I have a house that was built in 1980 and utilizes a subfloor of a
compund I have never seen in a house, before or after this house was
built. The material is a gray, almost pressed paper looking material
that is a full 1-1/2" thick. The problem is that over the last 32
years, the subfloor has drooped a bit between the floor joists, and the
joists themselves may not be the straightest, either. Nevertheless, we
are determined to put an engineered flooring in and I am at An impass. I
purchased and laid 3/8" plywood over about 250 feet of flooring (1
bedroom and 2 hallways and a bathroom) and began laying my flooring.
The plywood did not take out enough of the differences from the peaks to
the valleys to make the flooring install correctly. I realized this
after installing it in the bedroom, and I pulled it back up to address
the subfloor.

The subfloor has up to a 3/8" difference across different parts of the
floor. I am considering using a self leveling compound, but posts I've
read state that it's not the best on big floors and the bedroom itself
is about 150' and it needs it all over the floor. Additionally, There
is still some flex where the plywood did not completely adhere to the
original subfloor and I fear the self leveling compound will break up
over time.

I am at a standstill and completely clueless as to what to do. I have
already committed several sheets of plywood down with glue and nails, so
ripping out the original subfloor and replacing it is a bigger task.

Any thoughts or directions would certainly be appreciated.


*I have seen floors like you describe. They developed cavities over time
for whatever reason. I think that stuff is for sound and/or fireproofing.

What I have seen done is the cavities get filled in with a Dash Patch type
product such as this:
http://www.pacoa.com/dash-patch-25lb...ler-12125.html

Home Depot sells a similar product under the "Henry's" brand name (I think).