We are looking at fixing up our basement apartment (including kitchen/separate entrance) to rent out, but our big concern is the noise that comes from walking on the hardwood floors in our portion of the house. I don't WANT to build a drop ceiling or remove the current sheet-rock if I don't have to. Is injecting foam into the ceiling a horrible idea? I've read lots about it being a poor moisture barrier and such, but not much about sound and an option for only a ceiling between floors. Please let me know thoughts as I'm a DIY guy with a small budget to prep the space.

Injecting foam into that space will not go very far toward mitigating noise AND it would be expensive.

The most important factor is air sealing both the floor and the ceiling layers, as would blowing the cavities a low density cellulose, not dense-packing, and just barely filling the cavity. 


Using GreenGlue^tm and double-layering the ceiling gypsum would help too.


Foam is not a solution- it would mechanically couple the subfloor to the ceiling gypsum far more than low-density fiber.

Getting rid of all the squeaks & creaks in the floor would be a first and very important step.  Those are usually due to a flexing of floorboards on an nail where the floorboard & subfloor doesn't quite meet, or at a subfloor & joist fastener. There are snap-off screws that can be face-drilled into the squeaky section to keep the boards from moving relative to each other & working against the nail like a violin bow.

One of the few application where we use low-density foam, high density glass and as Dana suggests double drywall.

Folks have had good success using Homasote 440 SoundBarrier over the existing drywall followed by another layer of drywall in this situation.

As mentioned above, foam will do little to absorb the offending sound.