size. Nothing on your quote needs you to do the maths. What matters is that the quote names the spacer type, so you know which one is in the unit.

The effect is modest in isolation. It tightens the whole-unit figure a little, and warms the rim by a margin you can feel with a hand on a cold morning. What it does well is remove the worst edge behaviour. In a room with kitchen steam, a shower nearby or a bedroom that is slept in with the door closed, the edge is where moisture lands first. A warmer edge keeps it off the glass.

Heat crosses the gap between two panes in two ways: by the gas moving in slow loops, called convection, and by the gas conducting heat from molecule to molecule. A heavier gas that conducts less slows both.

Air is a mix of light molecules. Argon is an inert gas that makes up a small fraction of the air around us, and it is heavier and a poorer conductor of heat. Fill a cavity with it and the currents inside the unit move more slowly, and less heat passes across. Argon is also colourless, odourless, non-toxic, and inexpensive to extract from the air, which is why it became the standard filling for energy efficient glass. You will see it written on a specification as a percentage fill, because the factory cannot displace every trace of air. A good unit is filled to a high percentage, checked at the factory, and sealed.

An argon filled skylight behaves better than an air filled one, and the gain is steady rather than spectacular. It improves the centre of the glass, and it works together with a low-E coating, because the coating cuts the radiated heat and the gas cuts the conducted and convected heat. One without the other leaves a gap in the performance. The two together are how modern glass gets its figures. If you want the gas explained from the other direction, our answer on what argon does inside a rooflight is the shorter read.

There are heavier gases again. Krypton conducts heat more slowly than argon and suits very narrow cavities, where argon has less room to work. It costs considerably more, and it is used mostly in slim triple units or where a thin frame demands a thin cavity. For a typical Crawley kitchen lantern or loft rooflight, argon is the sensible gas. The wider picture sits in our energy efficient rooflights hub.

A gas fill has a best cavity width, and going beyond it makes the unit worse. That surprises people who assume a wider gap always insulates more.

In a narrow cavity the gas cannot circulate freely, so convection is small and conduction across the gap dominates. Widen the cavity and conduction falls, but the gas now has room to loop from the warm pane to the cold one, and the convection carries heat across. The two effects trade off, and the lowest heat transfer sits at a particular width. For argon that width is a little narrower than for air. Beyond it, more gap gives no gain and can make things slightly worse.

This is why a glazing specification lists the build as a string of figures: the outer pane thickness, the cavity width, the inner pane thickness, the coating position, the gas and the spacer. A rooflight unit with a good gap, argon, a coating and a warm edge is a designed unit. One with a wide gap for the sake of it is not. The numbers on the quote should describe one coherent build, and the installer should be able to say why the cavity is the width it is.

A written glazing specification folds all this into one line. Once you know the pieces, the line is easy to read, and you can compare two quotes on the same terms.

A quote that gives only a centre-pane figure and says nothing of the edge or the gas has left out half the story. A quote that names every line lets you compare honestly. Our page on rooflight U-values explained shows how the individual figures combine, and the quiet spec we write for every quote sets the glazing build down in full for your room, so nothing is left to a line that says "double glazed".

A warmer rim is the cure for one of the most common complaints about rooflights: the line of moisture that forms round the edge of the inner pane on cold mornings.

Condensation on the inside of a window is not a fault with the seal. It is water vapour from the room, meeting a surface colder than the dew point. Cooking, washing, drying clothes and breathing all add vapour to the air. When it meets cold glass, it condenses. A warm edge raises the temperature of the rim, so the point where droplets form moves out towards the frame or disappears. Good ventilation does the rest.

It helps to tell this apart from a failed unit. Condensation on the room side of the glass wipes away and comes back in cold weather. A unit that has lost its seal shows a fog or a film between the panes that cannot be wiped, because the moisture is inside the cavity. That is a different problem, and the answer is to replace the sealed unit or the whole rooflight. Our page on misted and failed rooflight replacement covers that case.

An old unit with an aluminium spacer and an air fill is the typical candidate for edge condensation. When we are replacing one, the new build addresses the edge and the gap together, and the written spec records it.

No sealed unit holds its gas for ever. A very small amount escapes through the seal over the years, and air diffuses in slowly to take its place.

That is a slow process in a well-made unit, and the performance drifts down rather than dropping off. The quality of the seal, and of the spacer that carries it, is the main factor. A rigid spacer that flexes under the thermal movement of a roof, which heats and cools more than a wall does, puts more strain on the seal than one that moves with the glass. This is one reason the spacer choice matters beyond the heat loss: it is part of what keeps the gas in, and the moisture out, for the life of the unit.

A rooflight works harder than a wall window. It faces the sky, so it takes the sun at midday and the full cold at night, and it sees rain, frost and snow sitting on the glass. The seal is under more stress, and a good unit is built for it. The 10-year workmanship guarantee we give covers our installation, and the manufacturer stands behind the glass unit under its own terms, which the quote sets out.

Building Regulations do not ask for argon by name, and they do not ask for a warm edge spacer by name. They set a limit on how much heat the rooflight as a whole may lose, and the build has to meet it.

