Warm-Edge Spacers Explained

The sealed unit inside your double-glazed window is more than just two panes of glass. Running around the perimeter is a spacer bar that holds the panes apart, contains a desiccant to keep the cavity dry, and plays a bigger role in energy performance than most people realise. For decades that bar was aluminium, which turned out to be a significant thermal weak point. Today, a warm-edge spacer made from a low-conductivity material is standard in any unit built to meet Part L whole-window U-value requirements, and understanding what it does helps you check that you are actually getting one. If you want to see what better glazing does for your heating bill, the energy savings calculator will run the numbers for your home.

What a spacer bar is and what it does

A sealed double-glazed unit is a factory-assembled package: two panes of glass bonded together around the perimeter with a spacer bar, the whole assembly hermetically sealed. The spacer bar has two jobs. First, it holds the two panes at the correct separation, which determines the width of the gas-filled cavity in the middle. Second, it contains a desiccant, typically molecular sieve beads, that absorbs any residual moisture inside the cavity. That desiccant is why a sealed unit stays clear on the inside even in cold weather. If the seal fails and moist air enters, the desiccant eventually becomes saturated and you get misting between the panes, which means the unit needs replacing.

The spacer also forms the perimeter of the unit, meaning it sits between the glass and the frame rebate. That position makes it part of the thermal path between inside and outside. In a well-specified unit, the glass itself is doing a good job of resisting heat flow through its centre, but the edge zone, where the spacer sits, has historically been a weaker point. This is the problem that warm-edge spacers were designed to solve.

Why aluminium spacers were a problem

Aluminium conducts heat very well. That is a fine property in a window frame if you are also including a polyamide thermal break (as modern aluminium windows do), but in a plain aluminium spacer bar there is no such break. The bar runs continuously from the warm inner pane to the cold outer pane, conducting heat directly across the edge of the unit. Engineers call this a cold bridge or thermal bridge: a localised path of high conductivity that bypasses the insulating cavity in the middle.

The consequences are twofold. The first is energy loss. Heat flows along the aluminium bar and out through the edge of the unit, raising the overall heat loss rate. Because this happens around the entire perimeter, the effect on the whole-window U-value is meaningful, even though the edge zone is narrower than the glass area. The second consequence is condensation. A cold bridge keeps the inner surface of the glass cooler near the edge than in the centre. When warm, humid room air meets that cooler surface, moisture can deposit as condensation along the bottom edge of the glass, a familiar stripe of wetness on older windows on cold mornings. This is distinct from misting between the panes (a failed seal) and from surface condensation on the room side (a humidity problem). For a full explanation of the different types, the condensation guide covers each one in detail.

The aluminium bar was used for decades because it is cheap, rigid and easy to work with. Nobody thought much about the fact it was also acting as a little radiator fin, pulling heat out around the edge of every unit. When U-value testing became more rigorous, the edge-zone penalty became impossible to ignore.

Tom Bradley, FENSA-registered installer

How a warm-edge spacer fixes the problem

The solution is straightforward in principle: replace the aluminium bar with a material that conducts far less heat. A warm-edge spacer uses a low-conductivity material for the body of the spacer, so the thermal path across the edge of the unit is broken or at least severely restricted. The edge of the glass runs warmer, which reduces both the rate of heat loss and the risk of condensation forming at that cooler perimeter zone.

The name comes from the effect on the inner glass surface. With an aluminium spacer, the edge of the inner pane is noticeably cooler than the centre, which is where the phrase "cold edge" originates. With a warm-edge spacer, that edge runs closer to the centre-pane temperature, hence "warm edge." This improvement feeds through to the whole-window U-value (Uw), which is the figure that Part L is judged on. The centre-pane U-value (Ug) might look fine with any spacer, but Uw accounts for the frame and the edge zone, and that is where the spacer type makes a difference.

Warm-edge spacers are used as standard in combination with Low-E glass and an argon fill. These three components work together: Low-E glass cuts the centre-pane heat loss, argon slows heat transfer through the cavity, and the warm-edge spacer reduces the edge-zone penalty. Together they allow a high-quality double-glazed unit to meet the Part L target of 1.4 W/m²K or lower, or WER band B or better, without needing triple glazing.

The types and how much they help

Not all warm-edge spacers are equal. The improvement over aluminium depends on what the spacer is made from. The figures below compare performance at the edge zone versus a standard aluminium bar. The same gap widths and desiccant loads apply across types; the difference is purely the thermal conductivity of the spacer body itself.

Source: web/lib/research/u-values-wer.md

Stainless steel was the first step away from aluminium. Its thermal conductivity is considerably lower than aluminium, which gives a useful but modest improvement of around 10% at the edge zone. It is still metallic and rigid, which makes it easy to fabricate with existing machinery.

Foam and thermoplastic spacers such as TPS (thermoplastic spacer) and Superspacer take a bigger leap. These materials have very low thermal conductivity, delivering an improvement of around 20 to 30% over aluminium. They are flexible, which simplifies the corner sealing and can improve the long-term integrity of the unit. Many mainstream sealed units now use a thermoplastic spacer as standard.

Composite spacers such as Swisspacer combine a stainless steel outer shell with a thermoplastic or foam core. The result is a spacer that is structurally rigid and precise to manufacture, but has very low effective conductivity across its cross-section. The improvement over aluminium reaches around 30 to 40% at the edge zone, making these the best-performing option currently available. They are the sensible specification for any unit where thermal performance matters.

I specify composite spacers on every job now. The cost difference between a thermoplastic spacer and a Swisspacer-type composite in a finished, fitted window is negligible. The improvement in edge performance is not negligible. There is no good reason to accept aluminium in a new unit.

Tom Bradley, FENSA-registered installer

Warm-edge spacers and condensation

The link between spacer type and condensation is direct. Condensation forms on a surface when that surface drops below the dew point of the surrounding air. The colder the surface, the more likely condensation is. With an aluminium spacer acting as a cold bridge, the inner glass surface near the edge is measurably cooler than the rest of the pane. On a cold morning, or in a room with elevated humidity, that cooler edge zone is exactly where condensation appears first, typically as a horizontal stripe along the bottom of the glass.

A warm-edge spacer raises the temperature of the inner glass surface at the edge, reducing the improvement in edge heat loss and condensation by up to 30 to 40% compared with aluminium. That does not mean condensation disappears entirely. If the room air is very humid, the inner surface may still drop below dew point, particularly in bathrooms or kitchens. But the edge stripe of condensation that was so common with old aluminium-spaced units is significantly reduced. For a full explanation of why condensation forms and what can be done about it, see the condensation explained guide.

This improvement in condensation resistance is one reason why warm-edge spacers became standard when manufacturers started engineering units to meet more demanding U-value targets. Lower surface temperatures at the edge are not just a comfort problem; they indicate higher heat loss, which feeds directly into the whole-window U-value. Fixing the cold bridge fixes both the energy and the condensation problem at once.