Low-e coatings, argon and warm-edge spacers

An insulated glass unit, the part of a rooflight that does the glazing work, is two or three panes of glass held a set distance apart with the gap sealed shut. That gap is the insulator. Air, or better still a heavier gas, conducts heat far more slowly than glass does, so most of the resistance to heat loss lives in the cavity rather than in the panes themselves.

Three components turn that cavity from an ordinary air gap into a genuine thermal barrier. The first is the low-emissivity coating, a metallic film applied to one of the glass faces inside the sealed space. The second is the cavity fill, either dried air or an inert gas such as argon. The third is the spacer bar, the frame that sets the gap width and carries the seal that keeps the gas in and moisture out. Each one is a lever. Pull all three and a basic double-glazed unit at 2.7 watts per square metre per degree becomes a good one at 1.2 or better.

None of these are visible once the unit is glazed into a frame and sitting in your roof. That is exactly why they get skimped. You cannot look up at a finished rooflight and see whether the cavity holds argon or plain air, whether the spacer is aluminium or a warm-edge composite, or which grade of coating is on the glass. The only place that information exists is on the manufacturer’s data sheet, and the only way to get it is to ask before you buy.

Emissivity is a surface property. It describes how readily a material gives off heat as long-wave infrared radiation. A plain sheet of glass has an emissivity of around 0.84, which means it is very good at radiating heat away from its warm inner face into the cold cavity, and from there towards the outside. That radiated heat loss is a large share of the total, larger than most people expect, because two panes of glass facing each other across a gap trade heat by radiation as well as by conduction through the trapped air.

A low-emissivity coating is a microscopically thin layer, usually based on silver, laid onto the glass to drop that surface emissivity right down, to somewhere around 0.03 to 0.10 depending on the grade. A surface that will not radiate heat cannot lose it that way. The warm inner pane keeps its heat instead of throwing it across the cavity, and the room stays warmer for the same energy. The coating is transparent to your eye, so the view and the daylight are barely affected, though a very slight tint is sometimes visible at an angle on the highest-performance grades.

There is a second job the coating does, and it depends on which type you specify. A basic low-E film mostly targets the long-wave heat your home generates. A solar-control low-E film is tuned to also reflect the near-infrared coming in from the sun, the invisible part of sunlight that carries most of its heat. That distinction matters enormously on a south-facing roof, and it is the point where a coating stops being only about winter warmth and starts being about summer comfort. More on that below.

Low-emissivity coatings come in two manufacturing types, and the difference shows up in both performance and price.

Hard coat, also called pyrolytic low-E, is applied while the glass is still hot on the production line, so the metallic oxide fuses into the glass surface as it cools. The result is durable and can be handled, cut and even used as a single-glazed surface. Its emissivity is moderate, typically around 0.15 to 0.20, so it improves the U-value but not dramatically. Hard coat sometimes carries a faint haze.

Soft coat, also called sputtered low-E, is applied to cold glass in a vacuum chamber, laying down multiple ultra-thin layers including one or two of silver. It reaches emissivity figures down around 0.03 to 0.05, so it is the higher performer by a clear margin. The trade-off is that the silver layers oxidise if exposed to air, which is why soft coat has to sit inside a sealed cavity and cannot be used on an exposed surface. It is the standard choice for a quality sealed unit, and it is what you want on a rooflight where the U-value has to satisfy Building Regulations.

For almost every rooflight and roof window installed today, the coating in question is a soft-coat solar-control low-E on the outer pane, sometimes paired with a second low-E layer on an inner pane in a triple-glazed build-up. When a quote says “low-E glass” with nothing further, it is worth asking which family and which grade, because the gap between a modest hard coat and a high-specification soft coat is the difference between meeting the regulations and comfortably beating them.

Fill the cavity with a heavier, slower gas than air and you cut the heat that conducts and convects across the gap. Air is mostly nitrogen and oxygen, both relatively light and mobile. The noble gases are denser, move more sluggishly and carry heat less readily, so they suppress the little convection currents that otherwise circulate inside the cavity and ferry heat from the warm pane to the cold one.

Argon is the workhorse. It is about 34 per cent less conductive than air, it costs very little, it makes up nearly one per cent of the atmosphere so there is no supply problem, and it is completely inert and harmless. A typical sealed unit filled with argon rather than air improves its U-value by roughly 0.2 to 0.3 watts per square metre per degree, which is a meaningful step for almost no extra money. For that reason argon fill is close to standard on any unit worth buying, and its absence on a modern quote is a warning sign.

Krypton is the premium option. It is denser again, performs best in a narrower cavity of around 9 to 12 millimetres, and is used mainly in triple-glazed units where three panes leave two thin gaps that argon cannot fill as efficiently. Krypton is expensive because it is scarce, so it turns up in low-energy and passive-standard builds rather than in a standard extension rooflight. Xenon exists too, performs better still, and is priced out of ordinary use entirely.

The gas does not stay put forever. A sealed unit loses a small percentage of its fill every year through the seals, and the industry design assumption is that a well-made unit will hold the great majority of its gas across a couple of decades. A poorly made one loses it far faster, and once the fill has largely gone the cavity is back to air performance while still looking identical from below. That longevity is set almost entirely by the third component.

The spacer bar is the frame running around the perimeter of the sealed unit, holding the panes apart and carrying the primary and secondary seals that keep gas in and water vapour out. It sounds like a minor part. It is not. It governs two things that matter a great deal: how cold the edge of your glass runs, and how long the whole unit survives.

For decades the standard spacer was a hollow aluminium bar filled with a moisture-absorbing desiccant. Aluminium is an excellent conductor, which is a virtue in a saucepan and a defect in a window. It forms a thermal bridge straight across the sealed edge, short-circuiting all the careful insulation the coating and the gas provide, but only at the rim. The result is a cold band around the perimeter of the glass where the inner pane runs several degrees colder than its centre. That cold edge is where condensation forms first, where mould takes hold in the reveal, and on a rooflight it is the strip most likely to bead with water on a cold morning.

A warm-edge spacer replaces the aluminium with a material that conducts heat far less readily: stainless steel, a structural foam, or a thermoplastic composite. The edge of the glass stays warmer, often by several degrees, and that single change pushes the point at which condensation appears down to colder, rarer conditions. It also lifts the whole-unit U-value slightly, because the perimeter is no longer a thermal leak. The table below shows the order of the effect.

On a rooflight this matters more than on a wall window, because a rooflight sits over the warm, moist air that rises off a kitchen or bathroom and collects at ceiling height. That air meets the coldest available surface, which is the edge of the glass, and if the spacer is running cold it will condense there first. A warm-edge spacer is not a luxury on overhead glazing. It is the difference between a unit that stays dry through an Essex winter and one that greets you with a wet rim on the frosty mornings.

The spacer also carries the seal, and the seal is what determines how long the argon stays in and the desiccant keeps the cavity dry. A quality warm-edge system with a good secondary seal is the reason a sealed unit lasts twenty years rather than eight. When a rooflight “blows”, misting up permanently between the panes with a haze you cannot wipe off, the seal has failed and moisture has got into the cavity. That is not something to live with. A failed sealed unit is replaced, and fitting a new unit into the existing frame is exactly the kind of replacement work we do.

None of these components works in isolation. The published U-value of a rooflight is the sum of what all three do together, plus the frame around them, and you can see the contribution of each by watching the number move as you add them.

Read the first two rows and you see the coating doing the heaviest lifting: a whole watt of improvement from one invisible film. The gas fill takes off another 0.3 or so. The warm-edge spacer trims the last couple of tenths and, more importantly, warms the rim so the improved centre-pane figure is not undone by a cold, wet perimeter. The jump to triple glazing buys the last stretch down towards passive-house territory, at a weight and cost penalty that only makes sense on the coldest or most demanding roofs.

Every rooflight installation is notifiable under Building Regulations, and Part L sets a maximum U-value that a new or replacement unit in a home has to meet. Any full-specification double as described here clears that bar comfortably, which is why the middle rows of that table describe the sensible default for most south Essex work rather than the top of the range. We make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the job, and the glazing has to stand up on paper for that notification to pass.

Everything above is the winter story: keeping heat in. On a south-facing roof there is a second story that matters just as much, and it turns on the type of low-E coating rather than the gas or the spacer.

A basic low-E coating is optimised to reflect the long-wave heat your home produces back into the room. It does very little about the short-wave and near-infrared radiation arriving from the sun. Put that coating on a rooflight facing straight up at a July sky and you have a unit that keeps the room warm in winter and then lets the summer sun pour in on top of you. A solar-control low-E coating is a different tuning of the same silver-based technology: it still reflects your indoor heat back inwards, and it additionally reflects a large share of the sun’s near-infrared before it enters the glass. The daylight still comes through, the invisible heat mostly does not.

This is the whole reason we survey the roof before we specify a unit. Southend-on-Sea faces south across the Thames Estuary, an unusual aspect for an English seaside city, with an open horizon and reflected light coming off the water onto every south-facing roof plane along the cliff and the seafront. A flat rooflight faces straight up and collects far more midsummer sun per square metre than any wall window. On those planes, the coating is not chosen for its U-value alone. It is chosen for its solar-control performance, because a room under unshaded south-facing glass can become the hottest in the house between early afternoon and evening. That is the argument our energy-efficient glazing work is built around: orientation first, then the glass specification, then the unit.

North-facing planes are the opposite case. There is no direct beam to control, every scrap of daylight is worth having, and a plain low-E coating with a high light transmittance is the right answer. On a house with a roof lantern over a south-facing extension and rooflights on a north slope above the same building, the two units should not carry the same glass. Specifying one coating across a whole roof means someone has not looked at the compass.

You do not need to become a glazing engineer to buy the right unit. You need to ask five questions and expect a straight number for each.

1. “Is the cavity argon filled?” The answer should be yes without hesitation. Argon costs almost nothing and its absence tells you the unit is built to a price rather than a specification.
2. “Is the low-E coating soft coat, and what is its emissivity?” You want soft coat, with an emissivity down around 0.05, not a modest hard coat quoted simply as “low-E”.
3. “Is it a warm-edge spacer?” On overhead glazing this is the component that keeps the rim dry through winter. An aluminium spacer on a rooflight over a kitchen is asking for a wet edge.
4. “Is the U-value centre-pane or whole-unit?” Centre-pane figures ignore the spacer and the frame and always flatter the glass. Whole-unit values are the honest comparison and the ones Building Control assesses.
5. “On the south-facing planes, what is the solar-control performance?” If the coating is only a basic low-E, the winter is covered and the summer is not. Ask specifically about the sun-facing units.

Every reputable manufacturer publishes a data sheet with these values calculated to the relevant British and European standards. Asking for it costs nothing and tells you a great deal about who you are dealing with. If the numbers come back cleanly, you are talking to someone who specifies glass rather than just orders it. If they do not, that is your answer about the rest of the job.

We have been installing skylights and rooflights across south Essex for over fifteen years, every installation backed by a ten-year workmanship guarantee and notified to Building Control on your behalf. The survey is where it starts, because the coating, the gas and the spacer only earn their keep once someone has worked out which way each roof plane faces and matched the specification to it. If you want those numbers written down for your own roof, on a pitched slope or a flat extension, ask us for a full specification with the U-value, the coating and the spacer set out for each plane separately.