Warm edge spacers and where condensation starts

The problem with aluminium

Traditional spacers are aluminium, which conducts heat extremely well. That creates a direct thermal bridge between the inner and outer panes around the entire perimeter of the unit.

Heat takes that route, and the result is that the inner pane is measurably colder within about 50mm of the edge than it is in the middle.

What that looks like

Condensation forming as a band around the edge of the glass on cold mornings, while the centre stays clear. It is the classic signature of an aluminium spacer, and people frequently assume the unit has failed.

It has not. Misting on the room side at the edges is a cold surface meeting humid air. Misting between the panes is a failed seal, and that is a different problem.

Warm edge

A spacer made of stainless steel, structural foam or a composite, all far less conductive than aluminium. The glass edge runs several degrees warmer, and edge condensation largely disappears.

It also improves the whole unit U-value by around 0.1 to 0.2, which is worth having but is not the main point. The main point is that the edge stays clear.

Why it matters more overhead

Warm humid air rises. A rooflight over a kitchen or bathroom sits in the wettest air in the building, so the dew point is reached at the coldest surface sooner than it would be on a wall window.

In those rooms a warm edge spacer is not an optional refinement.

Checking

Ask what spacer the quoted unit uses. Most decent manufacturers fit warm edge as standard now; the cheapest units still do not.

The band of water that is not a failed unit

A very common call: condensation forming as a band around the edge of a rooflight pane on a cold morning, while the middle of the glass stays perfectly clear.

People reasonably assume the sealed unit has gone. It has not. What they are looking at is the signature of an aluminium spacer bar, and it is a characteristic of the glazing rather than a fault in it.

Understanding why is worth a few minutes, because it explains a good deal about how rooflights perform and it changes what you should specify.

What the spacer bar does

A sealed unit is two panes of glass with a gas filled cavity between them. Something has to hold them apart at a consistent distance around the whole perimeter, contain the desiccant that absorbs residual moisture, and carry the perimeter seal.

That is the spacer bar. It is a small component doing three jobs, and traditionally it was made of aluminium because aluminium is easy to form, dimensionally stable and cheap.

It also conducts heat extremely well, and that is the problem.

Why aluminium produces edge condensation

An aluminium spacer connects the inner pane directly to the outer pane around the entire edge of the unit. Heat travels along it from the warm side to the cold side, continuously.

The consequence is that the glass at the edge runs several degrees colder than the glass at the centre. On a cold morning the edge drops below the dew point of the room air while the centre stays above it, so water condenses in a band around the perimeter and nowhere else.

That is exactly what you see. It is the thermal bridge made visible.

Why it shows up more on rooflights

Two reasons, and they compound.

A rooflight faces the sky. On a clear night it radiates toward the coldest thing available and its surface temperature drops well below the air temperature. Its edges therefore get colder than a wall window’s edges do, which is also why frost forms on rooflights first.

Warm moist air collects at the ceiling. The air touching a rooflight is the warmest and wettest in the room, so it has the highest dew point. Cold glass and wet air is the combination that produces condensation.

On a wall window the same aluminium spacer produces the same effect and less visibly.

What a warm edge spacer is

A spacer made from a material that conducts far less: thin walled stainless steel, a structural foam, or a composite.

It does exactly the same mechanical job, holding the panes apart and carrying the seal, and it runs several degrees warmer at the glass edge.

Two results follow. The whole unit U-value improves by around 0.1, which is modest. And the edge condensation largely disappears, which is the thing you actually notice.

The second is worth more to most people than the first.

How to tell what you have

Look at the perimeter of the glass from inside, at the strip visible between the two panes.

A bright silver metallic bar is aluminium. A dark grey, black or dull finish is usually a warm edge type.

On any unit fitted before the mid 2000s, aluminium is very likely. If a rooflight shows a band of edge misting on cold mornings and the centre stays clear, that is your answer without needing to look.

The four kinds of water at a rooflight

Worth setting out together, because telling them apart saves a great many unnecessary callouts.

Even condensation across the inner face of the glass, on cold mornings, unrelated to rain. Ordinary room condensation. Not a fault, and the answer is ventilation rather than glazing.

