Flow simulations with surfaces

The problem is that flow requires thickness to separate gases.  If you try to make 2 blocks touch only along an edge >< it sees zero thickness at the interface and gas passes right through.  With a surface of zero thickness the gas passes right through also.

How to get your surfaces to function right with thickness.... someone else would have to pop in, I don't work in surfaces, everything we do is machined from billet. 

First thing, if possible, post your models here (reply and click advanced editor in the upper right) would make it easier for others to attempt to help. 

You don't understand how the program works. Chris is essentially right but it would be more informative to say, since the program computes the flow volume, it can't do a bolean operation on something with zero thickness to establish the solid/fluid interface. The program doesn't care how thick it is but it has to have a thickness. If your set of surfaces are constructed with rigor then they should sew up into a solid body and enclose a volume - I get why that wouldn't be the case but at some point in your design process you will have to deal with it one way or the other. Use the roll back bar and put the thicken surfaces commands as you go before you stitch them up. If they will not thicken you have other issues. You could also try extruding a block and then cut the block with the surfaces - might require quite a number of trim and extend's to make it work - that may be even more work to deal with the "holes" but I would contend it can be done one way or the other. If you know ahead of time it should be pretty easy to avoid this issue.