I learned my lesson and wish I went with the thicker engineered wood like I did in 2009 as it is more sturdy, feels better to walk on and is warmer. I had to redo my engineered wood floors due to damage from my water heater.

It is a good question. I assume you referred to the engineered wood floors in floating installation. In fact, stapling down an engineered wood floor can be a good option for your case too with a good underlayment like Floor Muffler. Floor Muffler will definitely help to control the noise to the rooms below. We usually recommend the nailing or stapling down installation over the floating if you have a plywood subfloor. If you prefer solid hardwood floor, it could work the same way.

I honestly don't know the moisture content of the subfloor. Below is another floor (NOT a basement, it's a ground floor). The end joists are insulated with Polyurethane Spray Foam (4 inches = R24), so we're safe there. The upper floor walls are R24, the lower floor walls are R18-R22. As a matter of fact the whole house was insulated with SPF, the roof being R36. Of course we do get humidity fluctuations in Montreal, with our winters being dry and our summers very humid. We will be installing an air exchanger and a central humidifier. I'm guessing the humidifier would help? Would you still use glue? Thanks.

If you are on slab, I'm not sure what you would be nailing TO. Adding in a sleeper subfloor would be very expensive and will still require a vapour barrier underneath. *Technically a glue down SOLID hardwood can be done on slab - but it isn't recommended unless your HIGH END flooring installer has done it successfully in the past. A glue down engineered floor is technically "more stable" in the widths they are suggesting, but I would question a comparison to a 2.25" solid hardwood. Especially if the solid NARROW strip hardwoods are rift or quarter sawn. The same adhesive would be used regardless of plank structure (solid vs. engineered). The RIGHT adhesive (aka. EXPENSIVE glue) MUST be used over slab in TX. And then the proper SPREAD RATE must be used (lots and lots of the expensive glue will be required) as well as the PROPER TROWEL profile/depth will be required. These expensive adhesives ($100/gallon is common) will achieve 30-50sf per gallon. These adhesives will be the vapour barrier. The most expensive ones will have "limitless" moisture content as one of their key features. Even with the TOP adhesives, the installation will still be cheaper than a sleeper subfloor + nail down installation. The sleeper subfloor would mean a lower priced flooring professional can be hired to achieve the nail-down installation. The glue down method will require the higher skilled professional with all the knowledge required to install a glue down wood floor PROPERLY. And the next issue is the HVAC system. Regardless of the installation method, your HVAC will have to have the ability to maintain a CONSISTANT level of humidity. Sometimes A/C isn't enough in high-humidity environments. Sometimes a whole-home dehumidifier is required in extreme humid areas. Please review your HVAC system's abilities to TIGHTLY control humidity (somewhere around 40% is PERFECT...and it must be maintained 24/7 for the next 25-60 years). Start there. Spend more money for the upgrade on the HVAC system so you can have the wood floors - regardless of width/composition.