Can we have specifics on the floor and on the radiant system. Is this is a home or multi-family unit? Is it a staple up system or an imbedded system. How is the floor constructed? What level of noise reduction are you trying to achieve and why (you have a recording studio or just don't want to hear footfalls)?

ETA: Some sound dampening can typically be achieved with minimal effect of the heating system if you have access to both floors.

It is a two-floor single family home. I am most interested in minimizing footfall noise from the upper floor. No recording studio – I just want to make sure my parents won't be bothered by high energy children running around upstairs.

The hydronic radiant system is a staple up system nestled between the ceiling joists. The subfloor on the second floor was constructed in the mid 1900s and I think it's just wooden planks (see attached pic). Insulation is stuffed into the ceiling joists. The ceiling will be double-drywalled with Green Glue soundproofing compound. Still, I understand the underlayment will have the most impact on footfall noise.

The engineered wood floors will be compatible with radiant heating. I understand the heating will perform better with tile and stone but will compromise with wood floors.

Thank you for your help!

Is your staple up system heating both floors? If so, I would recommend adding sound isolation clips to the downstairs ceiling and ensure everything is sealed then call it a day.

There are a lot of good resources out there for soundproofing information. I suggest looking at some of the home theater boards as they have collected a wealth of great information. I would not use any type of resilient underlayment system as they generally work by converting sound energy to heat energy and tend to not be conductive to either. There are some soundproofing materials with decent thermal conductivity, but they cost too expensive to be practical in a home.

There are basically four principles to dampening sound... (1) Sealing, (2) Mass, (3) decoupling, and (4) resilient layers. With a staple up hydronic system, sealing and decoupling are likely your best options. Adding mass (thermal mass) can also be effective but your system is going to less responsive if possibly more efficient and I believe that one of the benefits of the staple up systems is their responsiveness.

There are other options to add mass that is thermally conductive, e.g. fiber cement (hardieboard), either as a subfloor or an isolated thermal mass between the joists, but they are probably not worth it. Sealing, drywall isolation clips, and two layers of drywall you will get a lot of deadening.

ETA: I am not an expert on this... Have a studio and a theater and was forced to do a lot of research to get around marketing B.S.

I hear you on the amount of research needed to get around all the marketing-speak. After doing the research, it’s hard not to become an expert minus the real-world experience.

That’s why I feel the GenieMat sounds too good to be true. It has low R-value AND deadens sound.

The downstairs will be heated with a warmboard. No sound deadening is needed so that’s easy.

And to be clear, the green glued double drywall will be installed beneath the 2nd floor subfloor, or the 1st floor ceiling.