Double glazing can be equipped with dog and cat flaps to let your pet leave and come as they please. They are also great to keep your pet's attention while you are away. It is possible to install a cat flap into an existing uPVC or double glazing sealed unit. https://cat-flap-in-upvc-door-pan78996.webbuzzfeed.com/31331204/installing-a-cat-flap-in-a-upvc-door-cost