I'm working on installing the new Lopi Greenfield direct vent gas. I have a question about the thimble. I have attached the photo. This is what the dealer ordered in for me with the stove. He basically just sells stoves and does not do installs so I can't really get any answers from him. He does sell at good prices though. What the 1/8" gap between the stove pipe and the spindle? Do I use caulk to fill that? What kind? The same gap will be on the outside too. What about "dead" or "open" area, is this supposed to be like this? Thanks!