What Is a Warm Edge Spacer?

Open up a double-glazed unit and you find two sheets of glass with a gap between them, held apart by a narrow bar running round the perimeter. That bar is the spacer. It is filled with a drying agent that keeps the sealed gap free of moisture, and it sits under the frame where you rarely see it, though sometimes you can spot a thin dark or silver line at the glass edge.

The gap itself, often filled with argon gas, does the insulating. The spacer's job is structural, yet it also touches both panes, so it can carry heat from the warm inner glass straight across to the cold outer one. That route is called a thermal bridge, and it is exactly what a warm edge design aims to cut.

• Spacer the bar that holds the panes apart
• Thermal bridge a route that lets heat skip past insulation
• Argon a gas between the panes that slows heat flow
• Edge seal the sealant that keeps the gap airtight

Because the bar is hidden by the frame, nobody chooses a rooflight for its spacer, and yet it quietly sets how the glass behaves in the first hour of a frosty morning.

Aluminium is light, strong and cheap, and for decades it was the standard spacer. It is also a very good conductor of heat, which is a poor trait in a part whose purpose is to sit between a warm side and a cold side. On a cold night the aluminium edge chilled the inner pane along its rim, and the air against that strip dropped below the temperature at which moisture condenses.

The result was the familiar line of mist or droplets round the edge of an old roof window, while the middle of the glass stayed clear. Warm edge spacers replaced the metal with a bar made of a stiffer plastic composite, sometimes with a thin metal layer, or from stainless steel, all of which carry heat far more slowly. Different makers use different materials, so the useful question is simply whether the unit has a warm edge design.

Another reason to move on was durability. A bar that conducts heat expands and contracts more as it warms and cools, which puts the edge seal under a little more strain each day. Gentler temperature swings at the rim are kinder to the sealant that keeps the unit airtight.

A rooflight has more edge for its area than you might think, because roof windows are narrow and tall and lantern roofs are made of many small panes. A bigger share of the glass sits near an edge, so the spacer counts for more here than on a wide patio door. It also faces the sky, so its glass runs colder than a vertical window on the same night.

The spacer feeds into the overall U-value that Part L measures, so a good spacer helps a unit stay under the Part L ceilings, which Part L fixes at 1.4 W/m²K for roof windows, and 2.2 W/m²K for rooflights. Our page on rooflight U-values explains how the pieces add up.

For a bathroom roof window, where steam meets cold glass every morning, the edge is usually where moisture shows first, so it is the place to put the money.

No spacer works alone. A warm edge helps most when it is part of a package: a low-emissivity coating on the glass, argon in the gap, and a frame that is itself insulated. Improve only one and the weakest part still sets the pace. If you are weighing a bigger upgrade, triple glazed rooflights add a third pane and a second gap, and the spacer matters twice as much there.

For a damp-prone room such as a bathroom or a kitchen extension, the edge is also where tidemarks start. An opening rooflight lets you air the room afterwards, and a warm edge reduces how much moisture lands on the glass in the first place. Our guide to how to stop rooflight condensation covers the habits that go with it.

If your rooflight faces north and the room is used all year, the edge has the most to do, because there is no sun to warm it back up.

Warm edge spacers have become the normal choice in good-quality sealed units, so you should not expect to pay a large premium for one. What you should expect is for the quote to say what the unit contains. On our written, itemised quotes we list the glazing build-up, the coating and the edge, so two quotes can be set side by side fairly.

If you are replacing only a misted sealed unit in a frame that is otherwise sound, a new unit comes with a modern spacer, which is a quiet upgrade on the one that failed. Our misted rooflight replacement service explains that route, and the energy efficient rooflights hub shows the other choices. Costs are on the costs page.