Protecting wood from heat

Im about to start construction on an RMH for my greenhouse and im wondering if anyone had some ideas about protecting the framing in here from the heat from the thermal battery.
Air gap with metal sheeting on the back of the thermal storage? Cinder block(holes in block would provide an air gap.... sorta)

Open to all ideas/ suggestions.

Hi Jeremy; In my greenhouse I placed the mass apx 4" away from the outside wall and filled the gap with loose perlite. Works perfect to insulate/isolate from the outdoors. Also, your mass really doesn't get hot enough away from the core to harm wood anyway.  

Dude.... perlite in the gap is genius.  Thanks that's awesome.