Triple wall pipe sealing...what can I use.?

I have a wood stove. I just replaced my fireplace cap with one that fits over my triple wall pipe. The previous one installed was for single wall pipe. There is single wall pipe from the wood stove to the ceiling, then there is a section of triple wall pipe through the hole in the ceiling. Since I replaced the cap I now get smoke coming down the triple wall pipe into the room. The people who built it put a ring/plate of metal around the bottom of the triple wall pipe to keep ashes from coming back in but its not sealed enough to keep smoke out. What product would be safe to use to seall the ring against the pipe without causing a fire hazard? The height of the ceiling is about 9 feet over the stove and the stove stands about 3 feet off the floor so the single wall pipe is about 6 feet from the top of the stove to where it meets the triple wall pipe. This didnt happen before with the single wall chimney cap. It was replaced with a triple wall cap so now the smoke (i think) gets trapped more by the cap and has nowhere to go but down. Oh..btw... this only happens when the doors are open. When they are shut, all the smoke goes up and out. I'm only looking for some kind of sealing solution/adhesive that would seal the ring/plate to the pipe. Any and all suggestions are welcome. I would like to do it this weekend as it is getting closer to winter. Thanks everybody.