Double glazing can be equipped with dog and cat flaps to let your pet come and leave at their own pace. They also help in keeping an eye on your pet whilst you are away from home. It is possible to install a cat flap inside an existing uPVC panel https://double-glazing-cat-flap97873.total-blog.com/installing-a-cat-flap-in-a-upvc-door-cost-56082743