The spacer bar is the strip you see between the two panes of a sealed unit. It's a tiny detail with an outsized effect on comfort and condensation.
A glazing spacer bar holds the two glass panes at a fixed distance apart and provides the surface the sealant bonds to. It sits around the perimeter of every sealed unit. It's a small strip but every millimetre of window edge has one.
Aluminium is a metal, and metals conduct heat. That means the edge of every aluminium-spaced sealed unit is a cold bridge — a strip where the inside surface temperature drops noticeably below the middle of the pane. That's why old double-glazing shows condensation at the edges first.
Warm-edge spacers (Swisspacer Ultimate, Thermix, Super Spacer) use polymer or stainless-steel construction with a fraction of aluminium's conductivity. The edge of the sealed unit runs 3–5°C warmer than an aluminium-spaced equivalent — enough to eliminate most edge condensation and improve the whole-window U-value by around 0.1–0.2.
Warm-edge spacers add £3–£8 per sealed unit — trivial in the context of a window replacement. Yet many quotes still specify aluminium as standard. Ask explicitly for warm-edge on the order and check the quote wording. Any reputable installer will either specify it as standard or offer it as a low-cost upgrade.
Warm-edge makes the biggest difference on north-facing windows, in high-humidity households (young children, drying laundry indoors) and on windows in bathrooms and kitchens. For a comfortable, condensation-free window, it's the cheapest meaningful upgrade you can specify.
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