Greetings Thatguy,
As most have said condensation will be your biggest problem. First defense is to stop it getting into the shed from under the floor. Make sure the site is well drained and laying polythene over the ground before you build the floor would be a good idea. Make sure that the sub floor space is well ventilated with the floor framing clear of the ground.Some perforated foil insulation draped over the floor framing would be good The second defense is to stop the condensation on the inside of the walls and especially the roof. Your idea of framing and plywood is good but this must include the roof as well. Keep the framing clear of the wall and roof cladding and fit building paper on the outside of the framing. I would not recommend using spray foam insulation as I have seen this fail badly in the past. If it is sprayed on the inside of the steel it tends to flake off, especially on the ceiling. If you have enough space some bat type insulation especially in the ceiling would be good but not essential. There will be some condensation in the wall cavities but the building paper will absorb it until it can dry out.
An alternative would be to use your shed for all the messy stuff, depriming, case trimming and possibly sizing and find somewhere inside the do the priming, powder charging and projectile seating. This is what I am working towards with the messy stuff in the garage and the clean stuff in an office like space yet to be built in the corner.
Regards and All the Best,
Grandpamac.
Bookmarks