I'm trying to minimize the amount of insulating board I'll use, because it's very expensive. (I'm not even sure yet if I will use board or vermicrete.) Can I make the vent arch and landing out of insulating fire brick? That way I wouldn't need board under it. Then I could probably get away with 18sq ft instead of 24. That would amount to one less FB board. Or, because other suppliers only sell by the box, that would mean one less box to buy and ship. Waddya think?