40% humidity sounds like a dream. Here in Georgia, the LOWEST we get is 40%, and that is mid afternoon, in the morning it usually 80%, whith about half the year at 90%. When I go out to the pole barn in the morning, the condensation can be so bad you can see the little drops of water all over the surface of the roof. I didn't do any insulation in there because the top half of the stall are open. The steel building contractors and pole barn contractors all are telling me I need insulation to prevent condensation if I fully enclose it and want to air condition it, because am dew point is 70-75 degrees. Of course AC will also decrease the humidity inside, so that will help too, I hope.
I looked into the SIPs first, and the insullam product kind of as an off shoot of that. I talked to a couple of SIPs producer/suppliers and they basically told me they weren't competative, because after you put up the walls, you still need a sheathing on the outside. The Insullam product was my way initial thought as to how to do a SIP, utilizing the steel wall for the outside. I haven't found an SIP that does steel on the outside and wood on the inside, though I have found one that does steel for both. But the Insullam is only an R14 with 4 inches of foam and 5/8 sheet.
Currently, I'm thinking of using some kind of combination vapour/radiant barrier between the metal sheeting and the frame (it ranges from $0.25-0.45 per square foot) followed by fiberglass, and 0SB or plywood.... but still researching all of this.
