Excuse me for double clutching this Thread...
(darn that 5 minute edit rule)
I understand that you already have a Doggy Door, but...
there has been considerable discussion on the pros and cons
of one, by some very well informed and concerned users on
this forum. You might drop by the link below, and do a quick
read on those issues.
I am totally satisfied in using a Doggy Door, but
it's not for everyone, and those with one, should be
very aware of the risks.
Ref:
http://www.yorkietalk.com/forums/yorkie-products-reviews/224269-doggy-door-safety-convenience.html