Low e coating on rooflight glass: the invisible layer that keeps the heat in

Emissivity is a measure of how readily a surface gives off heat as infrared radiation. Ordinary float glass gives it off very readily, and a coating that lowers that figure keeps more of the heat inside the room.

Every warm surface radiates. A sheet of plain glass on a winter night is colder than the room, but it still takes up heat from the air and from the people and radiators inside, then passes a large share of it to the cold outer pane across the cavity and on to the outdoors. Clear float glass has an emissivity of around 0.84, which means it is an excellent radiator. A low-emissivity coating brings that figure down to somewhere between roughly 0.01 and 0.15, depending on the type. The coated pane still lets visible light through, because the layer is tuned to pass the short wavelengths of daylight while reflecting the long wavelengths of radiant heat.

Think of it as a one-way mirror for heat rather than for light. Heat radiating from the room meets the coating and is sent back into the room. Because the change is made on the glass surface itself, it adds no thickness, no weight worth mentioning and no visible frame or film. That is why it became the standard way to lift the thermal performance of sealed units without making them thicker or heavier.

The coating does not heat the room. It slows the loss. Everything in this guide follows from that one idea: anything that lets heat leave by another route, a cold spacer, a poor seal or a bare frame, will reduce the benefit of the layer.

Low-e glass is made by one of two processes, and the choice affects the emissivity, the handling and the price of the pane. Both end up as an invisible film on the glass.

A hard coat, also called a pyrolytic coating, is applied while the glass ribbon is still hot on the float line. The coating fuses into the surface, so it is tough enough to be handled and cut without special care. A soft coat, made by magnetron sputtering, is deposited in a vacuum chamber after the glass has been made. It is a stack of several very thin layers, often including silver, and it reaches the lowest emissivities. The price of that performance is fragility: a soft coat must be sealed inside the cavity of the unit, where nothing can touch it.

For a new rooflight or skylight in a house, the soft coat is the ordinary choice. Hard coats still turn up in older double glazing, in some single-pane products and in glass that has to be toughened or bent after coating, where a tougher film is needed. When we write a spec, it names the coating type and the face it sits on, so you can see the choice rather than trust a general phrase such as "energy efficient glass".

A pane that faces up loses heat differently from one that stands vertical. Two effects work against it, and the coating answers both.

The first is radiation to the sky. A wall window looks at the ground, at fences, at neighbouring houses. Those things are not warm, but they are far warmer than a clear night sky, which behaves as a very cold surface. A rooflight looks straight at that sky. The outer pane cools fast, sometimes well below the air temperature, and that is also why condensation can gather on the outside of a good unit on a still, clear morning. The coating cuts the radiant flow from the inner pane to the outer one, which is where much of the loss happens.

The second is convection inside the cavity. In a vertical unit, the gas in the gap rises along the warm pane and falls along the cold one, in a slow loop. Tilt the unit towards horizontal and the gas circulates differently and more freely, so more heat is carried across the gap. That is one reason a rooflight's thermal figure is always worse than a vertical window built from the same glass. It is explained further in our piece on how heat escapes through a rooflight, but the practical point is simple. A roof is the hardest place to keep glass warm, so it is the place where the coating earns the most.

There is a human side to this too. You do not stand next to a wall window with the coldest pane directly above your head. If a room has a skylight that feels cold in winter, the cold pane is radiating towards you as well as losing heat outwards. A warm inner pane is part of comfort, not just of the heating bill. Our answer on why a skylight makes a room feel cold looks at that from the room's side.

A sealed unit has four glass faces, numbered from the outside in. The position of the coating decides whether it saves heat or shades the room, so the number is written on the spec.

Surface 1 is the outside of the outer pane. Surface 2 is the face of that same pane that looks into the cavity. Surface 3 is the cavity face of the inner pane, and surface 4 is the inside face you can touch from the room. In a double unit for a UK home, a low-e coating for warmth normally sits on surface 3. There it faces the cold outer pane across the gap and sends the room's heat back where it came from.

The coating for controlling the sun sits elsewhere. A solar control coating generally goes on surface 2, so that it turns away solar energy before it crosses the cavity. The two jobs are related but not the same, and the layers are not interchangeable. Our guide to solar control glass for rooflights covers that choice, and it is worth reading if the room gets hot in summer as well as cold in winter.

In a triple unit, the layout changes. The two cavities make four cavity faces, and coatings are placed to keep the middle pane warm and the inner pane warmer. The extra pane adds weight and thickness, so it is a decision about the frame and the roof as well as the glass. We set that out in when a third pane earns its place.

A low-e layer is one part of a unit. Four other parts decide whether its benefit reaches the room, and each has its own page in the energy efficient rooflights hub.

The spacer bar around the edge holds the two panes apart. A metal spacer conducts heat across the edge, so the perimeter of the glass runs colder than the centre, and that is where condensation collects first. A warm edge spacer uses a low-conductivity material to cut that loss. The cavity itself is usually filled with argon, which is denser than air and moves less readily, reducing the heat carried across the gap. Both are covered in warm edge spacers and argon fill.

Then there is the frame. Aluminium carries heat as easily as it carries the glass, so a rooflight frame needs a thermal break, a strip of low-conductivity material between its inner and outer faces. The glass can have a fine figure in the middle while the frame lets the warmth out around it. Figures on a data sheet that quote only the glass at its centre flatter the whole unit, which is why the U-values explained guide separates the centre-pane number from the whole-unit one.

