You have a water leak!
If you can locate the highest concentration of ants, or follow their path, they will lead you to the water source. This is often a roof leak, so the "nest" will probably be somewhere in a “drip line”.
Carpenter ants, unlike termites, have no ability to digest cellulose(wood) However, after it becomes water-damaged they will forage around inside the wood, feeding off of fungi and other bits of food.
Boric acid is effective, but I have seen big problems arise when attics are indiscriminately dusted, and the corrosive dust gets transferred to the breathing space of little children through leaking heating and cooling air ducts.
Dry the conditions out completely, replace the damaged wood and your ants will disappear.
Be sure to check the wet area for water damage, dry rot (a type of fungi) and mold.