One room is 1-1/2" lower than the next room. I suspect the room was an add on.
I’m trying to figure out the best way of not having that 1.5” drop. All the transition pieces don’t go that big. I considered 1” or two 3/4” plywood on the concrete on top of a vapor barrier to prevent concretes natural moisture but the plywood wouldn’t be secured. The plywood would be cheaper than pouring concrete to match.
I've removed the flooring in both rooms are already down to the concrete.
Any ideas?
Use closed cell foam insulation board as the vapor barrier, taping the joints. Overlay with sheathing (one layer for maximum stiffness or two layers for maximum flatness). Secure the assembly with flat head concrete screws. Consider using a countersink drill bit to minimize the likelihood of tear out or heads sticking up above the plane of the sheathing surface.
Alternatively you could use latex (foam-compatible) construction adhesive to secure everything without drilling. There are two slip planes, however: The concrete/foam slip plane and the foam/plywood slip plane. I suspect that the adhesive would be more expensive and less reliable.
I like popham's answer, but those particular materials are fairly expensive. Investigate gyp-crete companies in your area (the same material used as sound-deadening apartment building subfloors). They may be able to stop by and fill 'er up for roughly the same cost, saving you a ton of work.
