Yes, if you have stock inside then insulation is agood idea. Normaly we had a loft full of hay which provided for that but we can't presume you even have a loft in your design.

T&G lumber is very expensive and if your covering with steel anyway, propably simple rough planks at 5/4 x 10 or 12" wide with tar paper over them would likely be fine with the 48" spans.

I mentioned T&G in my last post but neglected to consider the costs involved.