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Because metal surfaces do not 'breathe', moisture can get trapped within the insulation when room moisture condenses on the cold metal. And the more weather-tight the building is, the greater the problem since the moisture can't escape. If the barn is galvanized metal, the rust process will be undeniably slowed but not indefinitely. Add to this the fact that wet fiberglass insulation is not much more useful than no insulation, all your work will be wasted.
The only way to avoid this is to either 1) install a moisture barrier over the insulation such as plastic sheeting or 2) use a type of insulation that has vapor barrier qualities. The only moisture resistant insulation that also acts as a vapor barrier is plastic foam insulation.
Plastic foam is available in rigid sheets or as a spray. Both are excellent insulators, but the spray foam will also deaden sound and add to the strength of the structure while keeping moisture away from the metal. Foam is not fire resistant, so it is important to keep fire away from it. I can only suggest that you consult with your local building inspector on this matter. Generally, these products should be covered with a more fire resistant material such as wallboard.
There is a pole barn builder online, BCI Barn Builders. Their site has many photos of complete barns and some construction detail. They told us that they use a rigid plastic foam insulation faced on both sides with reflective foil for their barns.
An extensive article on insulation is available on the NH website at https://www.naturalhandyman.com/iip/infxtra/infinsul.html.
Blowing insulation between the roof and the drywall in a cathedral or vaulted ceiling is the easiest way to insulate this space. This can be done as you describe, but it must be done carefully so that there are no voids… areas without insulation.
Unfortunately, this installation can be potentially damaging to the roof unless there is some sort of ventilation under the roof deck. This is accomplished in renovations or new construction by installing special moulded foam spacers that keep an air gap between the roof and the insulation. This allows a path for cooling air to move freely from a soffit vent to the ridge vent. Of course, many older homes did not have this sort of ventilation system built in, so if one wanted to insulate the ceiling, there was no realistic ventilation option except tearing down the finished ceiling... or ignoring the problem completely!
As mentioned in my article on insulation at https://www.naturalhandyman.com/iip/infxtra/infinsul.html, the two potential problems are 1) overheating of the roof deck, leading to premature aging of the shingles and 2) moisture from your home rising into the insulation, condensing and causing rot or mildew in the framing (along with lessening the efficiency of the blown insulation).
The results in YOUR home are, in gambling jargon, a "crap shoot"! In any given home neither or both consequences may occur, depending on all sorts of factors, such as the total amount of air infiltration into the home (which would lessen moisture problems), the number of shade trees (which would lower roof deck temperatures) and the region of the country you live in.
The experience of roofers who know your area and have seen the problems should be helpful in determining the best way to do this sort of insulation retrofit, as well as consultation with insulation installers who use a variety of insulation products. Be wary of articles on any home repair situation that give you "the answer", especially when there are regional or local concerns. Realistically, all we writers can do is give you a variety of options... many possible "the answers". Only an experienced local professional can give you a more definitive local solution for your problem.
Many older homes were built with a construction technique called "balloon" construction. The outside wall studs run in a continuous length from the sill plate to the roof. Then the upper floor is literally hung from the long wall studs. This method is quicker for the builder than standard "platform" construction where each floor stands on shorter studs, with the upper walls built above each floor. There is also less expansion and contraction in the house as a whole, since lumber tends to expand or contract most (percentage-wise) perpendicular to the grain rather than along its length.
Nothing is perfect. The problem with balloon construction is the hollow in the walls allows rapid spread of fire to the upper floor(s) and attic. To combat this problem, wood strips known as "firestops" are (or should be) installed between the wall studs at each floor level. Firestops act to limit the movement of fire within the walls.
If your home has firestops, then the insulation from the upper floor will stay where it belongs. If you don't have firestops, you should install them as part of the renovation. You can make this determination by cutting open a section of wall between the studs on any outside wall (inside the house... not outside) just below the ceiling. Pull out some of the insulation and look for a firestop. You might have to reach into the wall or use an unbent coat hanger (or similarly improvised probe) to feel for the firestop.
In a typical home whose roof is over an unheated attic space, the roof structure is not part of the insulation system. And you don't want it to be. In principle if not in practice, you want the temperature of the attic space to be the same as the temperature outside or lower... in harmony with nature if you will! That's why there are minimum ventilation requirements for attics to keep the outside air flowing through to equalize temperatures and allow moisture to escape to the outside.
A roof deck made of gapped horizontal boards as you describe would add somewhat to the ventilation of the attic and could lead to lower temperatures, since asphalt shingles do not provide a solid barrier against air infiltration. An even greater ventilation effect would occur with cedar, slate or tile roofing.
A plywood roof deck, on the other hand, seals somewhat tightly and therefore offers less ventilation regardless of the roofing material.
In either case, though, if there is a venting system in place, the difference might be inconsequential.