My son's came home for a visit recently and when going out to the backyard from the living room, it took a minute for them to figure out what was going on.  For the longest time we didn't have a lock by the handle of the sliding glass door, only by the floor, a lock that you do with your foot, whatever that's called.  So we recently (finally) put in a lock on the sliding door (which we delayed doing for years) - and my son's didn't know at first why the door wasn't opening when they tried to slide open the door, that's because now there's a lock they have to turn whereas before, like forever there was not. 

Anyway, we should have done this a long time ago, because now our power bill is $100.00 less a month, and in the heat of Summer, just from installing this lock making sure the door gets sealed when closed.  If you have doors or windows that you know air is escaping, get it worked on, it could be as simple as a good lock on it, but it will save you lots of money in the long run!