The short answer: A warm edge spacer is the strip that holds the panes of a sealed glass unit apart at the perimeter. A traditional spacer is aluminium, which conducts heat. A warm edge spacer is made of a material that conducts far less, so the edge of the glass stays warmer, loses less heat and is less likely to collect condensation. In a rooflight it is a small component that does a real job.
If you are reading about energy efficient rooflights, the warm edge spacer is one of the details that separates a decent unit from a good one. This page explains what it is and what to ask about.
What a spacer does
A double glazed unit is two panes of glass with a sealed gap between them. Something has to hold the panes apart at an even distance, and that something runs around the whole perimeter. It is the spacer, a thin frame hidden under the edge of the frame or glazing bead.
The spacer also holds a desiccant, a drying material that absorbs any trace of moisture inside the gap, and it forms part of the seal that keeps the gas in and the damp out. It is a quiet, structural part of the unit, and you only see it as a thin line around the edge of the glass when you look closely.
Why the edge matters
Heat takes the easiest route out. Through the centre of the pane, the gap and the coating slow it down. At the edge, it can travel along the spacer from the inner pane to the outer one, bridging the gap. A metal spacer makes this easy, so the edge of the glass is colder than the middle.
That cold edge has two effects. It lowers the overall performance of the unit, because the loss at the perimeter adds to the loss through the glass. And it makes the inner pane cold enough at the edge for moisture in the room air to settle there as condensation. Our guide to reducing condensation on rooflights goes into the causes.
What makes a spacer "warm edge"
A warm edge spacer is made from a material with much lower conductivity than aluminium, such as a structural foam, a composite or a plastic with a thin metal element. The aim is to break the bridge between inner and outer panes. The edge stays closer to room temperature, and the unit performs better as a whole.
The result is a more even glass surface, a smaller cold band round the pane, and less risk of the damp marks that gather along the bottom edge of an old unit.
How it relates to the regulation values
The Building Regulations do not ask for a warm edge spacer by name. They set a thermal performance for the whole unit. The figure is 2.2 W/m²K for a flat roof unit or lantern, and for a window in a pitched roof it is 1.4 W/m²K, or Window Energy Rating band B. How a manufacturer reaches that figure is up to them. Low-emissivity coating, gas fill, frame design and the spacer all contribute.
In practice the spacer is part of what lets a unit reach the figure with a normal double glazed build, rather than adding glass. That connects to the question in double or triple glazed rooflights. Our page on Building Regulations for skylights and rooflights explains how the rules apply when a whole unit is replaced.
What to look for when you compare quotes
You will not see the spacer from the room, so you have to ask. When you compare quotations for a replacement rooflight, check that each one states:
- the type of spacer used in the sealed unit
- the coating and gas fill
- the frame material and whether it has a thermal break
- the thermal performance figure for the whole unit, not only the centre of the glass
The last point is easy to miss. A "centre pane" figure looks better than the whole-unit figure, because it ignores the edge. A warm edge spacer narrows the gap between the two. Our page on rooflight U-values explained shows how to read them.
Where it makes the most difference
The benefit is greatest where the room is warm and humid, as in a kitchen, bathroom or open plan extension, and where the glass is large and has a long perimeter, as in a lantern with many panes. A small unit over a dry landing gains less, though the spacer is still worth having.
It will not rescue a poor installation. A warm edge unit set on an uninsulated kerb or fitted with gaps round the frame will still lose heat. That is why the fitting, the kerb and the seal count as much as the product, and why our work carries a 10-year workmanship guarantee.
Spacer, sealant and the life of the unit
The spacer works with the sealant around it. A sealed unit is usually built with a primary seal against the spacer and a secondary seal over the outside of it. If either breaks down, damp gets in and the unit mists. The spacer material affects how much stress the seal sees as the glass warms and cools through the day and the seasons.
A rooflight takes more of that stress than a wall window. It sits in full sun, faces the weather straight on and cools sharply at night. The edge of the glass moves a little with each cycle, and a well made warm edge system copes with that movement better than a rigid metal bar.
When a sealed unit does fail, the cloudy moisture between the panes cannot be removed, and the unit is replaced. Our guide to misted and failed rooflights explains how that is assessed.
A good spacer cannot be seen, which is the point. Look at the edge of a sealed unit with a torch and you will see a thin dark or grey line. Its colour and material tell an installer what was used, which is one reason the quotation should name it.
Next step
At the free survey we tell you what specification suits your roof, and the fixed written quote lists the glazing so you can compare it fairly. Book a survey, or return to the energy efficient rooflights hub.
