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  #11  
Old 02-01-2013, 11:13 AM
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Default Re: Alternative Insulation

Quote:
Originally Posted by Laurentius View Post
Why can't you just make a Void, its full of air?
Because as the air heats up it moves around taking the heat with it. It needs a barrier, or better still lots of barriers, to prevent this.
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  #12  
Old 02-01-2013, 01:49 PM
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Default Re: Alternative Insulation

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Originally Posted by david s View Post
Because as the air heats up it moves around taking the heat with it. It needs a barrier, or better still lots of barriers, to prevent this.
This is why double glazed windows are sealed as tight as possible: air is a pretty good insulator, but it does get heated, and if you let that hot air rise out of the way, then cooler air replaces it and that gets heated, etc.
End result, instead of a layer of insulating air, you get a current of air cooling things.
I reckon a void will work as long as it's sealed to stop air flowing.

The medium that traps the air, whether it be aerated concrete, glass bottles, ceramic fibres, scoria, what ever, conducts heat - some materials more than others. The idea though, is that there is not very much of this solid, in relation to the air trapped, so the aerated material should not conduct as much heat as the solid.
I believe even loose gravel could be used as an insulator if done right. If all the rocks were the same size, i.e. you screened out a very specific size fraction it could work.
Say you put a heap of gravel over a 3/4 inch screen, then over a 1/2 inch screen. If you kept the minus 3/4, plus 1/2 inch fraction, it would be quite uniform, wouldn't pack done much, and would provide lots of air spaces while imhibiting air flow.
The other reason you would want the particles all the same size is that this would mean the minimum of particle to particle contact, thus minimising heat transfer from particle to particle.
Anyway, this being my theory on insulation, I've been trying to think of some things that might help you out given you can't get even gold old vermiculite.
Bottles, yes.
Can you get anything spherical, like balls of clay or glass marbles?
A layer of say 1/2 inch clay balls or marbles, would have maximum air space, minimum point contact between particles.
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Last edited by wotavidone; 02-01-2013 at 02:15 PM.
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  #13  
Old 02-01-2013, 06:25 PM
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Default Re: Alternative Insulation

Sorry to butt into the conversation.
To consider which insulation material would work, you have to look at its application.
-exposed temperature range is probably most important and cycles of temp extremes.
Ice is a good insulator (if you use it in an igloo). Outside temp. -40F inside
above freezing!

Also, you have to consider application. If you put R-30 fiberglas insulation in a wall cavity with drywall on one side and other side open to elements, it is not as good an insulator as it would be if the backside were enclosed with almost anything- even a sheet of "Tyvek" paper.

I think the entire issue involves trapping air in a space, and not allow it to naturally circulate. This is accomplished in pericrete, styrofoam, glasswool, sheepswool, ceramic fiber etc. Each material perlite, vermiculite, styrene foam, extruded glass fiber, natural sheep wool fiber, extruded ceramic fiber all do the same thing (occupy a space otherwise filled with air). All are also low density.

Back to finding the alternative insulation- anything that can trap air, and not deteriorate from design elements. (temperature, wind, water, sun light etc.) should work. Add to that- the product should not off-gas something that would be dangerous or objectionable. (ie. water buffalo dung)
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Old 02-01-2013, 07:13 PM
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Default Re: Alternative Insulation

Quote:
Originally Posted by Laurentius View Post
Why can't you just make a Void, its full of air?
Yes it will work if it is fully sealed, which is difficult to do. better still evacuate the air and have a vacuum. The same principle as a vacuum thermos flask which insulate wonderfully.
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  #15  
Old 02-01-2013, 08:16 PM
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Default Re: Alternative Insulation

Quote:
Originally Posted by wotavidone View Post
Can you get anything spherical, like balls of clay or glass marbles?
A layer of say 1/2 inch clay balls or marbles, would have maximum air space, minimum point contact between particles.
Trouble with solid marbles or solid clay balls is that there is too much density. The only air is in the spaces.The beauty of bottles is that there is a large volume of trapped air inside the bottle.The melting point of glass is around 900c (although every glass is slightly different) this is way hotter than you would ever get on the outside of the inner brick wall so glass is ok in that respect. No chance of them melting.

Last edited by david s; 02-01-2013 at 08:21 PM.
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Old 02-01-2013, 09:17 PM
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Default Re: Alternative Insulation

When ever there is a temperature differential heat goes to cold. A molecule at a higher temperature transfers energy to adjacent cooler molecules. Heat transfer occurs by conduction, convection and radiation. Insulation materials generally used in WFO retard conductive heat loss by creating dead air spaces. Dead meaning no air flow thru the insulation i.e. the reference to Tyvek in the earlier post stopping air migration thru the fiberglass in a house wall. Convective heat transfer needs space for a convective loop to function. The easy way to understand this is that two sheets of glass sealed together in a window. One half inch space between the glass is better than one and one half inch even though there is more volume of trapped air with the wider space due to convection within the trapped air space. Radiant heat loss can be retarded by reflecting near infrared energy created by burning wood using reflective material like the aluminum foil you cover meat with when it comes out of the WBO. Some builds use foil over the blanket insul. The foil functions better if it can be kept from direct contact with the material between it and the source of heat. Hope these comments help others in planning their oven design. Burn less wood - cook, bake, grill more food.
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Last edited by silvfox; 02-01-2013 at 09:20 PM.
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  #17  
Old 02-01-2013, 10:10 PM
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Default Re: Alternative Insulation

Foil in the insulation layer is another issue. It has been found by a number of builders, me included, to be unsuccessful because although it may reflect some heat, it also acts as a moisture barrier. The steam condenses on the underside and it takes way longer to dry out your oven. I wish I hadn't done it on mine and have not since.
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  #18  
Old 02-02-2013, 03:02 AM
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Default Re: Alternative Insulation

Would it be better then to perforate the foil prior to installation? The tiny holes would not affect the reflective qualities (if they are important) but still allow water vapor to migrate out of the build.
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  #19  
Old 02-02-2013, 03:05 AM
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Default Re: Alternative Insulation

I will be facing this issue soon-
What is the recommended assembly if not using ceramic fiber blanket?
Dome, _______,pericrete,________,_________ exterior? What would you do if you were going to do it again?
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  #20  
Old 02-02-2013, 04:35 AM
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Default Re: Alternative Insulation

Quote:
Originally Posted by david s View Post
Trouble with solid marbles or solid clay balls is that there is too much density. The only air is in the spaces.The beauty of bottles is that there is a large volume of trapped air inside the bottle.The melting point of glass is around 900c (although every glass is slightly different) this is way hotter than you would ever get on the outside of the inner brick wall so glass is ok in that respect. No chance of them melting.
Your point about the density is fair enough, but I wonder how marbles or clay balls compare to say vermiculite in cement. Fairly dense there, too, I would suspect.
Bearing in mind we are talking about what could be used if the good stuff like fibre board, blanket, vermiculite, etc, can't be procured, I vote for something small and spherical under the floor bricks. Every photo I've ever seen of an oven built using glass bottles under the floor shows that they had to use loads of clay (definitely not an insulator) around the bottles to get a surface to lay the bricks on.
I've always wondered if anyone has trouble with glass bottles shattering.
I reckon, with a 4 to 6 inch depression cast into a concrete slab, you could fill it with say 1/2 inch clay balls or glass marbles, giving yourself a screedable loose fill that, because the spheres should all be the same size, has a bucket load of air relative to the mass of the balls, and you could consider putting the bricks straight onto this.
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