I noticed that my new Regency I2400 has firebrick all around the firebox except for the top surface which is vermiculite. I asked the company about this and they told me their warrantly claims dropped a lot when they switched to vermiculite for the top surface instead of firebrick all around. From this I conclude that vermiculite is more durable when it comes to withstanding the very high temperatures at the top of the firebox. So now my next question is why not use vermiculite everywhere? Is it a lot more expensive than firebrick? Does it have some other drawback such as not insulating as well or not being able to withstand the weight of a load of wood on the bottom of the firebox? What is the difference between vermiculite and firebrick in practice? Part of the reason I want to know is that I know the firebrick will eventually wear out and need to be replaced. When that time comes, would there be any reason to replace the original firebrick with more vermiculite? On a related note, would it be reasonable to replace any of the firebrick with soapstone bricks to increase the thermal mass of the stove? The company said I shouldn't: "The firebricks do protect the metal firebox and yes helps keep the heat needed for combustion to burn the wood thoroughly. They do not act like soapstone and radiate heat after the fire has burned down. The Stove has not been tested or certified to use other types of Bricks or soapstone and could cause early signs of wear and could also void the stoves warranty." What do you all think? What might be the result of replacing ordinary firebrick with either vermiculite or soapstone?