fiberfrax adhesive
This is what I used:
http://www.aircraftspruce.com/catalog/cmpages/fiberfraxadhesive.php
It is supposed to be good to 2300F.
I am NOT an expert, my plane is NOT flying yet (hopefully in a month or so) but this was an area of concern after Dan Horton's extremely helpful thread. Dan has not mentioned this product but I think the spirit of his contribution has been to get folks to do research on what they believe will be safe and then work with the information to everyone's individual comfort level. Thank you Dan for all your work.
Prior to Dan's thread I had installed the 3/4 inch black foam stuff that was advertised as heat and sound insulation. I wanted both so I thought it was good. I saw Dan's thread and after changing my underwear struggled with a solution. Did a lot of research and finally decided that I would be comfortable with a sandwich of the stainless steel, the carborundum adhesive, fiberfrax, more adhesive, and the black foam. I sealed all edges with the adhesive. I took a sample coupon of the sandwich and exposed it to a butane flame. There was a bit of smoke, but no open flame. The smoke was from the binder in the fiberfrax. Much less than the same test without sealing the edges with the adhesive.
I am not recommending this, but it is what I have decided I'm comfortable with. I would have preferred something with no outgassing at all. In retrospect, I wish that I had known about these issues before I hung the engine, as I would have put all heat and sound insulation on the engine side, thereby avoiding the whole outgassing issue!
My concerns with my solution as it stands now, prior to actual in flight experience, is that when the carborundum glue is dry it has a bit of a crumbly brittleness to it. The sandwiched panels I put on the cabin side of the firewall are close fit between the aluminum angles, so friction is the predominant fastener. If the glue works even a little bit, it should be O.K.
Again, be aware that this is all coming from someone with zero flight experience in the airplane they're building, so I'm expecting to be surprised by many things...
respectfully,
Jeremy Constant