Unfortunately, the building is close to a main road where you can hear the traffic, so yes, isolation is a must.
I'm curious to know why they couldn't put it on a concrete slab. As long as you can get the permits to build it you should not have had any problems.
Me too. Unless there were local code restraints, the foreman was full of shit. And it really is too bad that you didn't get what you wanted. The reason is LOW FREQUENCY TRANSMISSION. Now, you have TWO problems. Serious problems. I hate to be the bearer of bad news, but you might as well know the truth from the outset. Your building is a worst case scenario. Sheet metal buildings are difficult at best to improve Transmission Loss, impossible under budgetary restraints. Let me explain.
First problem. A wood framed floor is simply a membrane, and as such will structurally transmit low frequency sound like a drum head. From inside/out and vice versa. The ONLY solution is to add mass over the subfloor and depending on how loud the environmental noise is AT YOUR BUILDING SITE, you may even have to add layer of decoupling pad to the subfloor mass, and then add another 1/2" subfloor over that, then finish floor.
You can add mass by laying one or more layers of 5/8" drywall over the subfloor, staggering/caulking the joints and using GREEN GLUE to make the layers act as one. This is important, although expensive. The whole thing will probably be a real pain in the ass, and more expensive than a slab. However, if you REALLY need isolation, other than raising the building and pouring a slab under it, which is REALLY a pain in the ass, I see no alternative. MASS is the ONLY solution.
Second Problem. You must understand the principle of a TWO LEAF SYSTEM whereby EACH LEAF is comprised of MASS. Thin sheet metal envelopes have NO MASS. Roof AND walls.

Here again, adding MASS is your only alternative. Not only that but it must be SEALED, which sheet metal buildings are impossible to seal. That leaves only one way.
Walls:
Your best bet is to dampen the sheet metal at EVERY STUD CAVITY. There are products for this but I won't go there now. AND, they ain't cheap. Next, add two layers of 5/8" drywall BETWEEN the studs, within the stud cavity, right up to the sheetmetal. I see they didn't even put a visqueen layer(Tyvex) on the building, before attaching the sheet metal. Too Bad.

Again, all joints must be caulked, and the second layer SHOULD be glued to the first, and then caulked CLEATED to the studs to hold in place. Add a layer of insulation, and then two layers of 5/8" drywall on the interior. Again, staggered joints,
This all assumes that you are planning on supporting CEILING JOISTS on the stud wall assemblys. Since the roof is also made of thin sheet metal supported by small profile structural members, it will NOT SUPPORT additional weight, especially the MINIMUM mass that is required for a TWO LEAF SYSTEM. You can't even count the sheetmetal roof as one leaf. Therfore, you must create a Two Leaf system within the confines of the joists. Not easy, nor cheap. In reality, the ceiling system would have to be built similar to the walls. In all probability, depending on the spans involved, 2x8 joists would be the joist nominal dimension of choice, considering FOUR layers of drywall would be supported by these. 2 layers in the attic leaf, and 2 layers on the interior leaf.
And then you have doors/jambs/thresholds/HVAC-ventilation and THEN, the acoustical end of things. All in all, to isolate this building, your looking at a ton of difficult, expensive, and time consumeing details that MUST be done in order to succeed. Again, I hate to be the bearer of bad news, but you won't be the first owner of a sheet metal building that has come here and faced the truth.
Should you decide to tackle this as I've suggested, by all means keep us up to date and don't hesitate to ask questions BEFORE proceeding. And btw, I'm certainly no expert, but there are a few members here that ARE. You can depend on their expertise for conclusive advice.
Sorry bro.
fitZ
