Slab-on-grade floors are prefered by some builders and are common in certain geographic areas, like the southern states, where slabs help keep a home cool in hot weather. Slabs also have an advantage when it comes to reducing air leakage, although it’s still important to seal penetrations where pipes, wires and ducts enter the conditioned space. While many of the tightest homes are built on slabs and many Passive House designers prefer them, framed floors over crawlspaces can be very tight as well and generally cost less.