Yeah, in theory aluminum would soften and melt at certain pipe temps, but in reality there is not enough pipe surface touching the side of the stove pipe to ever make this happen. It is more expensive, though - so we never used it.
Cement board is not actually tested for use with stove pipes running though it, and as others here have reported, it contains everything from wood to poly foam pieces. I would not use it unless I had to.
There is really only a need for alternative materials when the preferred one cannot be attained…....on this board, we sometimes discuss everything to death…..it’s easy to see how committees and government rarely get anything done, but one person with some tools…now there is productivity!
We didn’t even discuss liners, etc…...but if the stove is being installed with a full liner, than the removal and reinstall of the plate is not going to be needed…..that was one advantage of the sheet metal - relatively easy removal and replacement.
If a person was going to insulate on top, even a small amount (1 inch) or fiberglass (no paper or foil) or koawool, rock wood, micore, etc. would do the job. Whether this does much depends on the stove and how close it is to the plate. In my fireplace at home, I was always able to touch the plate when the stove was running - evidence that not much heat was headed in that direction.
Elk has a good idea with that fireplace wall insulation - we should develop and package a new product - something that you can cut with a utility knife, does not throw fibers out, and simply glues or otherwise fastens on the rear and sides of exterior fireplaces (the interior walls of them).
