Building Regulations and Window Energy Ratings
5) Consider Aluminium and Warm Edge
When putting together a WER application, Installers should consider that a unit construction using Aluminium spacer (gas filled or not gas filled) with low-iron and/or float glass can still achieve a ‘C’ rating with many window profiles and can provide the cheapest option for everyone.
Warm edge spacers such as Thermobar should be considered should the traditional recipe of components not meet the Installer/fabricator WER requirements.
Building Regulations and WERs Page 11