I tried a few years ago to cut out a cast vent pipe with a saw-zall. Kinda like trying to kill a grizzly bear with a bb gun.....Went and got one of those ratching chain cutters, had it in two in less than 5 min. Worked like a charm. However, for a winter fix, drop by HD and get a small jug of Fiberglass resin, and the woven matt. Cut the matt in 4" strips, duct tape the top of it to the pipe and let it hand down. Brush a little resin on the pipe ( to make it sticky) lay a 3-4 layers of the matt. Depending on temp, it might take a day or two to set. But when it does, it's never comming off.