My wife and I are renovating our 110 year old home and we just poured 1.5" of concrete on the main floor tubing for radiant floor heat. Today is the first day of walking on the floor and we both noticed vibration in the floor when another person walked around. I can see that this would be very aggravating while trying to sleep, read or eat. Our engineer told us that the floor assembly has more than enough strength to accommodate the concrete (1.5" is at 65% of floor capacity) so we know the structure is solid enough. Admittedly, our house is completely empty as the reno is over the entire main floor so there is no furniture, no kitchen, no area rugs...just bare concrete. We had planned to do glue down hardwood however I am nervous about the vibration. Should we consider a floating engineered floor or will glue down with furniture, rugs and a kitchen temper the vibration?

Strength, stiffness and sound transmission are different things. The problem is something that you hear or feel?

Feel only.

What's underneath the floor? You may need more support in the way of columns. Look at it this way- a trampoline will easily hold your weight (strength) but you wouldn't want one for a floor (stiffness). Floor stiffness is usually defined as a fraction, such as 1/360. That means for 360 inches of span, the floor could deflect 1 inch. Not noticeable in a small room, but it becomes a major factor over bigger spaces.

As long as you are sure that the construction is all proper, I will tell you that adding things such as floor coverings and furniture will help damp out that vibration unless you are on the thin edge of code requirements, and in that case, I couldn't say. I laid some proper 4" reinforced floors that sounded uncomfortably drum-like, but either additional curing or other changes made the problem go away.

We are at 65% load capacity, well within structural safety and far from the limits. I have access to the joists from the basement so I can, if needed, stiffen things up later. Alternatively, I am considering glueing down cork and the glueing down the engineered hardwood. Have yet to check into costs on the cork but think that may be a good option. Trouble is, I won't know how the floor will perform without the cork until it is too late to go back. The floor is supported by old growth fir 1.75" by 9.5" on 16" centers with the longest span being 11' and sheeted with 3/4" by 8-10" fir planks.

Zad - how is your X bridging/blocking? If you are not going to finish your basement, try running some 1 x 4 strapping every four to five feet across the bottom of the joists. Use construction adhesive on each joist and nail. This prevents the bottom of the joists from moving sideways and may remove much of the vibration.