Fire safety with polystyrene
Why not use Roxul ComfortBoard instead of the polystyrene? It is fire proof.
Alternatively you could make a sandwich with the comfortboard on the roomside and the foil faced EPS on the other.
Are you trying to achieve the highest r-value? what r-value per inch is the foil faced EPS you are considering?  How many inches do you want this removable insulation panel to be?
I'd just go with a pure comfortboard solution and not have any EPS.