What Is a Warm-Edge Spacer?

Where the spacer sits

A double or triple glazed unit is made of separate sheets of glass with a sealed gap between them. Something has to hold the sheets apart and keep that gap a constant width all the way round. That something is the spacer bar, a thin strip hidden inside the frame, running around the edge of the unit. It also holds a drying agent that keeps the trapped air or gas free of moisture.

What the old ones did wrong

For a long time spacers were made of aluminium. It is cheap, strong and easy to bend into a frame. It also conducts heat very well. In winter that meant the outer rim of the glass, a few centimetres wide, was noticeably colder than the middle. Warm room air touched that cold band and dropped its moisture as condensation, which is why old double glazing so often shows a damp line around the edge.

What makes a spacer warm-edge

A warm-edge spacer uses material that passes heat slowly. Different makers use different routes.

• Structural foam or composite bars with little metal in them.
• Thin stainless steel, which conducts much less than aluminium.
• Plastic and metal hybrids designed to break the path of heat.

The principle is the same in each case: keep the edge of the glass closer to the temperature of the room.

Why it matters on a rooflight

A rooflight faces the sky, so its glass is exposed to cold nights and radiates heat outward more than a window in a wall does. That makes the edge condition worth getting right. A warm-edge spacer is part of the reason a modern unit can reach a lower overall U-value than the same panes would with an aluminium bar. Our page explaining rooflight U-values shows how the centre of the glass, the edge and the frame all feed into the final figure.

Rooflights also tend to be larger than a typical wall window, and the edge length grows with the size of the unit. A big pane has a long perimeter, so a poor spacer costs more in a rooflight than it would in a small casement.

Spacers and condensation

A warmer edge does not stop condensation completely. It reduces the chance of it appearing at the rim and delays it when the room is humid. Moisture in the house still needs somewhere to go, and extractor fans and ventilation matter. If you are seeing drips or misting, read our guide to condensation on rooflights.

Do not confuse this with a unit that has misted between the panes. That means the seal has failed and moisture has got into the gap, and no spacer will fix it. The unit needs replacing.

How to check what you have

You can often see the spacer by looking at the edge of the glass from inside. A bright, silvery bar suggests aluminium. A black, grey or white bar, sometimes slightly matt, is more likely a warm-edge type. The maker's data sheet or the quote tells you for certain. Our written quote puts the glazing build-up on a separate line, so the spacer is never left to guesswork.

Does the colour matter?

Only for looks. A black spacer is common because it shows least against dark frames, while a white or grey one blends in with pale frames. A warm-edge bar in any colour performs the same if the material is the same. If appearance matters to you, mention it at the survey and we can choose to suit the frame finish.

Questions worth asking a supplier

When comparing offers, ask which spacer is used and whether it is the standard or an upgrade. Ask for the declared U-value of the whole rooflight, because that figure already includes the spacer's effect. Also ask whether the gas fill and coating are included. A supplier who answers all three clearly usually knows the product well.

One practical point for the survey: if the frame is dark inside, a thin spacer bar is barely visible from the room. In a pale or white frame it shows more, so tell us your finish early and we can match the bar to it before the unit is ordered.

Lifespan and seals

The spacer is also part of the seal that keeps the unit airtight. Around it sits a sealant that bonds the glass to the bar, and over years sun, heat and flexing can wear that sealant down. A good spacer bar with a well made edge seal lasts a long time, and the maker's warranty on the sealed unit is worth reading. We hand the warranties over with the product details, so you know how long the unit is covered and what the cover includes.

If a unit fails, the fix is a new sealed unit, or a whole new rooflight if the frame is tired as well. Our page on the choice is energy efficient rooflights.

Working alongside the other glass features

A spacer is one part of an energy-efficient unit. It works with a low-E coating that sends warmth back inward, described in what low-E glass is, and with the gas fill between the panes. Together they decide how cold a rooflight feels in winter. For the full range we fit, see energy efficient rooflights.