Well, I'm still not sure if you mean invisible or invincible... but I'm guessing probably invisible (meaning you can see through it), in which case I believe you just select one side of the brush and change the texture/properties for that side.
There was a great tutorial on how to make a 1-sided-window, and, if I were a better person, I would post a link to it. I believe it is on slackkiller.
...then I got some ap, and shot a big ass lazar at everyone.
Then yeah, I think you should be able to do what I said. On one side use whatever texture you want, then on the other side use the texture thats completely see-through (been a while since I used hammer, otherwise I'd tell you exactly what texture I mean)
