These doors are grey and usually (always?) open into a "weapon" room, i.e. there is a weapon on the minimap for them. I've gotten in two, but it was very non-obvious what I did to open the doors. For other doors, I've killed every mob on the floor, put out every fire, and dropped a bomb on the door and it still didn't open. Does anybody know what the rule is for these things?
Binding of Isaac – How to Open the Barred Doors
the-binding-of-isaac
Best Answer
The doors are tied to your health. The door will only be open if you have full health. However, the temporary HP boosts (blue hearts) can serve as a equivalent of the missing red hearts.
For example: If you're missing one red heart but have one blue heart the door will open. But if you're missing two red hearts and only have one blue heart the door will still be locked.