Low emissivity coatings and argon fills

Low emissivity coating

A microscopically thin metallic oxide layer on one of the internal faces of the sealed unit. It is transparent to visible light and reflective to long wave infrared, which is the wavelength at which warm objects radiate heat.

So daylight comes in, and the heat your radiators put into the room is reflected back rather than radiating out through the glass. It roughly halves the heat loss of a double glazed unit and costs very little.

It goes on the inner pane, facing the cavity, where it is protected from cleaning and weather. Ordinary glass cleaner will not damage it because you never touch it.

Argon fill

The cavity is filled with argon rather than air. Argon is denser and conducts heat less readily, and it convects less within the cavity.

Worth roughly 0.2 to 0.3 on a U-value for almost no cost. Krypton performs slightly better in narrow cavities and costs considerably more; it is rarely worth it in a rooflight.

Does argon leak out

Slowly. A well made unit loses perhaps one per cent a year, so after twenty years it has lost a fifth and performance has dropped a little.

That is not the failure that matters. When the seal fails properly, the argon goes and moist air replaces it, and the unit mists. At that point the argon is the least of it.

Together

Low emissivity plus argon takes a plain double glazed unit from around 2.8 to around 1.4. Both are standard on any decent modern rooflight and neither is worth treating as an upgrade to be negotiated away.

What is inside a sealed unit

A modern rooflight glazing unit is two panes of glass separated by a cavity, held apart by a spacer bar around the perimeter, sealed at the edge, with the cavity filled with an inert gas and one of the internal glass surfaces carrying a microscopically thin metallic coating.

Three of those components do the insulating: the coating, the gas and the spacer. Understanding what each contributes explains why a modern unit performs three times better than one from the 1990s, and why the specification lines on a quotation matter.

How the coating works

Heat crosses a sealed unit three ways: conduction through the glass, convection within the gas, and radiation across the cavity. Radiation is the largest of the three, and the coating is what deals with it.

Emissivity is a measure of how readily a surface radiates heat. Ordinary glass has high emissivity: warmed by the room, it radiates that heat freely across the cavity to the outer pane, which sheds it outside.

A low emissivity coating is a transparent metallic layer, usually silver based and only nanometres thick, applied to one of the cavity facing surfaces. It reflects long wave infrared back into the room rather than letting it radiate away, while passing visible light through almost unchanged.

The effect is substantial. A low emissivity coating on its own accounts for the majority of the difference between a modern unit and a plain double glazed one.

Which surface it sits on matters. On a coating designed for insulation it faces the cavity from the inner pane and reflects room heat back inward. Solar control coatings work on a related principle in the opposite direction and reflect incoming solar infrared outward, and some products combine both functions.

The coating is inside the sealed cavity because it would not survive on an exposed surface. That has a consequence worth knowing: when a sealed unit fails and moisture gets in, the coating begins to degrade, which is what produces the permanent bloom that does not clear on a warm dry day.

Why argon rather than air

Argon is denser than air and less thermally conductive, and being a monatomic inert gas it convects less readily within the cavity. Replacing the air with argon therefore reduces both the conduction and the convection components of heat transfer.

The improvement is real and smaller than the coating’s: typically a modest reduction in the U-value. It is standard rather than an upgrade on any current unit, and there is no reason to specify a unit without it.

Krypton performs better again and costs considerably more, and it is used mainly where a very narrow cavity is required for a slim sightline. On domestic rooflights it is rare and rarely justified.

The important point about argon: it escapes slowly through the perimeter seal over the life of the unit, and rapidly once the seal fails. A unit that has misted has lost its argon, which is why a failed unit performs like plain double glazing long before anybody notices the misting. The two symptoms have the same cause and arrive at different times.

Cavity width

There is an optimum, and wider is not always better. Too narrow and conduction across the gap dominates. Too wide and the gas convects freely within the cavity, carrying heat from the warm pane to the cold one.

For argon that optimum is around 14 to 16mm, which is why most sealed units sit in that range. A very slim unit in a conservation frame trades some performance for the sightline, which is a deliberate choice rather than an error.

The spacer bar, which is where it comes undone

All of the above describes the middle of the pane. At the perimeter, the spacer bar connects the two panes directly.

