January 30, 2016 - 2:21pm
Perlite vs Vermiculite
New to this site and forum. Thanks for having me. I am starting the process of building an Alan Scott wood fired oven. Foundation will be poured on Tuesday. I am researching certain things and wanted to know what people thought was a better insulation for the hearth and the outer dome. Vermiculite is almost double the cost of perlite but wasn't sure what the trade offs were. Also, if anyone has used either in their hearth floor, what are the measurements of insulation to concrete. I noticed in the Alan Scott book, they call for a 6 to 1 ratio. Are they talking weight, cubic feet or ?
Thanks in advance for any help and guidance.
Hi Brian, I built a forno bravo style oven a couple years ago and I used vermiculite for the hearth and stuck with perlite for insulating around the dome. Not sure what the insulating properties of both are, I would have to say you may have a hard time mixing the perlite so that its distributed properly across the entire mix. Vermiculite on the other hand is fine and would get a good mix and distribution in the cement. When I last did it I used a small beach bucket and used 6 buckets vermiculite to 1 cement. i agree the stuff is expensive. Also for the very top of the dome you may want to dump in a bag of diatomaceous earth from a pool place, it fills in any cracks that may form over time, then pour perlite over that to fill. Good luck and post some pics of your progress.
I'd suggest you track-down Rado's site, in Australia, and "read-through" his builds. He has modified some of Alan's work, and has a bunch of general suggestions, ratios for mix, etc... You can find him with a "wood fired oven" google search. FWW, I used vermiculite, and mixed it in with the cement, lime, etc... If I had to do it again, I would put ceramic fiber board (2") on top of 4" of vercrete (vermiculite/concrete mix) under the hearth materials. Can get enough insulation.... repeat that phrase, often! ;)
Thanks Kevin. I looked at Rado's site a while back and forgot about it. Apparently, there is no insulation difference between vermiculite and perlite. Looks like 5 parts vermiculite, one part cement and one part water will do the trick for a 4" slab (multiple batches of course). Also noticed that he said to mix in a wheel barrel as a cement mixer would break down the vermiculite.
The ceramic fiber board I assume you are talking about is cement board? So what you are suggesting is to pour the 4"vercrete slab with rebar, add two layers of cement board and then pour the concrete hearth? Top with firebricks as the floor of the hearth? I noticed in Alan's book that he is showing only a 2" vercrete slab. I assume going 4" will only help to increase the thermal factor. Also, he mentions installing 4" common nails at a 45 degree angle in order to attach the vercrete slab with the hearth slab. Is that really enough for supporting the vercrete slab to the underside?
Thanks for the help and advice.
I’d suggest 4” of the vercrete, then 2” of the ceramic fiber board. I only put the 4” of vercrete, and wish I would have done both. I did use 6” of ceramic blankets over the top of the dome (barrel dome for me) and then loose vermiculite over top of that. I can upload a bunch of pics of my build to google photos, if you think that would help.
And, to answer your questions… You will need a cement base. Mine is cement pavers over top of cinder block. Then, I have 4” of vercrete (for insulation) then I have 2” of refractory cement (heat mass) then high aluminum (30%) fire brick (flat side) as the hearth. If I did it again, I would do: cement base as before, 4” of vercrete (insulation) then 2” of ceramic fiber board (more insulation) then the 2” of refractory cement and the fire brick.
Still working on the insulation ideas and thoughts. Looked up some R values and found 2" of cement board has an R value of 1. A 4 inch slab of concrete has an R value of 0.80. I am going to email a couple of companies who sell high temperature insulation and see if they have any recommendations for something that can be imbedded into the upper cement hearth pad or between the vercrete pad and the hearth pad. I know when you compact fibrous material, it will decrease the R value but what about sandwiching a thermal insulation between the layers of cement board? Or applying a aluminum backed tape to each layer of concrete board to help reflect the heat?
Thanks for the advice and info. I tracked down a company about 30 miles from me that makes calcium silicate fiber boards. Emailed the owner and he said he could order the flat pieces that I inquired about or stop by his facility and he would give me scrap stuff for free. Free is always better so I went there this morning and filled the back of my truck up with pieces that range from 2" to 6" thick. All of it was for insulating different diameter pipes up to 1.5 feet in diameter. He said I could smash them flat and they would work perfect with the concrete hearth poured over the top. The big round ones might fit the top and sides of the dome perfectly. Thinking of going with laying the pieces over the dome, wire mesh around that and back fill into the loose spaces with perlite. Just not sure if I should encase the entire dome in concrete before scratch coat and stucco.
My bad. You were mentioning ceramic fiber board when I was interpreting cement board. Looking into locations around Los Angeles that I might be able to purchase the ceramic fiber board or possibly calcium silicate board which might be cheaper and a little lighter.
If you are in LA look up McGills warehouse. I mail ordered mine from them on my Pompeii oven build and they were pretty reasonable in price. And it seams to be a very good performance from the product. It can take in excess of 10 days for my oven to drop below 100F. Good luck
I have had one Scott oven built and my husband and I have built one ourselves. Have you read From The Wood Fired Oven by Richard Miscovich? I found it right before we insulated and we went with flexible ceramic fiber blanket. I bought it online and had it shipped. I think it works MUCH better than the vermiculite I had in my old oven. On the hearth I would have to double check with my husband how he did the mix. I know he had more vermiculite in the ottom than closer to the top of the hearth. have fun building! We did!!