What is a warm edge spacer?

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 whole perimeter.

The inner pane runs measurably colder within about 50mm of the edge than it does in the middle.

What you see

Condensation forming as a band around the edge of the glass on cold mornings while the centre stays clear. The classic signature of an aluminium spacer, and it is regularly mistaken for a failed unit.

It is 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 a different problem.

Warm edge

A spacer of stainless steel, structural foam or a composite, all far less conductive. 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 and is not the main point.

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 on a wall window.

Checking

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

The bar that holds the panes apart

A sealed glazing unit is two panes 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 seal. That is the spacer bar.

Traditionally it was aluminium, which is convenient to manufacture and conducts heat extremely well. That last property is the problem.

Why aluminium is a weak point

An aluminium spacer creates a direct thermal bridge between the inner and outer panes around the entire edge of every pane. Heat travels along it from the warm side to the cold side, so the edge of the glass runs several degrees colder than the centre.

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

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

What a warm edge spacer is

A spacer made from a material that conducts far less heat: thin walled stainless steel, a structural foam, or a composite. It does the same mechanical job and it runs several degrees warmer at the glass edge.

The result is a modest improvement in the whole unit U-value, typically around 0.1, and a much larger improvement in the thing you actually notice: the edge condensation largely disappears.

How to tell what you have

Look at the perimeter of the glass from inside. A bright silver metallic bar is aluminium. A dark grey, black or dull metal bar is usually a warm edge type.

On an existing unit that shows a band of edge misting on cold mornings, an aluminium spacer is almost certainly the reason.

Should you specify one

Yes, and it should be standard rather than an upgrade. Any reputable current rooflight has one, and where a quotation does not say, ask.

Along with an insulated kerb on a flat roof, or an insulation collar on a pitched one, it is one of the small unglamorous details that decides whether a rooflight feels warm or merely complies.

It is not a repair

A spacer bar is inside a sealed unit and cannot be changed. Where an existing window has an aluminium spacer and shows edge condensation, the answer is a new sealed unit or a new window, not a treatment.

The component and its job

A sealed glazing unit has two or three panes held apart by a spacer bar around the perimeter. The spacer sets the cavity width and holds the desiccant that keeps the cavity dry.

Traditionally that spacer was aluminium, which is an excellent conductor. It formed a metal bridge around the entire edge of the unit, linking the cold outer pane to the warm inner one.

A warm edge spacer replaces the aluminium with a material of much lower conductivity, usually a structural polymer or a thin stainless composite. Same job, far less heat crossing it.

Why it matters more in a rooflight

Because of 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 edge carries a large share of the total heat loss.

This is exactly why the whole unit U value and the centre pane U value diverge, and why the divergence is worst on small units. The spacer is one of the main reasons for that gap.

What you actually notice

Condensation appearing at the edge of the glass first, in a band around the perimeter, while the middle of the pane stays clear.

That band is the aluminium spacer chilling the inner pane at its edge. A warm edge spacer raises that surface temperature and pushes the condensation point back, so the unit stays clear in conditions where the old one would mist.

It is a small line on a specification and one of the better value ones. Ask whether it is included rather than assuming.