vermiculite or perlite?

I am new to this forum, and I have been experimenting with rocket stoves and rocket mass heaters for about three years before visiting this site. I had run across Paul's videos and finally decided to come and visit his site, and I am glad I did and hope to be a regular contributor in many areas here as I develop this land (100 acres of south facing steep wooded slope with about 5 acres reasonably usable for on contour swales and hugleculture applications - all in very neglected shape as of right now) into a permaculture paradise. I hope to continue to learn from the rest of you fine folks, as well as sharing my insights and hard learned lessons along the way.

Now to my question. I first got into my head that vermiculite was the best aggregate for light weight concrete as well as a great insulator, and now that I visit this forum all I ever see talked about is perlite. Is there a difference in performance? Being an old school concrete superintendent and having built everything from bridges to refineries, to K Marts, I am stuck in the vermiculite frame of mind when thinking of concrete casting. I have recently built a rocket mass heater in my work shop, and I am quite impressed and satisfied with the performance, but I will soon be building one in the main house, and I want to do the best job I can do, so is perlite better? I have a 8" system in my shop with a kiln brick fire box and combustion chamber feeding a terracotta flue chimney with a 2" cast layer of vermiculite mixed 5 parts vermiculite/1 part refractory clay, and 1 part Portland cement. The burn box and combustion combustion chamber are finished out with 2"x 8*'x 16" masonry pavers set on edge and 2" of vermiculite between the fire box/ combustion chamber and the outside paver "skin." Over the burn chamber there is 3" of vermiculite covered with a 2" concrete slab, locking the tops of the outside paver "skin" together for a solid mass. The mass is formed with 8" ducting and an outer "skin" of the on edge pavers, filled with a sand/ clay mix, and topped with 2" of concrete. I have been running it for about 2 weeks now and I am quite happy with the performance, but I want to know if I can do better on the main house's RMH.

Also I would like input as to whether the terracotta flue is a mistake. I was buying my kiln brick, refractory clay, and vermiculite when I saw the flue sections, and the salesman said he thought they would work for my application. Like I said, I cast in place a 2" insulation on the outside of it, so I am hoping that this is not a mistake and wondering if I should repeat it in the main house RMH. Any comments would be appreciated because this was an impulse purchase and design change and I have never heard of anyone doing this in all of my research. The 24" sections were only $16 each and it took 1 3/4 for the height I needed for the drum I used. Like I said, I am very happy with the performance of the system, but it is only about 2 weeks old and I am now wondering if the terracotta flue will break down in time. I had a portable rocket stove made out of stainless steel, and it only took 2 winters and it was rotten and all but evaporated. I would hope that the terracotta flue pipe would hold up much linger.

I'm afraid your terracotta heat riser is not going to last either. It is made for high heat, but not fast changes in temperature, and the stress will crack it. I have seen video of someone who tried it, and the flue tile after disassembly was crumbled into bits. I have heard that first sawing a vertical joint into the tile will let it flex a bit and reduce stresses enough to work.

Perlite has a higher insulation value and so less material is needed, especially when suspended in thermally conducting masonry material. Vermiculite is best used as loose void fill, without clay or masonry binders.

Perlite can be harder to source than Vermiculite. I had no luck with masonry and refractory suppliers. But then checked with the local farmers coop who had Perlite in their catalog available for order; 4 cu. ft. bag @ $12.00 each.

Thank you Glen, I was afraid that the terracotta was not a good idea for this use, but I tried it anyway. I will take that into consideration when I build my big one for the house. I am thinking of doing a cast in place one with vermiculite and metal skins, and I just may make the interior skin removable so that I avoid the inevitable rot out of it. I will let this sleeping dog lay till the spring (it is working just too good right now to mess with) and in the spring I will open her up and report on what I find.

And thank you Byron for your input. That is about what I paid for the vermiculite and I will inquire at my co-op and see what they have. Being an old concrete guy, I am worried about the compressive strength of perlite if I use it to cast my next riser. Vermiculite has a proven track record for compressive strength and the formula I used was given to me by a guy that builds commercial kilns. It is 5 parts vermiculite, 1 part Portland cement and 1 part refractory clay. This is what I used to cast around the terracotta and I made a couple of balls about 2" in diameter and a "patty" about 1" thick and 8" square to play with and the balls can be thrown to the floor and not break and I can put the "patty" on top of the barrel and then put my hand on it after it has been sitting there. I could never touch the barrel itself but only 1" thickness of this cast vermiculite is enough insulation that I can put my hand on it for a count of 5. This stuff makes a pretty good casting medium and I think that it makes a great cast in place insulation. I used it loose around the fire box and burn chamber about 2" thick and 3" over the burn chamber, but since perlite has a higher insulation factor I think I will fill this area with perlite on RMH #2

I am new to this forum, but I have done a lot of research and built 2 portable rocket stoves out of metal. They worked great but neither lasted more than a season. I also want to give a hat tip to Ernie and Erica, and give them a lot of thanks for being the unselfish people they are and for freely sharing the way they have for so many years. May they be greatly blessed.