Finally, the structure around the glass. The kerb, the insulation and the airtightness of the junction between frame and roof all decide how much of the glazing's performance survives in the finished building. A fine unit on a bare, uninsulated upstand is a coated pane with a cold bridge round its edge.

1. Coating: cuts radiant loss from the inner pane.
2. Gas fill: cuts heat carried across the cavity.
3. Spacer: cuts loss at the edge of the glass.
4. Frame and upstand: cut loss around the glass.

The layer is clear, but it is not perfectly clear. It takes a small share of the light that would otherwise pass, and it can shift the colour of the view very slightly.

Light transmittance, the share of visible light that gets through the whole unit, falls a little with each coated pane. A modern soft coat is tuned to keep that loss low, and a typical coated double unit remains bright to anyone standing under it. You may notice a faint green or grey cast looking at the glass at a steep angle, and a slight change in reflections compared with plain glass. Put two samples side by side and the difference shows. Put one overhead in a kitchen and it is rarely the thing you remember.

Where light transmittance matters more is in a deep room with a single small rooflight. If the aim is to bring daylight into the back of a loft bedroom or a long hallway, the light figure on the spec is worth reading alongside the thermal figure. Our answer on what light transmittance means on a rooflight explains how to read it, and the related answer on the g-value shows how much solar heat the glass lets in.

Low-e for warmth and solar control for shade can be combined in one unit. The price of combining them is a little more lost light, and a room that gains heat in winter sun loses some of that free warmth. On a north-facing roof, the warmth coating alone is usually the right call. On a south-facing roof over a conservatory-style extension, the decision is closer and depends on how the room is used.

A quiet rooflight usually has a laminated inner pane, two sheets of glass bonded with a soft interlayer. That changes where and how the coating can go.

Rain on a flat pane is the noise most owners notice. Thicker, laminated glass damps it, and so does a mismatch between the two panes: using different thicknesses in the outer and inner glass makes the unit a poor conductor of certain vibrations. Laminated glass also holds together if it breaks, which is what good practice expects over a place where people stand. A laminated inner pane earns its place on that reason alone, and the quiet benefit comes with it.

The coating still sits on the cavity face of the inner pane, which with a laminated inner pane means the outer face of the laminate. The lamination does not alter the job the coating does, but the lamination adds weight and thickness, so the frame and the roof structure have to carry it. Our note on one glazing build for warmth and quiet walks through how the choices fit together, and the answer on whether acoustic laminated glass changes a rooflight's U-value deals with the thermal side of the same question.

This is why the written quiet spec that comes with a quote lists the coating, the pane thicknesses, the cavity, the spacer and the seals together. They are chosen as one build for the room, not as separate upgrades.

You will not find a clause in the regulations that says "use a low-e coating". What you find is a limit on heat loss, and the coating is how a double unit meets it.

Approved Document L sets maximum U-values for new and replacement glazing, with a separate figure for rooflights and roof windows, and these limits are written for the whole unit. A plain double-glazed unit without a coating will not reach them. A coated, gas-filled, warm-edged unit in a thermally broken frame will. The detail of what applies to a new extension and to a replacement is in Part L and rooflights, and we install to current Building Regulations, handling the Building Control notification where the work is notifiable.

Because the requirement is for the finished unit, a manufacturer's data sheet matters more than a headline about glass. The sheet should state the U-value for the whole rooflight, the coating type, the gas fill and the spacer. If a quote names none of these, the figure quoted for the glass tells you very little about the product in your roof.

A coating does not wear off a sealed unit, since it is protected inside the cavity. What fails is the seal around it, and once moisture gets into the cavity the unit is finished.

Soft coatings are vulnerable to moisture. If the seal of an old unit breaks down and damp air enters the cavity, the coating can be attacked from within and the glass clouds with condensation that cannot be wiped away. At that point the unit has to be replaced, and the replacement is a chance to specify the coating, gas and spacer properly instead of matching what was there. Older rooflights from before low-e was standard may have no coating at all, which is why swapping only the glass in an existing frame can give a clear gain in warmth if the frame is sound. Our guide on upgrading an old rooflight for energy efficiency looks at what the frame can and cannot support.

If you want the coating, gas fill, spacer and frame written out for your own roof, get a quote and the spec comes with it.

Does the coating wear off with cleaning?

No. In a sealed unit the coating faces the cavity, so cleaning the outside or the inside of the room does not reach it. Use ordinary glass cleaner on the faces you can touch. What shortens a coating's life is moisture entering through a failed seal, not a cloth.

Can I add a low-e film to my existing skylight?

Window films exist, but they change the glass surface you can reach, which is not where a factory coating sits, and they can cause heat stress in a sealed unit. They also cannot match a unit made with the coating inside the cavity. If an existing rooflight is cold, the reliable route is a new sealed unit or a replacement rooflight.

Is one coating enough on a rooflight?

For a double unit, a single soft coat on the cavity face of the inner pane is the normal build. A second coating is added in some triple units and in combined solar control builds. More layers lower the heat loss again but cost a little more light, so the extra layer needs a reason.

Will a coated rooflight look different from the outside?

Slightly, at certain angles. Coated glass can have a faint tint and a different reflection colour from plain glass, most visible on a bright day looking at the roof from the garden. From inside the glass reads as clear, and the tint is hardest to spot under an overcast sky.