the builder should have insulated the room floors/garage ceiling. why do you suspect inadequate insulation there as the cause of your problems...how old is the house? the walls should have been insulated properly too as should the attic space above the rooms should be well iinsulated too. what kind of flooring do the rooms have ,,,carpet or something else???
you can add more insulation in garage ceilings and room walls too but that will be kind of a hassle....make sure you isolate the source of your heat leaks first before deciding what to fix. LJ