Those large black ants roaming around your yard are probably carpenter ants. Carpenter ants do not eat wood, but the will excavate it for nesting and occasionally can cause structural damage. They will also nest in insulation. In most instances they are mostly nuisances. Many home infestations are satellite colonies from a nearby stump or log. The best way to prevent damage is to avoid soil or wood contact with wooden portions of your house. This would include branches touching eaves, firewood piled against the house, and bark touching wooden portions of the house.