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jersey 04-24-2011 09:21 AM

wow my build is starting to come along and i need to start thinking about insulation. my question is perlite or vermiculite??? can i use perlite and portland to insulate the dome and what is the ratio (sand,lime,cement and perlite) and for loose insulation is perlite ok as it is alot cheaper. or is vermiculite the way to go. oh and do i get a course bag and does anyone have ideas of where to get it in san diego thanks for all the help this forum is great!!!:)

david s 04-24-2011 01:36 PM

Re: insulation
The mix for under the floor needs to be richer for the under floor insulating slab (5:1) for the insulation over the dome you only need it strong enough so the stuff will hold together so you can render/stucco over it without nit fallingbapart. More cement in the mix means a denser mix which results in reduced insulating capacity. I find 10,3.5,1 vermiculite, water, cement mix by volume not weight works well.I prefer vermiculite to work with (fine grade) but perlite is a slightly better insulator. Where I live vermiculite is also slightly cheaper.

azpizzanut 04-24-2011 07:49 PM

Re: insulation
Hello jersey,

Shop for insulation at the FornoBravo store. You can't go wrong with ceramic blanket insulation. An additional benefit is that it is dry. I've built two ovens, one with Perlcrete insulation and one with ceramic blanket. The blanket is much easier to apply, maybe an hour total for three one inch layers. You can render over it if you wish.


SableSprings 04-24-2011 11:07 PM

Re: insulation
When I started my oven a couple of years ago, I thought the ceramic insulating blankets were way too expensive. I chose to use perlite and cement as my dome and base insulation. To give you a rough idea of costs to insulate my 39" dome--I used 8 bags of perlite (4 cu ft or 25# bags) at about $12.75 average cost per bag and three 90# bags of cement at $8/bag --brought me to about $125 in materials.

Beyond material expense, there's a LOT of labor time mixing, applying, and cleaning up the perlcrete that should be counted in the cost but usually isn't...azpizzanut's advice is great...go for the insulating blankets. Maybe later you can use an outer coating of perlcrete to give your dome some light-weight shape and form between the blanket and your final outer surface. If you really want to stick with using perlite and cement, you should be able to find perlite at a brick & block yard or at a garden supply. I did note that at the garden supply perlite was about $7 more per bag...

Rocko Bonaparte 04-24-2011 11:30 PM

Re: insulation
I managed to get some mineral wool inexpensively. IIRC it was something like $25 for enough to completely cover the dome, with enough scraps left over that I basically doubled up on towards the top of the dome.

Regarding perlite/vermiculite: they're almost interchangeable in terms of handling, insulating ability, and how they can screw up your lungs. They are at least kissing cousins in all those regards. For ratio I've seen anywhere from 2:1 up to 12:1, with 4:1 to 8:1 being more normal. I am doing 8:1 myself. That's ingredient:cement. Water is somewhat more subjective IMO.

david s 04-25-2011 12:41 AM

Re: insulation
The perlite that is available to me has more fine particles which are an irritant to the lungs so I prefer to use vermiculite which seems to be free of of fine particles. Perlite is actually marginally better as an insulation but for me is also slightly more expensive. Judging the amount of water to add to the mix is very difficult if you just keep adding it until you think it it's right you'll usually end up adding too much. The water tends to carry the cement to the bottom of whatever you are mixing it in, with the perlite or vermiculite floating on top. I've found that for every 10 parts vermiculite or perlite I add 3.5 parts water by volume. This produces a good workable mix but to the person who's doing it first time you'd think it's too dry.

azpizzanut 04-25-2011 08:28 AM

Re: insulation
Hi All,

San Diego area builders can buy 4 cu ft bags of perlite at Grangetto's Farm Supply for Less than $14 per bag.


stonylake 04-25-2011 09:31 AM

Re: insulation
one of our local refractory suppliers sells boxes of scrap pieces of ceramic blanket for $50.00... it was enough to put 3 layers of 1" on my 36" oven..

azpizzanut 04-25-2011 11:19 AM

Re: insulation
Hi stonylake,

That is a great way to be thrifty. It comes with a caveat though. Here's some information I picked up from others that may be useful. Not every ceramic blanket is created the same. If you plan well and know the specifications for the product you got at a bargain price then you can accommodate it in a build. Some ceramic blankets don't take well to wet environments so it shouldn't get exposed to rain, water spray, soupy perlcrete/vermicrete, or wet render. FB ceramic blanket holds its shape when damp/wet and maintains its insulative qualities when it dries out. Some other ceramic blankets can turn to mush. If you know this in advance you can plan ahead use bargain priced insulation in your build with great success.


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