Approved Document L sets maximum U-values for rooflights in new extensions and for replacement elements in existing homes. The requirement is on the performance of the finished unit, measured or calculated for the whole rooflight. How the manufacturer reaches that figure is their choice: a coating, a gas, a spacer, a thicker frame or a third pane. In practice, meeting the current limits with a rooflight means using most of these details together. A plain air-filled double unit with an aluminium spacer would be unlikely to get there on its own.

Our page on Part L and rooflights sets out what the energy rules ask for, and the answer on whether a replacement rooflight has to meet current U-value rules deals with the replacement case. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, so the compliance paperwork is not left for you to chase.

The warmest edge spacer in the world cannot rescue a frame that conducts heat straight through. The unit sits in a frame, and the frame has its own route for heat.

A rooflight with a warm edge spacer in an unbroken aluminium frame puts a cold edge back in at the next layer. The glass rim may stay warm, but the frame behind it carries the cold through. This is why the better lantern and flat rooflight frames include a thermal break, a strip of low-conducting material that separates the inside face of the frame from the outside one. Our page on thermally broken rooflight frames explains how the break works, and it is the natural partner to a warm edge spacer. One treats the rim of the glass. The other treats the frame that holds it.

Below the frame sits the upstand, and that needs insulating too. A well-built kerb with insulation inside it carries the benefit of a good unit right down to the ceiling. Our guide to insulating around a rooflight goes through the build-up.

Only a little, and it is worth being plain about that. Warmth and quiet both come from the same glazing build, but they do not come from the same parts of it.

Sound crosses a sealed unit in a different way from heat. What slows it down is mass and the mismatch between layers: thicker glass, panes of different thickness, and a laminated pane that damps the vibration. The gas in the gap has little to do with it, and a warm edge spacer has very little to do with it either. So an argon filled skylight is not a quiet skylight by virtue of the argon. The quiet comes from the pane thickness, the laminated inner pane, the seals and the upstand.

What the two do share is the unit. A build that uses different pane thicknesses and a laminated inner layer can carry a coating, argon and a warm edge at the same time, so you do not have to choose between a warm room and a hushed one. Our page on one glazing build for warmth and quiet shows how the pieces go together, and the answer on whether acoustic laminated glass changes a rooflight's U-value deals with the trade-off from the quiet side.

In Crawley that combination is easy to justify. A loft bedroom under the Gatwick approach, a kitchen lantern in a Three Bridges extension and a flat rooflight over a home office each need warmth and quiet from the same hole in the roof, and each deserves a glazing build chosen for both.

The benefit of a warm rim and a gas fill is largest where the glass is small, the room is humid, or the room is used in the coldest hours.

• Loft bedrooms. A cold band of glass over a bed is felt at night, and the breath of a sleeping person adds moisture to the air. A warm edge keeps the rim dry, and the gas fill keeps the pane closer to room temperature.
• Kitchens and bathrooms. Steam is the main source of edge condensation. A warm edge spacer rooflight over a cooker or a shower is the detail that keeps the glass clear.
• Small flat rooflights and roof windows. On a small unit the rim is a large share of the area, so the spacer has a bigger effect on the whole-unit figure.
• Large lanterns. The centre of each pane dominates, so the gas and coating matter most, but the long run of spacer round many panes adds up.

On a south-facing rooflight that gets strong sun, the question turns to overheating, not heat loss, and there the answer is solar control glass and not more insulation. Our page on solar control glass for rooflights covers that side.

When we survey a room, the glazing build goes on paper before the order goes in. It names the spacer, the gas and the cavity, alongside the noise we listened for and the seals and upstand we propose.

That matters because the details in this guide are invisible. Once the rooflight is in, nobody can look at the rim and tell whether it is aluminium or composite, or test the cavity for argon by eye. The only place they exist is on the specification. We write the spacer type, the gas and the coating into the quiet spec for that room, so you can check the unit against the paper, and the paper against any other quote you hold.

If you are replacing a rooflight and want to know what the new build would do differently from the old one, get a quote and ask us to set the old and the new side by side. You can also message us on WhatsApp with a photo of the room.

These come up when a quote mentions a warm edge or argon and the homeowner wants to know what they are paying for.

Is a warm edge spacer worth having on a small rooflight?

Yes, and more so than on a large one. The edge is a bigger share of a small unit, so the spacer has a larger effect on the whole-unit figure and on whether the rim mists on a cold morning.

Can I tell by looking whether a unit has argon?

No. Argon is invisible and leaves no mark on the glass. The only proof is the specification and the manufacturer's label, which is why we put the gas on the written spec.

Will the gas leak out and make the unit useless?

A sealed unit loses gas very slowly, and the performance drifts rather than failing. A good seal and a spacer that moves with the glass are the things that slow the loss. A unit that fogs between the panes has lost its seal and needs replacing.

Does argon help with noise from aircraft or rain?

Not in any way that you would specify for. Quiet comes from pane thickness, a laminated inner pane, the seals and the upstand. Argon belongs on the warmth side of the specification, and sits comfortably beside the quiet build. To see how the costs fit together, our page on rooflight and skylight costs in Crawley explains what drives them.