i'm making a side-draft forge and, pardon my lack of appropriate terminology, the box or flue or chamber or whatever you wanna' call it will be going through the wall of my garage. i'm looking for some suggestions on what kind of barrier to put between the sheet metal and the wall itself...not on the flat drywall interior, but lining the hole that goes to the outside...i've thought about cutting up a sheet of "wonderboard", the concrete sheets used for laying tile down on but wondering if anyone has any better suggestions. i'm assuming that direct contact with the interior of the wall is NOT a particularly good idea, given the potential for a fire...

I made a 3 layered tube as described by building codes in my area. There are stainless pass through fittings available in various sizes but all were more than I wanted to spend. I even put a temp sensor in the chimney box with a readout at my forge so I could monitor the temp in the roof penetration. The three layers in the tube are open and a draft to the outside is created from heating in the tube and thusly cools the center layer of the pass through.

Side draft forges are very British so congratulations. Have a look at the on line catalogue of the major british manufacturer Vaughans (not vaughns who are a different organisation). Vaughans (Hope Works) - For all your Blacksmith, Foundry, Farrier needs. Now available Ironworkers and Bending machines

Hope the pictures there are of help.