I found and sealed the interior air leaks around my windows, water pipes, and front and back doors years ago and mulled over what could be wrong when the cold air hit me.
I didn’t weatherize, weather strip, or check the sliding glass door for air leaks when I did the rest of the house.
I didn’t need to light a candle or stick of incense and run it along the patio door to find warm escaping because the air leak was so big and blatant it almost smacked me right in the face. There is absolutely no weather stripping between the stationary and sliding glass doors. Take a look!
There’s yer problem!
Sliding doors can be tricky to weather-strip and seal to prevent drafts and leaking air if you need to use the door during the winter but it is not impossible.