Are warm edge spacers worth it in a rooflight?

A sealed glazing unit is two or three panes held apart by a spacer bar that runs round the perimeter. The bar sets the width of the cavity, holds the drying agent that keeps the gas dry and carries the seal that stops moisture getting in. It is invisible once the unit is in its frame, apart from a thin line at the edge of the glass.

Because it runs round the whole edge, the spacer is also a bridge between the inner and outer panes. If it conducts heat well, it carries warmth from the inside pane to the outside one, along the very strip where the glass meets the frame. That strip is the weakest part of the unit for heat loss, even when the centre of the glass is excellent.

The traditional spacer is aluminium, which is strong, cheap and a good conductor of heat, which is exactly the wrong property here. A warm-edge spacer is made from a material that conducts much less, such as a composite, a plastic-based product or stainless steel in a thin section, often with a flexible profile. It does the same mechanical job and lets less heat through.

The effect is on the edge of the glass, usually in the first strip of a few centimetres in from the frame. There, a warm-edge spacer keeps the inner surface warmer in cold weather. Manufacturers describe the improvement with a linear thermal transmittance, a figure that measures heat loss along the edge. A lower figure is better. You do not need to memorise it, but you may see it on a data sheet.

The gain across the whole rooflight depends on its proportions. A long, thin rooflight has a lot of edge for its area, so the spacer matters more than it does on a very large pane. A rooflight made of several small panes, such as a glazed bar roof lantern, has many edges and gains the most.

You are most likely to notice a warm-edge spacer through condensation. On a cold morning, an aluminium-spaced unit can show a line of moisture round the edge of the pane while the middle stays clear. Warm the edge and that line often shrinks or vanishes. Our page on whether rooflights can cause condensation sets out the other causes, because the spacer is one factor among several.

It also contributes to the rooflight's rated U-value. Window energy ratings include the effect of the edge, and so do the U-values quoted for the whole rooflight. If you are working to a target, a warm-edge spacer is a straightforward way to help meet it without changing the frame. The targets themselves are discussed in what U-value should a rooflight have? and in our guide to rooflight U-values explained.

The limiting U-values that new and replacement rooflights must achieve come from Approved Document L, Volume 1, in its 2021 edition. Most current products reach them with a warm-edge spacer as part of the unit, so you are unlikely to be offered the old aluminium kind on a quality rooflight, but it is a good question to ask.

A spacer does not make up for a poor frame, a single pane, a failed seal or missing insulation round the opening. It will not stop condensation caused by a steamy room with no extractor, and it will not warm a cold patch on plaster caused by a gap behind the lining. It is one part of a specification, and the best results come from the parts working together: low-e coating, gas fill, number of panes, spacer, frame, insulation and finish.

It is also not a guarantee of durability by itself. The edge seal's long-term life depends on the quality of the sealant, the manufacturing and how the unit is fitted and supported, and a unit left standing in water or stressed by a twisted frame can fail whatever its spacer. Good installation protects the unit, and our workmanship guarantee covers the way we fit it.

The price difference between a warm-edge spacer and an aluminium one is small next to the cost of the rooflight, the access and the finishing. For that reason it is rarely worth specifying the cheaper spacer to save money. The saving is slight, the performance penalty is permanent, and the unit lasts for years once it is in the roof.

There may be a case where it matters less: a small rooflight in an unheated outbuilding, say, where nobody minds a cold edge. For a home, in a room you heat and live in, the argument is hard to make.

The more useful question is whether to spend on a better spacer, a third pane, or a different coating. If you are choosing between them, the spacer is the cheapest of the three and the least visible, and the guide to double or triple glazed rooflights helps with the larger decision. The wider question of how much heat glass gives away is answered in do rooflights lose a lot of heat?

Ask for the glazing build-up in writing: the panes, the coating, the gas, the spacer type and the whole-rooflight U-value. A quote that says only "double glazed" tells you very little. If you are replacing a misted unit in an existing frame, ask the supplier which spacer the new unit uses, and whether it suits the depth of your frame.

You can sometimes see the spacer through the glass by looking at the edge. A bright metallic strip suggests aluminium, while a dull, grey or black strip suggests a warm-edge product, though looks alone are not proof. The written specification is the reliable record.

For a new or replacement rooflight, ask for a quote and we will write the glazing specification, spacer included, into the quote alongside the finish schedule. The rest of the glazing choices are collected on our energy efficient rooflights page.