I am installing a pellet stove and placing the exhaust vent through the window. I will be closing up the window in order to facilitate this, with a "removable" closure. My issue is that I need the material I build the "wall" out of to be non combustible as it will be touching the vent pipe from the stove. The vent pipe is double walled, but I'm not sure how hot it will get so I would like to be safe. Any Ideas? I was thinking of using wonderboard for the outside, and a sheet of insulation on the inside (not sure about its fire rating). I would then cut the 4 inch hole in the board and insulation to run the pipe through. I am going to seal it with the caulk that I am using on the inside of the pipes to connect them together. If it is rated for the inside (it came with the stove) of the vent pipe, it should be fine on the outside.