Just met with the builder again... his first description of the multi zone HVAC was a "manual" system where each zone just has a main trunk coming out of the furnace. Dampers are set to control how much goes to each zone. That didn't seem like nearly as good of a solution as a thermostat in each zone that could call for heating/cooling for that zone only. Sounds like between a REAL multi zone system and the best insulation, I could be ok here. Given the fact that he quoted out over $30k for in floor heat in the basement alone, I may have no other choice. We could also rough it in for $6k and add the boiler down the road.

So I am going to answer the question first and then explain the answer later. It would be very expensive to keep all the flooring the same height without adjusting the floor depth. Serenity mat in your entire house is not going to really do it, and the cost of full sound proofing the entire floor is quite high and unnecessary. Many of the solutions such as Serenity mat are largely made for retrofit applications. It is much cheaper and better to properly design your space than it is to buy stuff to put on it. Background: We are only concerned with limiting bass, highs will automatically limit as we limit bass. Bass waves do work. There are two ways to deaden bass travel. The first is to limit the work - this is what all absorption products do, any compressible material will work, you don't need to spend the money for serenity mat in fact a lot of people (myself included) use horse stall mats from Tractor supply as a cheap and easy alternative. The second way to limit bass travel is to increase the work required, simply add mass. The best solutions will use both. For the serenity mat to work well you really need layers that look like subfloor - green glue - serenity mat - green glue - 2 layers of 3/4 plywood glued together - flooring, also nails into the plywood shouldn't penetrate the mat. This creates a big mass over the mat and limits the absorption needed by the mat. However you are still going to have sound travel above the room and through the walls. Making sure you joists don't create sound travel channels and soundproofing the garage ceiling will be a lot cheaper than sound proofing the floor. You can get away with a Mooney wall type garage ceiling (with double drywall) and Mooney walls (with double drywall) in the garage and essentially eliminate the bass travel and spend a lot less than you would on the mat. To be fair attic and HVAC travel is going to be a lot more significant and likely than bass gain from a garage. I run two SVS PB 13 Ultras for Home theater and two Velodyne DD18+ for music. Occasionally I will throw them all on for a movie with a good soundtrack. I doubt your husband gets more bass than that. The only thing I use between our theater and the living room above it is 2 layers of drywall glued between the joists along with sound isolation clips with 2 layers of drywall hanging from them. With all 4 subs going you can barely feel it in the living room (during big explosions you can feel a very low deep vibration probably in the 20 - 30Hz range (which you are not going to get rid of.)) So back to your question. You are talking a lot of glue, serenity mat and plywood to get the floor the same height. It is just not worth it.

Unless the roof or pipes leak or ducts discharge into the attic, during hot humid weather moisture can get into an attic through oversized wall vents that allow moisture laden air to enter the attic and/or from a bathroom directly below the attic floor. Moisture from a bathroom can be controlled with an exhaust fan and a vapor impermeable ceiling paint. Moisture from vents can be reduced by substituting smaller eave and ridge vents In a sealed conditioned attic with a moisture barrier at the ceiling of bathrooms, moisture is not expected to enter the attic but if it does the conditioned air should dry it. Even HVAC ducts in the attic will leak enough dry air to keep the space dry. These issues can be solved regardless of whether the attic is vented or sealed so you should decide what you want, then design the attic. Since it is expensive, the primary reason people seal an attic is to allow HVAC equipment and ducts to be located there but they might also want to achieve a higher R value without changing the structure or the create a better environment for storage. If you are planning to replace the roofing I recommend applying Grace Ice & Water Shield over the entire roof surface and using coated aluminum or copper step flashing at side walls.

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.