Actually, you only need about 3/16 for the siding and 1/4 for any dividers. If it is not holding humidity, add some dividers or blocks of SC. This diagram shows what I mean:



When I built my walk-in, I bought blocks that were about 24 x 6 x 48. I ripped them to 1x1 for the shelfing supports and 3 x 1/2 for shelf railing, as you can see here: