Window NATURE OPTIMO XL is made entirely of wood and combines optimal combination of high energy efficiency, attractive appearance and affordable price. It is the most thermally efficient window of Nature Optimo family, which can be upgraded into passive window with the use of thermally modified spruce instead of normal spruce – NATURE OPTIMO XLT which was awarded as world most cost effective PHI certified passive timber window. Window has very nice outlook due to aligned frame and casement which is represented by letters L (Line) and X (Xclusive).