I already ahve a few threads about the insulation in my new home.  Here is another question:

I have a large bonus room over my large garage.  I have radiant floor heat in my garage floor and would like to benefit from that in the room above.  I plan to insualte the floor system of the bonus room with dense packed cellulose on top of the ceiling of the garage (WHERE I CAN)  There are several of the bottom cords of the trusses that aren't going to be open once I install the ddrywall so I need to do those first.  Do I spray open cell in there and call it good, or will that transmit noise like crazy and not help with warming?  Or do I hang a net and dense pack the cellulose in there?  Which is better?

Why don't you leave it uninsulated, and instead insulate the ceiling of the bonus room? That is if you want to maximize the heat gain from the garage.

Thats a good point to consider. One of my other threads is addressing the whole non-vented attic space above this bonus room. I am concerned about open cell foam against the roof deck and a vented attic. I am pretty certain that I would like to at least have some noise reduction from the garage below simply for future potential uses of the room up there. I suppose I could do that with batts of safe and sound Roxul though.

I believe a double layer of drywall plus Green Glue in between would be better at soundproofing than adding insulation between the trusses, as the trusses themselves transmit a lot of the noise. Sound resilient flooring (such as cork) above would be a good choice too. Floating floor even better.

You said " I am concerned about open cell foam against the roof deck and a vented attic". Not sure if mean you have soffit \ vents already and it will become unvented if you place foam directly against the deck? If that's the case I'd leave a ventilation gap (say at least 2") between the foam and roof deck. Different options for this -- install rigid panels or batts that don't extend all the way to the roof deck, or by adding cardboard\plywood channels creating a gap against the roof deck before spraying foam being sure to support cross-ventilation between the rafters too if you don't have a continuous ridge and soffit vent.

Also it may get hot up there in the summer... You probably at least want a vent, window or something to open.

I do already have soffit and ridge vent installed. Trying to decide of FOAM is the answer and allowing the 2" space above OR if I want to block off the soffit and ridge and make it unvented That's what I meant

OK, I see the other thread you have now, and that you're in Climate Zone 4, close to 5.

I don't believe raw open cell against the roof is recommended in Climate Zone 5 or colder, due to risk of condensation\mold in winter against the roof deck, most likely due to unwanted voids in insulation against the deck allowing moisture to condense and accumulate. So be sure to have a vapor barrier of some kind (drywall + approved paint\poly, etc). I suppose you could use closed cell instead too, but not going there for this conversation.

So if I were you I would probably make it vented especially since you already have the soffit and ridge. My bias in the colder zones is I would only do unvented if I had a concrete or metal roof (nothing to rot). Vented also helps prevent a roof leak (from above) from silently turning your decking and rafters into mulch, although if you care about that then you probably also want open cell and no poly so that the leak will be found in the interior ceiling drywall in roughly where the leak is above (which is good).

So open cell with the air tunnels above it is ok for the roof deck of the bonus room. Does each rafter need to have the air tunnels? Those things can get pricey.
What's funny is that around here they are blowing he open cell directly on the roof decking and filling the entire rafter area. They quoted me about 6 inches deep in this area. They have been doing this in my area for 20 years and haven't had issues with roof decks rotting out or anything like that. Does it stand to reason that I could spray on the roof deck and not have an issue? I mean what are we using here to substantiate that e roof deck will rot and have posture issues?????

Still wondering about the floor....i've heard 3.5 inches of open cell, i've heard put the ceiling in and blow it under the floor, i've also heard do nothing since I have the radiant floor heat. I am about to make this decision final and would like to know if 3.5 inches of foam is going to help me at all or just block the radiant heat from the floor below? I would like an inexpensive method of keeping the floor warm