Double glazing can be equipped with dog and cat flaps to let your pet come and leave as they please. They also help keep your pet safe while you are away. It is possible to install a cat flap inside an existing uPVC panel or double glazing glass sealed unit, https://double-glazing-cat-flap41269.eedblog.com/30874406/installing-a-cat-flap-in-a-upvc-door-cost