Vermiculite for heat riser insulation in an 8" build with a 55 gallon steel drum

In Erica & Ernie's builders guide, it's suggested that either perlite or vermiculite can be used to insulate the heat riser.  If perlite, 2". If vermiculite, 4".  For an 8" build using a 55 gallon steel drum, is there actually room for 4" of insulation here - or is vermiculite an option mainly for smaller builds?  My rough sketching and the firebricks I have (which I think are standard size) suggests that 4" of insulation will run right up against the barrel at the corners of the heat riser and not leave a lot of room for gas flow.

So is perlite the way to go here?

So Jean Paul;   Are you planning on using bricks for your riser ?  Heavy fire brick ? Insulated fire brick ?  Or were you thinking a fireclay /perlite riser ?  Are you aware of the five minute riser ?

thomas rubino wrote:So Jean Paul;   Are you planning on using bricks for your riser ?  Heavy fire brick ? Insulated fire brick ?  Or were you thinking a fireclay /perlite riser ?  Are you aware of the five minute riser ?

I'm going with half-firebricks in the "sunwheel" configuration as described in the book.  I wasn't aware of the five minute riser...  I see a lot of discussions about it but I don't see an actual description.  Can you link me?

Check this out  https://permies.com/t/95849    

Oh, wow.  That looks easy...

Yes , Fast and easy. Not cheap but worth every penny.  No time spent heating up a riser , Basically impervious to the temperatures it is subjected to.

Beware the cheap ceramic blanket it is very hazardus (but does work) look for Morgan super wool its the best choice.

Also if you are not aware.  Ceramic boards to make your core rather than firebrick.   This and the riser were both inovated after the Wisners published their guide.

Here is a long link to my shop build.   In it you will find where I discuss using ceramic boards in my core build and how I would do it slightly different on my next.    https://permies.com/t/94980

I prefer perlite for being incompressible, so there is that, but whichever you use, if you have a square brick riser, the corners don't need as much thickness of insulation wrap as the sides. Heat has farther to travel through the brick and then has two faces to escape from. My RMH bell, 6" of cob over a layer of brick, almost never gets warm on the corners even when the faces are very warm, unless I have had to burn for many hours in bitter cold weather.

And the best plan is to start with a low-mass, highly insulating material in the first place. If all you can get is brick, don't worry too much about the corners.