What I was think about using them for was a lightweight alternate to mass my exterior
Lightweight alternate to mass?
There is none Seeker....mass is mass....1 lb is 1 lb whether you have 1 lb of feathers or 1 lb of rock, its still 1 lb. Same with Mass. As far as weight on your trusses, well let me put it this way.
I'm certainly no acoustic engineer, but heres the deal as I understand it. Your studio walls are a two leaf(if you fill the blocks with a certain type sand) assembly with a massive exterior leaf and a lightweight interior leaf, but combined with a isolated slab, I believe you'll have good transmission loss through these wall assemblies as long as you pay attention to all flanking/leakage issues. However, his means in order to keep the ceiling/roof from being a "weak link", it needs to match the transmission loss(TL) of the wall assemblies. But how do you know what your walls TL actually is? You guessed it, you don't. This is why studio designers use TL rated assemblies so they guarantee a certain assembly will meet thier TL goals . They know assemblies are tested in a lab and given a TL rating, usually in STC(Sound transmission Class), which is a "speech" frequency range rating. Music is much more demanding, because of impact and low frequency sounds transmit much more readily. In this respect, your walls should handle these TL grimlens pretty wlell, but who can say exactly... how well? However, your ceiling roof may be impossible to add the mass it takes to match the walls TL IF your trusses arn't engineered for the weight. If you are haveing these trusses engineered, they should be designed to support a TARGET WEIGHT of an assembly that would do the job in the first place. This means specifying what the assemblies are supposed to do, transmission wise. Since you don't know, you're spittin in the wind in that regard. This means going for as much mass as you think you can afford and how much weight you can afford in an engineered truss......BUT
Here is where its gonna get tricky. And please, I'm only suggesting you consider this. You are the only one who knows what your TOTAL circumstance are. When you decided to build a massive wall assembly, I'm sure you didn't know HOW MUCH transmission loss you actually needed, correct? Very few people do, and fewer yet with the skills it takes to translate this data into a building envelope that meets the goal.
In that respect, you designed for a WORST CASE scenario, in which you decided to build the walls out of concrete block thinking if ANYTHING would be
the best material for the job, concrete would, correct?. And you were right as long as everything else is in line with this thinking. But now you have to deal with the ceiling/roof. And this is where "only you know" comes in loud and clear.
Seeker, there is something to consider here. From what I understand, when sound encounters a rigid mass barrier such as your walls, sound will diffract up and over it, which is exactly why massive wall barriers along hiways, to get this diffraction high enough to lower the db profile of sound reaching adjacent residential buildings. And this is the issue. Not being an acoustician, I can't say for sure the exact connotation this implies as it relates to the mass required in your cieling/roof leafs . But it just might be the little acoustical knowlege detail that allows a LIGHTER MASS cieling/roof assembly. However, this doesn't mean HOMOSOTE would do the job either. At least in my opinion.
Here is the bottom line.
Only your ears/knowlege of the situation, can define what you THINK you need. Distance to neighbors, conditions of the space between you and neighbors, type of music, db profile of the music you are recording, and a few other issues have a bearing on this final decision. But here is a few suggestions.
If you are buying premade trusses, they are usually designed to support standard residential construction as per span specs. If you don't know this info, after the fact guessing as far as your trusses support limits are concerned, is asking for trouble if you overestimate their limits. I would find out EXACTLY what these limits are, and if they don't do the job...well, I don't know what to say other than...
I think you are going about this backwards. If you haven't already ordered these trusses, good. Here is what I suggest. BEFORE you order them:
1. DEFINE your ceiling/roof assembly MINIMUM TRANSMISSION LOSS requirement in db.
2. Tranlate this into an assembly that meets this minimal TL need.
3. Calculate weight of these LEAF assemblies.
4. Have structural framing/truss engineered to meet this load support requirement.
5 Order trusses.
6. Build roof/cieling assemblies as per design.
7. START RECORDING
Other than that, you are guessing, plain and simple.
fitZ