Alternative Insulation

[silvfox]

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.

[david s]

Re: Alternative Insulation

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.

[david s]

Re: Alternative Insulation

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.

[mikku]

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)

[wotavidone]

Re: Alternative Insulation

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.

[david s]

Re: Alternative Insulation

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.

[Laurentius]

Re: Alternative Insulation

It is the air that does the insulating. The volume of air inside the volume of a bottle is way more than the volume of the glass. Plus you get extra air spaces between the bottles. It is all about the resulting density of the layer. Glass is not a great insulator on its own but add a large quantity of air to it and it becomes a good insulator. Fibreglass insulating batts are another good example. Likewise ceramic fibre blanket which uses clay fibres separated by air.

Why can't you just make a Void, its full of air?

[TropicalCoasting]

Re: Alternative Insulation

A mate built a corrugated iron ring around his dome and filled it full of stubbies over 1 metre high over the dome.
His oven definitely holds its heat

[david s]

Re: Alternative Insulation

[QUOTE=
If you enclose your dome you can use glass bottles on top or anything that wont melt but traps air.[/QUOTE]

It is the air that does the insulating. The volume of air inside the volume of a bottle is way more than the volume of the glass. Plus you get extra air spaces between the bottles. It is all about the resulting density of the layer. Glass is not a great insulator on its own but add a large quantity of air to it and it becomes a good insulator. Fibreglass insulating batts, which are made of glass are another good example. Likewise ceramic fibre blanket which uses clay fibres separated by air.neither glass nor clay are wonderful insulators but when combined with lots of air they are.
If you have free bottles then you have free insulation.

[Laurentius]

Re: Alternative Insulation

What the....?

It means that your statement is true. It means the glass bottles have proven to be a good insulation in the construction of so call earth houses, thick walled(1&2 meters thick) adobe houses, using, straw, bottles, shredded car tires and other things to trap air pockets for insulation. The misinformation is that the insulation properties of those thing applies across the board to heaters, kiln, ovens and furnaces.

[brickie in oz]

Re: Alternative Insulation

No, You don't miss much. I stand corrected, you miss, misinformation.

What the....?

[Laurentius]

Re: Alternative Insulation

No, You don't miss much. I stand corrected, you miss, misinformation.

[brickie in oz]

Re: Alternative Insulation

you can use glass bottles on top or anything that wont melt but traps air.

Sorry but I still fail to see how glass bottles can thermally insulate, if you heat glass up it glows until it melts, each adjacent bottle would also get hot and so on.
In the end you would end up with hot glass bottles sucking all the heat away.

Or am I missing something?

[wotavidone]

Re: Alternative Insulation

Scoria, or pumice if you can get it. Lots of air holes in those little rocks, and when you make concrete with scoria as the coarse aggregate, the air holes get sealed over by, but do not fill up with, the cement and sand paste. It worked for me. My floor sits on it, the dome is covered with vermicrete, I seem to have no trouble getting the floor very hot.
Failing that, the dome could be covered with a mix of sawdust and clay, when the sawdust burns away you have aerated clay insulation.

[TropicalCoasting]

Re: Alternative Insulation

People have traditionally used straw, sand, clay and sawdust for insulating kilns.
There was talk of using lava stone "scoria" in the cement mix hear too.
If you enclose your dome you can use glass bottles on top or anything that wont melt but traps air.