Macfrog,

1 - Ceilings are never visible in the 3D window. Ceilings are one-sided, meaning that they are visible when seen from underneath (that is from inside the room), but are transparent when seen from above.
2 - Visible - invisible
- click in the top-right window to select it
- type CTRL-A to select everything at one
- do a right-click you can then change all walls, or all rooms, or all furniture in one go
- this is a great way to make everything (in)visible

3 - To make it even easier, you could decide to group all furniture.

Hans