Depending on the rebate of the windows, you may be able to get double-glazed units manufactured to fit. Talk to your glazing supplier about the options available, such as low-emissivity K Glass and argon gas-filled units. If these aren’t feasible, remember your SAP energy rating will not fail if your windows do not fully comply with the required standard, as they form only part of the building. You can compensate elsewhere – such as using extra insulation – to reach an acceptable standard.