John -
Your question was unclear as to whether you were looking for a fire shield, cooling airflow control, or thermal protection. Please defer to answers from guys who know the real answer to your question. However, here are some things to think about.
First things first. I am referring to the firewall as the protective layer/layers that protects the airframe from fire. I am referring to the bulkhead as the structural element that carries the loads. They are not usually the same part. Normally they are layered. But usually we call the bulkhead "the firewall", even though it is incorrect to do so.
1) Fire shielding -- Thin stainless steel sheet (0.023 thick) is commonly used for aircraft firewalls. The purpose of this element is to protect load-carrying structure from direct flames. The qualification test is to expose it to a 2000 F flame for 15 minutes, or something very similar to that (I'd have to reread the advisory circular). If the sheet doesn't burn through, it passes. In a metallic airplane, or even a tube-and-rag airplane like a Citabria, the fact that the back side of the steel gets close to 1000F does not matter. The load carrying bulkhead or tubing can still hold together. Heat shielding is not needed.
2) Heat shielding (thermal insulation) -- Those of us with composite airplanes need to keep firewall backface temperatures low enough for the bulkhead to maintain its structural integrity. This is typically something like 180-200 F. The Lancair 320/360 that I have has a plywood bulkhead with fiberglass cloth reinforcement. I do not know what the critical temperature is but it is probably similar, since the plywood bulkhead is attached to the fuselage with fiberglass. Later model Lancairs have fiberglass honeycomb bulkheads, as I understand it. For either of these aircraft a heat shield is needed. In the early days our firewall consisted of an 0.023 stainless fire shield on top of 3 plies of Fiberfrax heat shielding. Later models used a quilted ceramic blanket that appears to be aluminized on one side (the hot side) and covered with a red RTV impregnated cloth on the other. (That's probably not what the outer lyes really are, but that's what it looks like.) (I took one look at Angier Ames' beautiful quilted firewall and took my steel/fiberfrax out and replaced it. I saved 3.5 pounds in the process, too.)
3) Engine baffling -- These are the aluminum sheets that direct cooling air thru the cylinder head cooling fins. These don't get very hot and take very slight loads, so aluminum works fine.
I'm not entirely sure why I barfed out all these words but they might help someone. Have fun at Oshkosh for me this week if you are there. I plan to join y'all next year -- maybe even fly there in my own Lancair! (Don't panic -- I have threatened this before...)
- Rob Wolf