If by models, you mean the walls of your house model, then check 3D view->Modify 3D View Walls transparency.

If you mean furniture models, if you created your own furniture, check the .mtl file for a d transparency value - see http://www.sweethome3d.com/support/foru ... hread,1196

If neither of the above, I haven't a clue. Perhaps post a screenshot so that others have more to go on.