The bay has been replaced with high specification uPVC, not all Windows are the same! Our uPVC profile has 5 chambers to create extra insulation, the gasket and hardware work together to form a tight seal to keep valable heat in. The glass has a very low iron content which captures free energy from natural sunlight bringing it into your home, it is also filled with argon to provide another layer of insulation. The glass also features a warm edge spacer bar which helps limit the amount of coldness passing through your glass and into your home.