Recommended insulating concrete ratio?

[kebwi]

I believe (I don't have it in front of me right now) that the Pompeii directions suggest a 1:5 ratio (portland to vermiculite/perlite of course). On the forum I know I have read mention of some other ratios, at least at divergent from Pompeii as 1:7. I don't recall specifically.

What is your recommended ratio and why?

Followup: Do you believe the ratio should be varied with vermiculite/perlite "grade", which refers to the size of the grains? That is, would larger or smaller grains require an alteration to the cement ratio?

Standard grades, as I understand it, are in four steps (generally labeled 1 to 4, 1 being the smaller grains), the smallest being sand-sized, the largest being dime-sized, the two intermediate sizes being, I dunno, BBs and half-dimes respectively, I suppose. What "grade" did you use and what are your thoughts on it?

[david s]

Re: Recommended insulating concrete ratio?

For the vermicrete insulation under the slab use 5:1 for strength. For vermicrete on the sides and top of the dome (for igloo style) use 10:1 because this layer does not need to be strong, but just enough to hold it in place. Remember that the more cement you use, the more you reduce the insulation value.