Warm-Edge Spacer Bars: Why Swisspacer Ultimate Cuts Cold-Bridge Fogging on Double Glazing

Aluminium spacer bars conduct heat at around 160 W/mK, and that number sits behind much of the perimeter fogging found on otherwise sound sealed units. A conventional box spacer draws warmth from the room-side pane, pulls the edge glass several degrees below the centre-of-pane temperature, and leaves a narrow band where indoor moisture condenses first. Swisspacer Ultimate uses a stainless steel foil of roughly 0.1mm over a glass-fibre-reinforced polymer body, giving the combined spacer an effective conductivity near 0.14 W/mK. In a Rehau uPVC frame with a well-seated gasket, the psi-value at the glass-frame junction normally improves by a wide margin against an aluminium spacer, and the visible fogging band is where that change shows up.

Where the edge temperature falls

The important reading is the interior glass surface about 20mm in from the sightline. In cold weather, a triple-sealed argon-filled unit with an aluminium spacer can have an edge band 4 to 6 degrees colder than the centre of the glass.

Room air at 21°C and 55% relative humidity has a dew point near 11.6°C. If the centre pane is holding 17°C while the edge drops to 11°C, water forms at the perimeter although the middle of the pane remains clear. Homeowners usually describe this as fog creeping in from the frame.

Independent thermal modelling carried out under EN ISO 10077-2 procedures shows warm-edge composites raising the coldest interior surface temperature by a meaningful margin over aluminium. That rise is often enough to put the perimeter above the dew point under the same indoor conditions. The stainless foil stays thin enough to maintain the argon gas barrier the sealed unit needs, so the surface-temperature gain does not cost the desiccant seal life that keeps the cavity dry.

When a room fogs at the edges before the centre pane, the spacer is usually the weak part of the unit, and the low-E coating usually is not at fault. A warm-edge bar changes the mechanism producing the cold band.

Ventilation changes the dew point

Edge condensation starts with humidity. A sealed room containing cooking vapour, drying laundry, and two occupants can climb past 65% relative humidity overnight, and no spacer bar can keep every perimeter clear under that load.

Trickle vents rated at about 5000mm2 to 8000mm2 equivalent area per habitable room let moisture-laden air leave without the heat loss of an open window. Building regulations in England, through Approved Document F, now expect background ventilation on most replacement windows because tighter frames hold vapour that older leaky frames used to lose.

A Swisspacer Ultimate spacer in a damp room is still working against a high dew point. If overnight humidity falls from 65% to 50%, the dew point drops from roughly 14°C to about 10°C. That four-degree move can clear a perimeter that would otherwise fog, and it may have a larger visible effect than the spacer change by itself.

Rehau frames offer vent options that pass through the head profile without breaking the thermal break, so the vent does not create a new cold bridge in the frame. Placing the vent at the head rather than the sill keeps incoming cold air away from the lowest glass edge and cuts the chance of condensation forming around the vent outlet itself.

Bay windows put more cold bridge in one place

Bay windows tend to fog worse than flat window runs. Three or five facets meet at mullion posts that are structurally deep and thermally exposed on several sides, so heat drains more quickly at those junctions. A bay also projects beyond the wall line, catches more radiant sky-cooling at night, and can run colder than an in-plane window.

The corner posts are the sensitive points. Aluminium spacers and metal reinforcement combine their conductivity there, so the first fog band often appears beside the mullion before it appears along the straight sections of the sash.

A Luxaflex Silhouette shade softens incoming daylight with floating fabric vanes and works as a genuine glare control in a south or west bay, but any close-fitted blind can trap still air against the cold glass. That still air holds humidity close to the surface where condensation forms.

Warm-edge glass reduces that blind penalty because the interior glass surface begins warmer. A Silhouette shade over a Swisspacer Ultimate unit is therefore less likely to create persistent edge condensation than the same shade over an aluminium-spacer unit.

Perfect fit blind clips add another restriction to the airflow. They grip the glazing bead and hold the blind frame tight to the glass, giving a clean appearance and avoiding drilled fixings through the Rehau profile. The tighter the blind sits, the more it closes off the air pocket beside the cold pane.

On a bay with aluminium spacers, perfect fit blinds can turn occasional edge fog into a persistent strip of moisture. Pairing those clips with warm-edge spacers keeps the local surface temperature high enough that the sealed pocket does not routinely fall below dew point.

Depth makes the bay problem more obvious. A five-facet Victorian bay with metal reinforcement at every mullion carries more cold-bridging linear metres than any flat window in the house. Bays are often the room element where a spacer upgrade is most visible, and a single bay can justify the warm-edge specification even when the rest of the property is less troublesome.

Secondary glazing through the reveal

Where the sealed units are sound and the main fault is edge conduction, a secondary glazing rebate built into the reveal adds a second still-air layer inboard of the existing window, with a rebate depth of 20mm or more typically setting the insulating air gap. This leaves the spacer in the original unit unchanged, yet it warms the room-side surface the occupant touches and can suppress visible condensation without replacing the sealed unit. Secondary glazing suits listed buildings and heritage timber frames where a Rehau uPVC replacement is not permitted, with two surfaces to clean and a slightly deeper reveal offset by a lower cost than full unit replacement.

Reading the pattern before specifying the unit

The shape of the fog names the failing part. When the perimeter fogs and the centre stays clear, the spacer is the culprit, and Swisspacer Ultimate in a new sealed unit deals with the cold edge directly. A pane that fogs across its whole interior face has lost its cavity seal and saturated its desiccant, so no spacer type will save it and the unit needs replacement. Fog that shows up only behind a fitted blind is trapped humidity, which makes blind fit or room ventilation the more effective lever.

Take a west-facing bay with three facets, aluminium-spacer units, 21°C indoor air, and 58% relative humidity: the dew point sits near 12.6°C. On a still cold night, the edge surface can read 10°C and the perimeter fogs nightly. Add trickle vents that pull humidity down to 48%, and the dew point drops to about 9.4°C, leaving that 10°C edge clear except during the coldest nights.

Upgrade the same bay to Swisspacer Ultimate and the edge temperature moves closer to 13°C. That clears the perimeter even when the room drifts back toward the higher humidity that used to fog the glass. The vents and the spacer each add temperature headroom at a different part of the failure, and the visible result appears first at the sightline.

Rehau profiles carry both changes cleanly, since the multi-chamber frame already provides a strong thermal break and the improved spacer is not undermined by a weak frame junction. What no drawing captures, though, is how tightly the finished blind frame presses against the bead once the installer has clipped it home, and that clearance is worth checking on site before the first cold night.