The spacer holds the distance between the panes of an insulating glass unit. When it has low thermal conductivity, the perimeter of the unit behaves as a warm edge.
Compared with a conventional metal spacer, the thermally improved solution limits the thermal bridge at the edge, helps the overall Uw and keeps the internal temperature higher around the perimeter of the glass. That also reduces the likelihood of condensation when conditions are unfavourable.
Among the available solutions, products such as Swisspacer or Thermix can be used, depending on the build-up and on the supplier of the glazing unit.
The spacer holds the panes apart; when it conducts little heat, the perimeter of the glazing unit works as a warm edge. With the reference values of EN ISO 10077-1 the linear transmittance Ψg is about 0.08 W/(m·K) for a metal spacer and of the order of 0.05 for a thermally improved one. How much that counts depends on how large the perimeter is in relation to the area: in a window with many small panes, noticeably more than in one large glazed screen. And it counts in the calculation only if it is declared — if the quotation does not state the spacer type, the declared Uw has most likely been calculated with the least favourable values.