A traditional aluminium spacer conducts heat extremely well, creating a thermal bridge around the entire edge of the unit. Heat travels along it from the warm pane to the cold one, and the glass edge runs several degrees below the centre.

The visible result is condensation forming as a band around the edge of the pane on cold mornings while the middle stays clear. It is very common on units fitted before the mid 2000s, it is regularly mistaken for a failed sealed unit, and it is nothing of the kind.

A warm edge spacer, made of thin walled stainless steel, a structural foam or a composite, conducts far less. It improves the whole unit U-value by around 0.1, which is modest, and it largely removes the edge condensation, which is the thing you actually notice.

On a rooflight this matters more than on a wall window, because a rooflight faces the cold night sky directly and its edges get colder.

The desiccant

Inside the hollow spacer bar sits a desiccant, which absorbs the small amount of moisture inevitably sealed into the cavity at manufacture. That is why a new unit is clear rather than misted.

When the perimeter seal begins to fail, the desiccant absorbs the moisture entering the cavity until it saturates. Only then does condensation start to appear. That delay is why a unit can go from apparently fine to visibly misted in a single winter, years after the seal actually went.

What all of it adds up to

A current double glazed rooflight with a low emissivity coating, argon fill and a warm edge spacer lands at a whole unit U-value of around 1.2 to 1.4.

A unit fitted twenty five years ago has none of the four: no coating, no argon, an aluminium spacer and no solar control. Its whole unit figure might be 3.0 or worse.

That is the honest case for replacing an old rooflight, and it is why the room feels different immediately rather than marginally.

What to specify

Low emissivity coating and argon fill, which are standard and should be stated. A warm edge spacer, which should also be standard and is worth confirming because it is the one that shows.

Solar control where the unit faces south or west, and nothing extra where it faces north.

And the whole unit U-value in writing rather than the centre pane figure, because centre pane describes precisely the part of the unit this page has been explaining and excludes precisely the part where the heat gets out.

What a low emissivity coating does

Heat moves through a sealed unit three ways: conduction through the materials, convection in the cavity, and radiation across it. Radiation is the largest of the three in a modern unit, and the coating addresses it.

Emissivity describes how readily a surface radiates heat. Ordinary glass has high emissivity, so a warm inner pane radiates energy across the cavity to the cold outer pane, which sheds it outside.

A low emissivity coating is a microscopically thin metallic layer that reflects long wave infrared. Heat radiated from the room is bounced back into the room rather than crossing the cavity.

It is invisible in normal use and it makes a substantial difference to the U value.

Which surface it sits on

The surfaces of a double glazed unit are numbered from outside in. A coating for retaining heat sits on the cavity face of the inner pane, so it reflects room heat back inward.

A solar control coating sits on the cavity face of the outer pane, so it reflects incoming solar energy before it enters the cavity.

Some units carry both, on different surfaces, which is what a good rooflight for a south facing slope should have: heat kept in during winter, solar energy kept out during summer.

The coatings are always inside the cavity, protected from cleaning and weather. A coating on an exposed surface would not survive.

Why the cavity is filled with argon

Argon is denser than air and less thermally conductive, so it conducts less heat across the cavity and convects more sluggishly.

The convection point matters particularly in a rooflight. In a unit tilted towards horizontal, the gas in the cavity circulates more freely than it does in a vertical window, which is one reason the same glass specification performs slightly worse in a roof. A denser, more viscous fill reduces that circulation.

Krypton performs better still and costs considerably more, and it is normally reserved for narrow cavities where argon has too little room to work.

Does the gas leak out

Slowly, yes. A well made unit loses a small percentage a year, which over a service life is not enough to matter.

What matters is seal failure. When the perimeter seal goes, the fill is lost quickly and the cavity fills with humid air. That is the point at which the unit mists, and it is why misting also means the thermal performance has gone even before the appearance becomes a problem.

Where these sit in the priority list

On a rooflight, behind the warm edge spacer and behind the perimeter detailing on site.

Coatings and gas fills improve the centre of the pane. On a small unit the centre of the pane is not where the loss is. Get the spacer, the insulation collar and the reveal right first, then take the best coating you can within that.