rockwool.com claims a thermal conductivity of 0.033 W/m K. Many stainless steel alloys seem to be around 16 W/m K (and mild steel is quite a bit higher). So,

rockwool.com claims a thermal conductivity of 0.033 W/m K. Many stainless steel alloys seem to be around 16 W/m K (and mild steel is quite a bit higher). So, very roughly (and assuming all heat transfer is conductive, etc), if the fraction of the inside that’s steel is well under 0.2%, the overall thermal conductivity through the panel will be comparable to that of rock wool. Otherwise it will be higher. In any case, the thermal conductivity along the sheet (as opposed to through it) will be quite high due to the solid metal face. One could work out the fill factor from the drawings in the OP.