The Gemara (Bab. Shab. 146b) discusses certain rules that pertain to a house that is entirely sealed. What kind of house is entirely sealed? If it means what it seems to mean, that it has no entrance or exit, no windows, no doors, what kind of a house is that? What is the point of deriving and teaching a law that can't be applied?
According to (unattributed?) notes in the Soncino edition, "closed" doesn't necessarily mean entirely sealed. Here's what they've got:
First the text:
This seems to be saying "closed (sealed up) on that side", rather than "entirely sealed". "Closed" is achieved by walling off an intact doorway (but if you break through the door-frame first it's not "closed").
The cross-reference to Bava Batra 12a doesn't appear to add anything to our understanding.
The second note also says that "closed" means walled up in this way: