Back to basics: understanding warm-edge performance
“Warm-edge” is typically used as a descriptor for an insulating glass spacer category that improves fenestration's thermal performance and condensation resistance.
A spacer, as described by the National Fenestration Rating Council, "is a component that separates and maintains the space between the glazing surfaces of an insulating glass unit (IGU), excluding any sealants." A warm-edge spacer has lower thermal conductivity than an aluminum spacer. Because of lower conductivity, heat flow is reduced between the interior and exterior surfaces of the IGU, lowering the heat loss and improving the U-factor of the fenestration assembly.
Warm-edge spacers also reduce the risk of fenestration condensation. In windows with regular aluminum spacers, heat flows unhindered from the warm room side to the outside. Therefore, during cold weather, the room-side edge of glass (EOG) is significantly colder than room temperature. If the interior glass temperature is lower than the dewpoint, condensation will occur around the edge of the glass. In windows with warm-edge spacers, the interior glass surface remains warmer than those with aluminum spacers (hence the name warm-edge) because of reduced heat flow.
Because there isn't a quantitative definition for warm-edge performance. This leads to several challenges in making comparisons.
For example, a stainless-steel box spacer is considered warm-edge even though it typically only reduces fenestration assembly U-factor by ~0.01 BTU/oF.hr.ft2. Warm-edge spacers, including those comprised of non-metals or hybrids of stainless steel and plastic, typically reduce the overall fenestration assembly U-factor by 0.02-0.03 BTU/oF.hr.ft2 in captured systems and up to 0.05 BTU/oF.hr.ft2 in structurally glazed curtainwall.
Also, comparing warm-edge spacer performance should not be done based on conductivity alone. Two warm-edge spacer types with seemingly large differences in conductivity can have a similar overall impact on the assembly U-factor.
EOG U-factor also depends on sealant quantity and the size of the frame bite. The larger the bite and the less sealant used behind the spacer, the lower the U-factor. However, sufficient sealant must be used because it’s vital for durability. Fabricators typically certify the IGU durability to ASTM E2190 based on a specific sealant quantity. That quantity also should be used when simulating the performance of the EOG.