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

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.

[mikku]

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.

[mikku]

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?

[wotavidone]

Re: Alternative Insulation

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.

[wotavidone]

Re: Alternative Insulation

Just out of interest, we have a product we call "black sand". It's furnace slag that has been granulated by pouring the molten slag into a stream of cold water. You end up with a product that looks like coarse black sand. Particle size around 4-5 mm. A litre of this holds nearly half a litre of water in the voids. True story - when I fill a measuring cylinder to the one litre mark, I can pour nearly half a litre of water in.
Maybe this explains how some people came to recommend sand as an insulator under the bricks?
And maybe the same logic behind the people who recommend broken glass under the bricks?

[mikku]

Re: Alternative Insulation

Sand has to be an insulator! If not, how would you explain that on a very sunny day at the beach--the sand on the surface can be scorching hot, but dig down just a little bit and it is cool?

Or is it such a good conductor that the deep underlying sand sucks all the heat out of everything except for the topmost layer?

If the first case is true--being a good insulator, then it would make sense to have a thick layer of sand beneath the firebrick floor!

[silvfox]

Re: Alternative Insulation

David s makes a good point about foil trapping moisture. If you use foil you would certainly want to wait until the the oven has cured and then use a perforated foil if at all possible. Water vapor will pass thru much smaller holes than liquid water. The biggest advantage of foil is that it doesn't take any space.
Related thought some of you WFO pros may want to confirm; the ability to a WFO to bake a pizza in 90 seconds is that it utilizies all three means of heat transfer. Conductive heat from the deck, radiant heat from the live fire and convective heat caused by domed ceiling.

[Tscarborough]

Re: Alternative Insulation

Sand is not an insulator, the airspace between grains is. Sand on the surface is hot because there is no moisture, once you dig through to where there is moisture evaporative cooling takes over.

[david s]

Re: Alternative Insulation

The thermal conductivity (k) of dry sand is 0.15 - 0.25 W/mK.
Compared to dry loose vermiculite 0.058 which is about three times better.
But add cement to the vermiculite and it drastically reduces the k value.
A 4:1 vermiculite, cement brew has a k value of 0.16, similar to the sand.

[brickie in oz]

Re: Alternative Insulation

Sand has to be an insulator! If not, how would you explain that on a very sunny day at the beach--the sand on the surface can be scorching hot, but dig down just a little bit and it is cool?

Because its wet further down.

[wotavidone]

Re: Alternative Insulation

The thermal conductivity (k) of dry sand is 0.15 - 0.25 W/mK.
Compared to dry loose vermiculite 0.058 which is about three times better.
But add cement to the vermiculite and it drastically reduces the k value.
A 4:1 vermiculite, cement brew has a k value of 0.16, similar to the sand.

This is the thing: most of the solids we are discussing, sand, glass (which is, after all, pretty much just fused sand), vermiculite, ceramic fibres, etc., are thermal conductors, to varying degrees.
What makes any of them insulators is the ability to hold air in place so that it insulates, because if it's allowed to move it becomes a cooling medium.

In my not-so-humble opinion, insulating over the dome should be OK.
A sealed, empty enclosure will help a lot, or there's bottles inside the enclosure, there's clay mixed with saw dust, etc.

The BIG question. Have we yet found NeilB a decent alternative to go under his oven?

[david s]

Re: Alternative Insulation

Perlite or vermiculite both have tiny holes in them that the cement does not penetrate so each grain in effect is a mini bubble of air. A marble is solid and dense.
If using glass bottles as an underfloor insulator the usual method is to break the glass and lay the floor bricks directly on the bed of broken glass. Bottles are better than flat glass because they create bigger and more air spaces.

[mikku]

Re: Alternative Insulation

I agree with everything that has been said, at least somewhat.
But I think in a desert, on top of a dune if you dig down just a little the sand would be much cooler....(a location lacking a lot of moisture). So the air is the insulator!

As far as "NeilB" alternative insulation question goes, I think a bit of history research is necessary. Then the question is "are there OR have there been "traditional types of ovens" in your location?" And how were they insulated?

That holds true for anyplace on earth. The native peoples developed techniques from available materials to cook or bake. If it worked for them, as a means of survival--then it surely would work for you. The price would be very inexpensive as well! If you have to modify their design a little to reach pizza making temperatures, then that is another challenge.

[mikku]

Re: Alternative Insulation

I really like to know more about "K" factors. I searched last night for a long time to get anything--then could not understand the numbers when I saw them.
"R" values are understandable, "U" are inverse, but "K" is another bird???