I have been trying to address a few squeaky areas in my hardwood floor. I first thought these were original floors (1905) but as I've continued to investigate I think the offending portion of the floor was installed later (~1950).
Looking at the floor cross-section at a return air penetration, it appears that the current floor was installed on top of the original floor with furring in between. I'm not sure what the purpose of the furring was...to reduce the transmission of creaks from the original floor or perhaps to level it for the new floor? The cross-section is:
-3/4" hardwood floor (~1950)
-1/4" furring strips, I don't know the distance between strips
-~3/4" hardwood floor (probably the original floor from 1905)
The current floor is largely in good shape but there are a few spots, mostly in high traffic areas, where there is deflection as you walk over the boards. This causes a lot of squeaking. I assume the tongue and groove interface has worn over time allowing greater deflection in these unsupported areas between furring strips.
Has anyone addressed a problem like this before? Might it be possible to inject an adhesive into the void between the current floor and old floor to shore up the floor and reduce deflection? Perhaps something like this. Any thoughts or suggestions would be much appreciated.