I have a cast iron gas burning fireplace. I recently put wood trim on the walls of that room and, even though the clearances are all within code and done properly (so there is no risk of the fireplace making my walls catch fire) I do know that the metal surround of that fireplace still gets hot, VERY HOT. My fear is that it could start to warp or otherwise affect the expensive wood trim.

I examined things closely and I see where I could easily put a thin strip of material along the top of the fireplace opening (completely unseen from the outside) which would buffer the wood walls from the heat of the fireplace and the hot metal.

My question, without getting too lost in the details of my specific issue and fireplace configuration, is what type of insulating, but also fireproof material could I use for that strip? I thought of cutting a strip of 1/2 inch durock or wonder board, but I don't think those are technically fireproof.

Suggestions?