Excerpt from Rat Control
In Old buildings walls may be, stopped by fitting sheet metal between the joists and nailing it to the sill. Sealing hollow walls by these means is commonly called fire-stopping. It is now required by the building regulations of many cities and is of great value not only in eliminating rat harbors but also in retarding the spread of fire and in improving insulation against heat and cold. Wooden floors in buildings without basements, particularly on farms, provide free shelter for rats. Where possible these floors should be replaced by concrete, with retaining walls extending 2 feet underground to prevent burrowing under the ?oor from the outside....