Im rebuilding my firebox that i tore down to insert a gas direct vent insert. Would i need to rebuild the firebox with firebrick or can i use regular bricks due to the fire being contained in the insert?

So install date will be soon for my 23x23x33 size firebox, cast iron stove in my tiny home.

It has an extra plate of cast iron on the floor of the firebox, i presume to protect it? It's a very basic stove. No ash pan, the door has a small window with an air intake below it.

It did used to put out lots of heat when i had it installed before, more than enough for this size home. But now i've done a little research and people say that adding fire bricks will make it burn more efficiently by keeping the heat inside the stove so it somehow burns better. Or is that just for certain types of stoves with secondary burn, etc?

And how does the thickness of firebricks affect all of this? As i see a good few different thickness of them all.

And different materials too! Vermiculite, clay and many more! Does material matter?

How do firebricks affect the heat output of the stove? I've seen people say that without them the stove is putting out less heat, but that confuses me.

To cover the back and side walls, also the floor, it'll cost a fair bit. So i wanted to make sure on the what, why and how.

And bearing in mind, mine is cast iron with no clip/holdy things for bricks. So the only way to fit them would be fire cement or maybe that sealant stuff if that's as good as it's claimed to be.