Open door policy? Not in winter

During most of the year, I have my office door propped open at pretty much all times. It's a bit more social! People can see you're in, and just pop in. But my office gets cold in winter. And the corridor is always even colder! So in late October I started keeping my door closed. I also insulated my window again, of course. It did lead at least twice to people not calling me for lunch because they thought I wasn't in. I am a bit more isolated this way, but also better insulated, and I think that's more important. If people want me they can knock!