I have seen multiple houses sitting on concrete basements or slabs where additions were added on to the house in the 1970s-80s and air flows directly from under the addition to inside the house. The additions are not on concrete slabs, they appear to be just on piles with a crawlspace between the floor of the addition and the dirt below. Air then seems to get sucked from the crawlspace into the house when the furnace runs.
Is this the correct way to do an addition? Would it be better to pour a concrete slab under the addition which is connected to the concrete slab the rest of the house sits on, and build on that? Would that eliminate this air flow?
There is high radon around here and it always concerned me that the air sucked in from under the addition probably results in much higher radon levels inside the house.