What is a warm-edge spacer?

What the spacer does besides hold the panes apart

It has four jobs and only the obvious one is structural. It sets and maintains the cavity width, which decides how the gas fill performs. It carries the desiccant, a granular drying agent sealed inside the hollow bar, which absorbs the small amount of moisture trapped at manufacture and keeps the cavity clear. It forms the first of the two perimeter seals, a butyl bead that is the actual gas and vapour barrier. And it provides the bed for the second seal, a polysulphide or silicone that gives the unit its structural strength.

Everything the sealed unit does depends on that perimeter holding. When a unit mists internally, it is the seal at the spacer that has gone, not the glass.

Why aluminium was used, and why it stopped being good enough

Aluminium is cheap, easy to roll into a hollow section, dimensionally stable, and it takes butyl well. For thirty years it was the only spacer anybody used, and while glazing units were thermally mediocre in the middle as well, nobody noticed what the edge was doing.

Then the middle got much better. Low-emissivity coatings and argon fills took centre-pane values from around 3.0 down to around 1.0, and the aluminium bar, which had not changed, became the dominant weakness. Aluminium conducts heat at around 160 W/mK. Structural foam is nearer 0.2, stainless steel around 15. An aluminium spacer is a metal bridge a few millimetres wide connecting the cold outer pane directly to the warm inner one, right where the room can see it.

What “warm edge” actually means numerically

It is not a marketing word. The standards define spacer performance by a linear thermal transmittance called psi, and there is a widely used threshold: a spacer qualifies as warm edge where the product of its thickness and its conductivity stays below a stated limit. In practice suppliers quote either a psi value in W/mK for the glazing edge, or the two-box conductivity figure the calculation uses.

Typical psi values for a rooflight edge run around 0.08 W/mK for aluminium and 0.03 to 0.05 for warm edge. Ask for the figure rather than the label. Some products marketed as warm edge are thin-wall stainless steel, which is a genuine improvement over aluminium but a long way behind foam.

The materials you will see quoted

Foam and thermoplastic types have a further advantage on a roof: they are formed as a continuous run round the unit rather than cut and cornered, so there are no butt joints at the four corners, which is where aluminium units traditionally fail first.

What it changes on the whole-unit figure

The whole-unit U-value averages glass, edge and frame across the actual product, weighted by area. The edge zone is a band roughly 65mm wide all the way round, which is how the calculation treats it, and on a small unit that band is a large share of the total.

On a 1500 by 1000 rooflight, going from aluminium to foam typically improves the whole-unit figure by around 0.08 to 0.10 W/m²K. On a 550 by 780 unit, where the edge zone covers most of the glass, the same change can be worth 0.15 or more. The smaller the rooflight, the more the spacer matters, which is the reverse of most people’s assumption. Since rooflights tend to be smaller than windows, warm edge earns its money here more reliably than it does downstairs.

The visible benefit at the bottom of the glass

The thermal figure is the argument on paper. The reason we specify it is the surface temperature at the edge of the pane.

With an aluminium spacer, the glass within 50mm of the frame can sit four or five degrees below the middle of the unit on a cold night. That band reaches dew point while the rest of the pane is still dry, so you get a rim of mist that runs down and collects at the sill. A warm-edge spacer lifts that band by two to three degrees, which is frequently enough to keep it above the dew point of a normally run room. The effect is covered further on why a rooflight drips in winter and on the warm-edge spacers page.

The secondary benefit is unit life. A cooler, wetter edge is a harder environment for the seals, so the same change that keeps the glass dry also extends the working life of the sealed unit.

How to tell what you have got

Look at the bar through the glass at the edge of the pane. Bright silver with a mirror finish and small visible perforations along the inner face is aluminium. A dull grey or black matt bar is usually foam or thermoplastic. A dull metallic bar with a fine seam is likely thin-wall stainless. Some manufacturers print a product name along the spacer, readable through the glass with a torch.

The four corners give it away too. A bar with a visible mitre or a small corner key at each corner is a cut and cornered metal spacer. A bar that turns the corner as a smooth radius is a continuous warm-edge type. If the unit dates from before about 2005 and is on a budget product, assume aluminium.

Where it matters most on a roof

Small units, because the edge zone dominates. Bathrooms, kitchens and utility rooms, because the dew point in those rooms is high and the edge is the first surface to reach it. Rooflights in vaulted ceilings and over stairwells, where the warmest and most humid air in the house collects permanently against the glass. And shallow-pitched or flat units, which run colder than steep ones because they face more of the open sky.

It matters less on a large fixed unit on a steep north slope in a dry hall, where the edge is a small proportion and the room humidity is low. Even there it is a modest cost on the order, and we would still take it.

Asking for it on a quotation

Warm edge is standard on most current rooflight ranges from the manufacturers we fit, but it is not universal, and budget replacement sealed units are still supplied with aluminium. The word to use is the psi value: ask what spacer is in the unit, what material it is, and what its psi figure is at the size quoted.

Then check that the whole-unit U-value you have been given was calculated with that spacer rather than borrowed from a different build. A quotation that will not say what is in the perimeter of the glass is unlikely to have thought hard about the perimeter of the opening either. We install VELUX, Fakro, Keylite and Roto units and specify the spacer as part of the order. Fifteen years in the trade, Oxford and twenty five miles by road, installations and replacements only, ten-year workmanship guarantee. Call 01865 704245.