Bookshelving

[Dragster Racer]

I'm looking to start my insulation in my pole building, and have decided on batts for the walls. I think I will be going with bookshelving on 48" centers, which will aid in my attatchment of OSB later.
My question is:
Do you have to use 2X6's for the bookshelves, or can you face 2X4's to the inside? Is the 2X4 enough support for the insulation? Not sure why they recommend 2X6, but 2X4 would save some money. What do you think?

 

[Ezzie]

How thick are the batts you want to use?? R12 is normally used in 2" x 4" walls, R20 in 2" x 6" walls.

 

[Dragster Racer]

6" thick, but I would face the 2X4's to the front of the cavity. There would be a little unsupported insulation towards the outside wall of the cavity. I think I decided on the 2X4's last night when I looked at it. The 2X6" would have to be notched around many diagonal supports.