I have decided to convert my crawlspace into a conditioned room. An article from the Building Sciences website shows different methods as far as supplying conditioned air and recirculating it, but I was wondering what people here think. I have an almost full basement, with the exception of one part that is a crawlspace. The crawlspace is in a corner, so the finished part of the basement shares two block walls with the crawlspace. One of these shared walls contains a return air at floor level that leads to the air handler, which is in the crawlspace. Since my air handler is in the crawlspace, it is easy enough for me to cut a register into the plenum to supply conditioned air to the crawlspace. My question is what to do about circulating the air? One method I read about in one of the Building Sciences articles was to use passive transfer grills. They showed transfer grills being placed in the floors of the rooms above. From reading posts on this website, however, I thought it was a fire hazard to have open grills? In my case, I thought it would make the most sense to place a passive transfer grill in the shared basement wall at ceiling level above the return air grill. This way, the air would be passively drawn from the crawlspace through the shared basement block wall with the return. Or would it be better to place the passive transfer grill in the other shared wall, so that the transfer grill is as far as possible from the basement return air? Does this make sense to do, and is it OK in that the transfer grill in the block wall would have no ductwork attached to it? Elk? And as far as the foam board used to insulate the crawlspace walls, have people here used fire-rated foam board when doing this since it isn't covered with drywall?