In brief
A warm edge spacer is the bar that holds the panes of a sealed glass unit apart and runs around its edge. In an older unit it is metal, usually aluminium, which carries heat across the edge of the glass. A warm edge spacer is made of a material that conducts far less, so the edge of the pane stays warmer, loses less heat and is less likely to mist with condensation on a cold morning.
- The spacer is the bar between the panes, hidden in the frame edge of a sealed unit
- Metal spacers carry heat to the outer pane; warm edge spacers are made to slow it
- A warmer glass edge lowers the unit's overall U-value and reduces edge condensation
- It also holds the desiccant that keeps the gap dry, so it matters for the unit's long life
- Ask for the spacer by name in the quote, as it is not visible once the unit is fitted
What the spacer is and where it sits
A double or triple glazed unit is made of two or three sheets of glass with a gap between them. The gap is kept at an exact width by a thin bar that runs around the perimeter of the unit, set back slightly from the cut edge. A sealant bonds the glass to the bar on both sides and a second seal closes the outside of the unit.
Most of the time you cannot see it. The bar sits under the frame or the glazing bead, and on a flat rooflight it is covered by the upstand of the frame. If you look into the edge of a unit from a low angle you may notice a dark or silver line, which is the spacer or its sealant.
The bar has a second job. It is hollow or filled, and it holds a drying agent that absorbs any moisture trapped in the gap when the unit was made. That dryness is what keeps the glass clear for as long as the seal holds.
Why the material matters
Heat travels along whatever offers it the easiest path. In a sealed unit the centre of the glass is insulated by the gap, the coating and the gas. At the edge, though, a metal bar bridges the gap and gives heat a direct route from the warm inner pane to the cold outer one. This is a thermal bridge.
The effect is a cold strip around the inner pane, a few centimetres wide, which can sit below the temperature of the room air. Warm moist air that touches a cold surface leaves a film of water, so the strip is where condensation shows first. A warm edge spacer is made of a material such as structural foam, or a stainless steel or plastic composite, that conducts much less heat. The strip stays warmer and the bridge is weaker.
The words on a product sheet vary between makers, but the idea is the same: a lower-conductivity bar in place of plain aluminium.
What it changes in a real rooflight
- Heat loss. The spacer is part of the figure for the whole unit. A warm edge bar lowers the overall U-value a little, and the gain is greatest on small panes, where the edge is a large share of the glass. Our guide to rooflight U-values and Part L shows how the unit figure is built up.
- Condensation at the edge. Because the inner pane stays warmer, the damp line that forms along the bottom or sides of the glass on cold mornings is less likely. The causes of that line are set out in our guide to condensation on rooflights.
- Comfort. A warmer edge makes the area beside the glass feel less cold on a winter evening.
- Comparison between quotes. Two units can look the same and have different spacers, so naming it makes a quote easier to compare.
A warm edge spacer does not stop condensation from forming on the frame or a cold kerb, and it does not replace insulation round the opening. It improves the glass edge. The rest of the opening still needs its own attention, which we cover in our answer on heat loss through skylights.
How to find out which spacer you are getting
Because the spacer is hidden, the only reliable way to know is to ask, and to see it in writing. A good quote lists the unit with its glazing, coatings, gas fill and spacer. If any of these is missing, ask for it. That is a fair question for a builder to put to their rooflight supplier as well, since the unit is often supplied through the main contractor.
When we survey a roof and quote for a new rooflight, the specification page names the spacer type along with the glazing. For a replacement, the sealed unit has a label printed on the spacer, often with the maker, the date and the glass build-up, and we note what is on it so the replacement is a real step up from the old one, not a copy.
If you already have a rooflight and are not sure what is in it, a photograph of the edge of the glass and the label on the spacer helps us identify it.
Spacer, glass and frame: where to spend
A warm edge spacer is one of several parts that make up the performance of a unit, and a sensible order of priority is to start with the coating and the gas fill, make sure the spacer is warm edge, and then consider a third pane. On a standard modern unit the spacer is often already included, so the question is more about confirming it than paying extra.
A third pane lowers the heat loss across the whole glass but adds weight, which our answer on triple glazing explains. Solar control coatings deal with summer heat and are in our guide to solar control glass. Each can be combined with a warm edge spacer.
A range of options sits under energy efficient rooflights, and we are happy to talk through them against your roof and your room ahead of ordering.
Related questions
- Can a warm edge spacer be added to an existing rooflight?
- Not on its own, because the spacer is built into the sealed unit when the glass is made. The way to get one is to replace the sealed unit, or the whole rooflight, with a unit that specifies it.
- Does the spacer colour tell me what it is made of?
- Not reliably. Warm edge bars come in black, grey and other colours, and a silver bar is not always aluminium. The label and the written specification are better guides than the colour.