A band at the edge of the pane with a clear centre. An aluminium spacer. Not a fault, and only a new unit changes it.

Moisture between the panes, unwipeable from either face. A failed perimeter seal, which is end of life at fifteen to twenty five years rather than a defect.

Water on the plaster reveal rather than the glass. This one usually is an installation fault, and the cause is almost always a missing insulation collar leaving the reveal cold enough to condense.

Only the fourth needs anybody to come and put something right, and it does not involve the glazing at all.

Can a spacer be changed

No. It is inside a sealed unit, and a sealed unit is a manufactured assembly rather than a serviceable one.

Where an existing window shows edge condensation and it bothers you, the answer is a new sealed unit or a new window, not a treatment.

Whether that is worth doing depends on the age of the frame. Under fifteen years and sound, reglazing makes sense where the model allows it. Over twenty, replace the window, because most of the cost is the access rather than the component and new glass in a worn frame buys the shorter of two lifespans.

What to specify

A warm edge spacer, and it should be standard rather than an upgrade on any current unit from a reputable manufacturer.

Where a quotation does not mention it, ask. Where a quotation is noticeably cheaper than the others, this is one of the places worth checking, because it is exactly the kind of specification line that comes off a price under pressure.

It sits alongside the other two questions that matter more than headline numbers: the whole unit U-value rather than centre pane, and whether the insulation collar or the kerb insulation is included.

Reducing condensation generally

The spacer deals with the edges. Two other things deal with the rest.

Warmer glass overall, which means a good whole unit U-value with a low emissivity coating and argon fill.

Ventilation, which is what actually removes the moisture. An opening at the highest point of a room lets warm moist air leave and draws drier air in through anything open lower down, and it clears a room in minutes where a wall window alone takes hours.

In a kitchen or a bathroom, an extract fan sized for the room and ducted to outside does the heavy work. A trickle ventilator at the top of the sash, with a clean filter, provides the background provision.

No glazing specification stops a room condensing if the moisture has nowhere to go.

Why the edge is colder than the middle

The spacer bar runs around the perimeter of a sealed unit, holding the panes apart and carrying the desiccant.

Traditionally it was aluminium, and aluminium conducts heat extremely well. That put a metal bridge around the entire edge of the unit, directly linking the cold outer pane to the warm inner one.

Heat crosses that bridge. The inner pane loses warmth at its perimeter, so the glass runs colder there than in the middle, in a band a few centimetres wide.

That band is where condensation appears first, and it is why people describe a unit as misting round the edges while the centre stays clear.

What a warm edge spacer changes

It replaces the aluminium with a material of much lower conductivity: a structural polymer, or a thin stainless composite formed so the metal path is minimal.

The spacer still does the same jobs. It sets the cavity width, holds the desiccant, and carries the seals. It simply conducts far less.

The result is that the inner pane runs warmer at its edge, so the temperature at which condensation forms is pushed lower. In conditions where the old unit would mist, the new one stays clear.

Why it matters more in a rooflight than anywhere else

The ratio of edge to area.

A rooflight is frequently small, and a small unit has a great deal of perimeter relative to its glass area. So the spacer influences a much larger proportion of the total performance than it does in a big window.

This is one of the main reasons the whole unit U value and the centre pane figure diverge so sharply on small rooflights, and why the centre pane number is particularly misleading here.

Where it sits in the priority list

Near the top, above triple glazing on any small unit.

A third pane improves the middle of the glass, which is already the best performing part. A warm edge spacer improves the edge, which is where the loss is concentrated and where the condensation appears.

It also costs a fraction of what triple glazing does, adds no weight and no thickness, and does not reduce light transmission.

The condensation diagnosis it helps with

Knowing which surface is wet tells you what is wrong.

A band at the glass edge with the middle clear points at the spacer. Condensation on the frame or the reveal with the glass clear points at a thermal bridge in the construction, not the glazing. Water between the panes is a failed unit.

Each has a different remedy, and getting the diagnosis right first saves buying the wrong one.

What to ask

Whether the unit has a warm edge spacer, by name, rather than assuming. It is a line on a specification and it is not universal on entry level units.