Hi there Destt, and Welcome to the forum!
We have looked into the strength of the existing room and it can hold 2.8 tons (2840kg) so we think it should be OK.
Wellll.... yes and no. Just because a floor can hold 2.8 tons does not mean that you can have four walls resting on it that total 2.8 tons... It's not that simple. A wall is line load, where all of the weight is concentrated on a very small area along a single line, basically. That's not the same as having the same weight spread out across many square meters all over the floor. You need to do some more figuring out here, to make sure that you can place heavy line loads on that floor. The only real way to do that, is to hire a structural engineer to come take a look at it. As Greg said: it would be said if you built a nice studio, then it collapsed the first time you used it due to structural overload.
"3 psf rigid fiberglass" floar with 2 layers of board on top, does anyone know if the floor just sit on the fiberglass board, or would it be supported on joists?
No joists! It rests ONLY on the fiberglass. If you put joists in, then you would totally bypass the purpose of the insulation, which is to decouple and damp the floor. So, also take care when you lay the floor on the insulation, that the wood does not touch the walls anywhere. The wood sits ONLY on the insulation and does not touch the walls or anything else. Leave a small gap around the edges, then later caulk that with a highly flexible caulk.
And does anyone know a source for the "3 psf rigid fiberglass" I asked about it at our suppliers but they are not sure
I think you mean "3 pcf", not "3 psf". Big difference! The "3 pcf" part means "3 Pounds per Cubic Foot", which refers to the density. That's roughly 48 kg/m3. "3 psf" is meaningless for insulation. psf measures pressure. pcf measures density. That's probably what confused the guys at your hardware store. Asking for "3 psf insulation" is sort of like asking for "3°C of bananas" at the grocery store...
Owens Corning OC-703 would be about right for that.
and in your opinion is it worth it, or should we just add more layers of board to the floor?
That method is not really a true floated floor, but it does help greatly with impact noise, as well as providing some effect for airborne sounds, so it's worthwhile. However, you are going to loose several inches of ceiling height like that.... can you afford to lose that?
As for the HVAC, I am installing a split heat pump in the left and side of the live room, with a active extract and passive intake fed through a baffling box, probably have the extract on the back wall with the fan on the outside and the intake running down the length of the room in a flexible duct, to try and avoid the air short cycling.
I normally prefer to bring in the fresh air at the rear of the room, and extract the stale air at the front. Also, do make sure that your air supply duct is directly above the mini-split indoor unit's air intake, so the incoming fresh air goes directly into the unit, for cooling/heating/dehumidifying before it reaches the room. It's a common mistake in many home studios to not do that, then the owner can't figure out why the air in the room never seems to be at the right temperature, even though the HVAC is running full bore....
Just to a reasonable level, so I can practice with my band in the evenings.
As Dan said, you need to put real numbers on that. What one man considers to be a "reasonable level" might be considered by another man to be "screaming hell"... and the cops might have a different opinion too! Get yourself a simple hand-held sound level meter, and do some testing to find out how loud your band is, and how quiet you need to be to meet the local legal noise regulations. If you don't meet those, then everything you do is wasted. The cops will show up on your doorstep and shut you down.... as well as possibly fining you.
It's important to identify how much isolation you need, in decibels ("transmission loss"), and design your isolation system to produce that amount of isolation. A typical band can easily put out 115 dBC, and noise regulations likely impose some kind of ridiculously low limit, such as 35 dBA, so it's important to do the research and the testing to make sure that your studio actually will do what you want it to do. Just as sad as having it collapse under your feet, would be to have it shut down by the cops the first time you use it, because it isn't isolating enough. Almost as sad would be over-building it, wasting a lot extra time, money, and effort on getting more isolation than you need (however, that practically never happens! I've never heard anybody say "my studio isolates too much".....).
- Stuart -