We moved into an ICF home last year. ICF homes are known for blocking sound, and it is true that sound is blocked from outside. But, we’ve noticed that sound transmits really well inside the house. From our bedroom on the second floor, we can clearly hear people in the basement, 2 floors below. At first I assumed the noise was transmitting through the floors, after all, the walls are solid concrete surrounded by inches of styrofoam and drywall, but when I put my ear to the floor I don’t hear much. On the other hand, putting my ear to the walls allows me to hear the voices even more clearly, to the point I can understand words. There aren’t any cavities or utility runs. And it is on all exterior walls. I simply can’t figure out what is transmitting the sound. Concrete and styrofoam aren’t normally good sound transmitters. Maybe any rebar inside the concrete? I tried googling it, but everything online about ICF and sound has to do with transmission from outside to in, not within the house. Does anyone have any ideas what could be transmitting the sound?

The ICF normally doesn't transmit sound through it. You aren't dealing with it that way....It could be reflecting it throughout the house. Most sound is transmitted through heat ducting. We have a ICF ranch and the outdoor sound comes through the standard roof.

It’s not coming through the roof (which wouldn’t make any sense from that basement to the top floor) or the heating ducts (the top floor ducts are in the attic, while the main floor and basement share a completely separate duct system in the floor of the main floor and the ceiling of the basement). We have two completely separate furnace systems in the house - one in the attic and the other in the basement. There is nothing connecting the attic to the basement.

That sounds very strange. While it is true that ICF is a great barrier to stop outside sounds from entering the home, these properties are not one directional. While outside noise cannot enter, inside noise can also not easily escape. I always recommend adding sound barrier insulation to interior wall(s) to help reduce sound transmission from one side of the home to the other to stop this echo effect with the trapped sound throughout the home. I'm not sure how you could possibly use the wall to hear conversations 2 floors down....not calling you a liar, but I think there has to be something else causing this as the concrete, EPS, and drywall are all infamous for sound resistance (with the varying density of each respective material effectively blocking a different frequency of sound).