I just measured one of my medium 10-frame Langstroth boxes and a corresponding frame. The box and the frame came from Western Bee Supply, who I believe is the supplier for all of Dadant's woodenware. Anyway, in this particular example, there is 1/8" of space from the top of the top bar to the top of the box and there is also 1/8" of space from the bottom of the bottom bar to the bottom of the box. Obviously, this leaves 1/4" between layers. I know that, "A sample of one, doth not a conclusion make", but I'm willing to bet that this is pretty standard.