Rocket mass heater insulation

I'm playing with my first rocket mass heater build, and I have a question about insulation.  When using vermiculite or perlite, do I have to use clay slip with it, or is that just to stick it together?  I'm planning on completely enclosing the insulation container and would rather not have to mess with clay slip if there isn't a real need.  Thanks for any info. 

Todd

I was just reading through some other RMH threads, including one about heating water.  http://milkwood.net/2011/10/28/rocket-stove-water-heater-redux/ ;

In the case of the Milkwood water heater, the vermiculite was dry when it was installed and after several years had settled several inches, leaving open air that caused a loss in heat.

Lolly K wrote:
I was just reading through some other RMH threads, including one about heating water.  http://milkwood.net/2011/10/28/rocket-stove-water-heater-redux/ 

In the case of the Milkwood water heater, the vermiculite was dry when it was installed and after several years had settled several inches, leaving open air that caused a loss in heat.

I've done this, stuffed the vermiculite between two tubes. But i've compacted and squashed it with a stick.

Part of the reason that they make the clay slip is so the insulation will not fall apart once the inner metal burns/rusts away.