Advice on shop build...

[westerner]

I built a new house and shop last fall. I was the general contractor and up in Northern Wisconsin, insulation is extremely important. The largest delta (ROI) for a new construction is insulation. It will pay itself back rather quickly, but more importantly, if done correctly your living space will FEEL better.
I did closed cell spray foam in my house. The contractor charged my $0.85 per "board" foot. That is a 12x12x1" area. So, to fill up my wall to the 5" or so cost me $5 per square foot of wall space. Yes, expensive but.... I spent $650 to heat my 3400 sq ft home last winter on LP, which is one of the more expensive sources of fuel.

This is the guy to listen to. NOT a contractor, simply a smart guy who asks a bazillion questions of the contractors.

 

[StevSmar]

Vapor barrier ALWAYS goes on the inside of the building...

It's my understanding that in a cold climate this can be largely true. In a hot climate the vapour barrier can be on the outside (so it's more of a rain-barrier then). I believe it depends on where the dewpoint is within the wall assembly.

I live in Winnipeg, so we put our vapour barrier about 2m in from the wall...
(that was a joke on how cold it is in Winnipeg. We put it under the drywall if there is no foam insulation. If there is foam insulation plus batt insulation, then we don't use a vapour barrier since the dewpoint is somewhere inside the foam. As long as the wall assembly can dry to either the inside our the outside everything is good.)
(I've seen walls where there is foam on the outside, batt over that and then a vapour barrier on the inside- it's very concerning to see the amount of moisture that can get trapped under the plastic in a new house)

 

[HMiller]

The point is to keep moisture out of the wall as best as possible. The hotter side typically has more moisture so you want the vapor barrier there if possible. Of course there are other considerations and compromises. If you have hot and cold seasons you can't move the barrier twice a year (at least not for a reasonable price). If you have foam on the outside that forces the vapor barrier outside too, but foam on the outside is such a win that it is worth it.

I'm more concerned with the total insulation above the rest though, the more you add the better.

 

[7milesup]

The vapor barrier placement IS dependent on dewpoint. As I mentioned earlier, control of humidity inside the building via conditioned air will go a long ways in mitigating moisture issues within the building envelope. One of the concerns though is you do not want two vapor barriers (actually, the correct term is vapor retarder). You do not want a vapor retarder on the inside wall and the outside wall because moisture will become trapped and create issues withing the wall cavity.
Closed cell spray foam is considered air impermeable at .1 perm (or less). In my house, I sprayed over 2" of closed cell foam and then covered the studs with traditional drywall. No vapor barrier was installed because the wall needs to breath between the sheetrock and foam. Even if I painted the drywall with non-permeable paint, that would have created in issue inside of the wall cavity during winter months.
So, after saying all that, I am thinking that your vapor barrier needs to be on the outside if you live in a hot climate. There are zones for determining where to put the vapor barrier. I am much more versed for cold climate construction, so I apologize for that.
I would suggest checking out this article in Journal of Light Construction. Also, Walls that work in the South. And, Moisture management.