June, this is probably because the floorplate of your outer wall was put down without sealant or with gaps in the seal. Sometimes this can be general airflow from other areas, but if you can readily feel a strong draft from beneath the baseboard in many areas that's likely to be your source. With the home being this new, I'd check into my homeowner's warranty first and see if the builder wouldn't take care of this. If not, about the only thing you can do to stop this is to pull floor coverings back from the walls and caulk the floorplate where it meets the floor with a good sealant. Ideally you would do this from the outside edge of the framing, too, but it's rare you could reach that area with much success without some siding removal.