Dead Air as an Insulator
Are you considering building a climate controlled post-frame building? If so, then proper insulation is (or should be) at the top of your list.
If you have not seen ridiculous claims of double digit R-values from reflective radiant barriers yet (aka ‘bubble wrap insulation”) you will. Read more about these claims here: https://www.hansenpolebuildings.com/2014/04/reflective-insulation-wars/
Reflective radiant barrier manufacturers base their R-value claims upon an assembly including a 100% sealed dead air space on one or both sides of their products. In all reality, it is impossible to achieve this in real world construction.
For many years buildings have been built with an air space between building cladding and batt insulation in wall cavities. This air space did, in fact, help circulate air inside the wall and ventilate humidity through the wall. Now, as we increase wall air tightness quality and increase insulation levels, this air space no longer serves a ventilating function. Being on modern heavy insulation outside, it is too cold to help much with ventilation, and convection currents in this air space can actually make condensation problems worse. In addition, this air space is not a very good insulator. It is now recommended that all space between inside wall finishes (such as gypsum wall board) and outside cladding be filled with insulation, leaving no air space. Again – when insulating an exterior wall, don’t leave any air space.
Improper installation techniques with batt insulation can cost you 20% of an exterior wall’s insulating value from air spaces in hidden corners. This radically increases thermal bridging through framing members. If, on these same walls, you have an accidental space between insulation and vapor barrier, an air current can loop around insulation taking heat directly from warm interior finishes to cold cladding.
For an air space between wall insulation and interior finishes, vapor barrier location is critical. If an air space is between insulation and vapor barrier, air will rise because of building warmth. This air movement will find its way through or around insulation to cold side, where it will fall due to cladding’s colder surface. When insulation completely fills space between wall girts this looping is minimal. When insulation is installed less than perfectly, this looping force will accelerate. If there are open triangular corner spaces as mentioned above, this becomes a pump moving heat from interior finish to cladding as if there was no insulation there at all.
When there is an air space between vapor barrier and interior finish, nothing happens. Temperature goes from cool on bottom to warm on top but air in this space has no access to cold exterior cladding. It may circulate but it has no more effect than room air circulation.
Years ago walls were constructed to leave an air space between exterior wall framing and interior finishes. This was enough thermal break to stop condensation from forming on interior finishes in line with wall girts. With modern construction and heavier insulation, there is no longer a condensation problem on interior finishes caused by girts being cold. (There still is heat loss and in some climate zones building codes now actually require sheet insulation over all wall girts, either inside, or outside.) An air space’s insulating value is very small compared to the same thickness of any insulation.
Trapped air is an excellent insulator. Air moving freely carries heat. Circulating air, such as in a wall cavity, is effective at pumping heat from warm side to cold side. Not an insulator, in other words.
To be effective at isolating heat, air must be confined, trapped in tiny spaces, like in fibers of fiberglass, rock wool, or cellulose. Foam is particularly good at trapping air. So you take a not a very good heat conductor product and arrange for it to have many tiny cells able to capture air.
Your builder sadly did you no favors in not having a means to prevent condensation on your roof steel underside. It also requires having an actual thermal break, not just a vapor barrier. Your best solution now is to have two inches of closed cell spray foam applied directly to the underside of your roof steel, making certain they do not block ridge vents. Without this thermal break, expect to end up with damp insulation.
Now – on to your question at hand. For your walls, it appears most folks do exactly as you propose and build a 2×4 wood stud wall inside of their PEMB’s (pre-engineered metal building) steel wall girts. You will want to completely fill your wall cavity with insulation – I would recommend rock wool, as it is not affected by moisture (here is information on one particular product 
You need some provision for condensation control below roof steel – easiest would be to order roof steel with an Integral Condensation Control factory applied. In Tennessee you should not have a vapor barrier between steel ceiling liner panels and blown in fiberglass attic insulation. Placing a barrier on the underside of roof purlins does not appear to make a noticeable difference in performance of attic ventilation. Although you did not ask, you should have a well-sealed vapor barrier (6mil or greater) and R-10 EPS insulation under slabs (even in non-conditioned areas) to minimize potential for condensation on top of slab. 
#1 Controlling source – if there is not a well-sealed vapor barrier under your concrete slab on grade, you should be planning on two coats of a good sealant for it. If you do not know if there is a vapor barrier under it or not, leave a wrench on it overnight. Next day, pick the wrench up and if there is a dark place on the floor surface where the wrench was, you have no under slab vapor barrier.
#4 Ventilation – you are going to create a dead attic space above an insulated ceiling. If it is not adequately vented you are going to have problems. Best combination is vented eaves and ridge. Your building will require at least 1944 square inches of net free ventilation area (NFVA), distributed equally between eave and ridge. If this is not possible (building has no sidewall overhangs), then your choice is limited to gable vents and it will take many of them to provide adequate NFVA.
Commonly (when addressed at all during construction) solutions to this problem have often involved creating a thermal break. A thermal break reduces contact between a structure’s warm interior air and cooler metal roofing, thereby reducing or eliminating overall condensation. Installing a reflective radiant barrier, often termed Vapor Barrier, involves laying rolls of faced ‘bubble wrap’ across your building’s purlins prior to roof steel installation. Ideal weather conditions are required for this as even a slight wind can make this a challenging or altogether impossible task. This can cause jobsite delays and may bring progress to a halt while a structure remains unprotected to weather. Even when ideal weather conditions are present, installing a reflective radiant barrier can be a very dangerous task, requiring builders to expose themselves to awkward material handling on a building’s bare roof framing. These risks and delays often generate additional costs for both owners and builders, but have often been necessary with reflective radiant barrier being the only relatively affordable option to prevent interior dripping. 
Provided your building has roof trusses designed to adequately support a ceiling load, your best bet will be to blow in insulation above a flat level ceiling. If you do not have original truss drawings available to determine if they have a bottom chord dead load (BCDL) of three or more, then you will need to find the manufacturer’s stamp placed on truss bottom chords and contact them with your site address. With this information they should be able to pull up records and give you a yes or no. If you are yet unable to make this determination, a Registered Professional Engineer should be retained to evaluate your trusses and advise as to if they are appropriate to carry a ceiling and if not, what upgrades will be required.
If your building does not have some sort of thermal break between roof framing and roof steel (a radiant reflective barrier, sheathing, etc.) you should have two inches of closed cell spray foam applied to the underside of roof steel, or else you will have condensation issues (even with the ventilation). With trusses every eight feet (again provided trusses can carry ceiling weight), I would add ceiling joists between truss bottom chords every four feet and run 30 foot long (verify from actual field measurements) steel panels from wall to wall. 
In an ideal dream world every 6×6 column would measure exactly 5-1/2 inches square. However lumber comes from trees, and trees are organic and tend to have a certain degree of variability. Rarely are timbers going to be dried after being milled, other than by nature. As such, they most usually start off being cut slightly over-sized in order to allow for shrinkage hopefully ending up with a 5-1/2 inch dimension.
