What is a warm-edge spacer?
It is the bar that holds the two panes of a sealed unit apart, made from a material that conducts heat poorly. Look at the edge of any double glazed rooflight and you will see a strip a few millimetres wide running round the perimeter, just inside the frame. For decades that strip was aluminium, which is an excellent conductor and therefore a small metal bridge round every pane of glass in the country. A warm edge spacer does the same structural job with a material that does not carry heat nearly as well.
The point of the change is narrow and worth understanding: it does not improve the middle of the glass at all. It improves the temperature of the glass edge and the frame beside it, which is where condensation starts and where the whole unit figure is dragged down. On a rooflight that matters more than it does in a wall. The full thermal picture is on energy efficient rooflights.
The bar you can see between the panes
A sealed unit is two or three panes of glass separated by a hollow perimeter bar, filled with dry air or an inert gas, and sealed twice. The bar sets the cavity width, typically sixteen or twenty millimetres in a domestic unit, and it contains the desiccant that keeps the cavity dry.
It is also structural in a modest way. It holds the geometry of the unit while the sealant cures and while the assembly is handled, transported and lifted onto a roof. Whatever material is used has to be dimensionally stable, gas tight, compatible with the sealants and capable of carrying desiccant. That is a narrower set of options than it first appears.
What warm edge actually measures
The term has a definition rather than being marketing shorthand. A spacer qualifies where the product of its material conductivity and cross sectional area falls below a defined threshold, which in the European convention is expressed through the linear thermal transmittance at the glass edge.
In practice, manufacturers publish a psi value for the edge, in watts per metre kelvin. An aluminium spacer in a typical unit produces a psi around 0.08, a good warm edge product around 0.03 to 0.05. Lower is better, and the improvement applies per metre of perimeter, which is why it matters more on small units than large ones.
Why aluminium was used for so long
Because it is cheap, it rolls into a hollow section easily, it is dimensionally stable, and it takes desiccant and sealant without argument. For thirty years there was no commercial reason to use anything else, and units built with aluminium spacers are perfectly sound assemblies.
The problem only became significant once the rest of the unit improved. When the glass was clear float with a wide air gap, the edge effect was lost in the general poor performance. Once soft coats and argon brought the centre pane figure down towards 1.0 W/m²K, the metal bar around the perimeter became one of the largest remaining weaknesses, and it also became the coldest interior surface where water forms first.
Materials in current use
| Spacer type | Construction | Behaviour |
|---|---|---|
| Aluminium | Rolled hollow metal section | Highly conductive, cold at the glass edge |
| Stainless steel | Thin walled rolled section | Better than aluminium, still metal |
| Thermoplastic | Extruded polymer applied as a bead | Very low conductivity, flexible on large units |
| Structural foam | Silicone foam with a vapour barrier film | Low conductivity, accommodates movement |
| Composite | Polymer body with a thin metal vapour film | Low conductivity with rigid handling |
The rigid composite and the flexible thermoplastic types dominate current domestic glazing. Which one appears in a given rooflight is generally decided by the unit manufacturer rather than specified by the buyer, but the class of spacer is worth knowing because it appears on the declaration.
What it does to the whole unit U-value
The centre pane figure does not move. The whole unit figure does, because a whole unit calculation adds the frame, the glass edge and their interaction to the centre pane number.
On a typical domestic rooflight, moving from aluminium to a good warm edge product is worth somewhere in the region of 0.1 W/m²K on the whole unit figure. That sounds small until you consider the proportions. On a rooflight measuring 600 by 900 millimetres the perimeter is three metres against about half a square metre of glass, so the edge is a large share of the total. The smaller the unit, the more the spacer counts.
Why the edge of a rooflight is colder than the edge of a window
Two effects add to each other. The warmest, wettest air in a room collects at ceiling level, so the air presented to a rooflight edge is more demanding than the air at a window in the same room. And the outer face of a rooflight radiates directly to the night sky, which drops the outer surface below the ambient air temperature on a clear night.
So the edge of the glass in a roof plane sits at a lower temperature than the same detail in a wall, in front of air with a higher dew point. That is the argument for a warm edge product overhead being stronger than the argument for it in a window, and it is why the specification is worth confirming rather than assuming.
Desiccant, the double seal and the working life of the unit
The spacer holds the desiccant, which is a molecular sieve that absorbs whatever moisture is sealed in during manufacture and whatever slowly finds its way in afterwards. Two seals contain the cavity: a primary polyisobutylene bead against the glass, which is the gas barrier, and a secondary structural sealant around the outside, usually polysulphide or silicone.
The desiccant has a finite capacity. When it is saturated, moisture condenses inside the cavity and the unit is misting internally, which is the end of its useful life rather than a temporary condition. Twenty to twenty five years is typical. Warm edge spacers do not extend that clock much, but the reduced thermal cycling at the edge is gentler on the seals than the sharper gradients across a metal bar. The failure itself is covered on skylight replacement.
Working out what is in an existing unit
Look along the edge of the glass in daylight. An aluminium spacer is bright and metallic, often with a visible seam and small perforations facing the cavity. A warm edge product is usually black or dark grey, matt rather than shiny, and on a foam type the surface has a slight texture to it.
The other route is the unit itself. Many sealed units carry an etched or printed code along one edge giving the manufacturer and a date, generally hidden under the frame but sometimes visible on an older domed unit. Where an old rooflight is coming out, the edge is fully visible for a few minutes and it is worth a look, because it tells you the era of the whole assembly and what to expect from anything else fitted at the same time.
What to ask for on a quotation
Three lines settle it. The glass make-up, including pane thicknesses and cavity width. The cavity fill, air or argon. And the spacer type by name, together with the whole unit U-value and the plane that figure was declared in.
A quotation that gives a centre pane figure and no spacer information is describing the best part of the unit and staying quiet about the rest. Sunspire specifies units with warm edge spacers as standard and puts the make-up in writing at the fixed price stage, because it is one of the few glazing decisions that cannot be changed afterwards without replacing the unit.
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