#11  
Old 05-18-2011, 11:38 AM
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Default Re: For experts and/or experienced builders!

perlite or vermiculite are interchangeable in this case.
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  #12  
Old 05-18-2011, 02:19 PM
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Default Re: For experts and/or experienced builders!

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Originally Posted by Onyxx View Post

I was wondering what was the reason you had to tear your original oven down and start again...I would love to hear that story because it might have valuable data to learn from. Although I’m sure you are tired of telling it.
Here is a thread about pulling down the old oven, it sort of explains things.

http://www.fornobravo.com/forum/f28/...ild-15263.html (The reverse build.)

.
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  #13  
Old 05-18-2011, 02:38 PM
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Default Re: For experts and/or experienced builders!

Perlite is a slightly better insulator than vermiculite, but the more cement you mix with it the more you reduce it's insulating value. For the under floor slab go with a 5:1 mix for enough strength to hold the weight.
Fiberglass insulating bats tend to compress way too much (compressing will reduce it's insulating capacity) and are difficult to work against if you are doing an igloo, they might be ok for an enclosure. Some people have reported that the binder burns away, but I used them on my first oven and when I finally pulled it apart the fiberglass was still in good condition with no sign of any burning.maybe mine was a different brand.

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Old 05-18-2011, 02:53 PM
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Default Re: For experts and/or experienced builders!

In the absense of insulating board, blanket vermiculite or perlite, do not discount the broken glass method, it's been used for ages . It gets it's insulating value from the trapped air between the glass. A bucket full of broken glass, particularly if you use broken bottles with unsmashed necks and plenty of curved forms to create big spaces, is not particularly heavy and contains lots of good insulating air. A fiberglass bat is after all simply glass and air too.a thicker layer of a poorer insulator will give you the same result. Eg 1" insulating blanket=. 2" Vermicrete approx.
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Old 05-18-2011, 03:44 PM
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Default Re: For experts and/or experienced builders!

Hi david s... you know, when i started researching about brick ovens a couple of months ago here in Argentina (they are called Horno de Ladrillos here), there wasn’t a single website that didn’t mentioned the use of broken glass as an insulation material. So I thought, OK, that is easy to get so I’ll go with that, I even had in mind using some empty wine bottles I have kept from ¨special¨ occasions… yes, I know, I`m a remantic fool.
But then I started finding more and more people against the use of it (without really explaining why tho) so I then decided not to use it.

So now that I know someone out there is not completely against it, let me ask you:

What would you pour on top of the broken glass?

If the air in between the glass is what we are looking for, wouldn’t be better to use the whole empty bottle?

Would you choose glass (broken or whole bottle?) for the oven floor insulation over glass wool?

What do you think about a bed of empty bottles laid down with perlite concrete poured on top of that?

I am very, VERY interested in your answer (and everyone’s answer) as I am about to start my oven floor this weekend (hopefully)…
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Old 05-18-2011, 09:16 PM
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Default Re: For experts and/or experienced builders!

Quote:
I will go with glass wool batts!
Bad idea. They have an organic binder that burns and stinks at oven temperatures.
Quote:
I was able to find through an eBay like website (MercadoLibre) a supplier of perlite instead and it is much cheaper than the vermiculite… So my question is; how does perlite compare to vermiculite as an insulation material and can I go with the perlite without losing insulation properties?
They are functionally identical. I used perlite concrete on top of a thin layer of refractory blanket over my dome. Perlite concrete works fine, you just need twice as much of it as refractory board/blanket.
Quote:
Would you choose glass (broken or whole bottle?) for the oven floor insulation over glass wool?
I wouldn't use either one.
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  #17  
Old 05-19-2011, 01:54 AM
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Default Re: For experts and/or experienced builders!

Onyxx,
If you have access to perlite then use it mixed with cement 5:1 as your floor insulation. It is a slightly better insulator than vermiculite so if you can get it cheaper you"re onto a double win. I did say the broken glass should only be considered in the absence of other, better materials. Don't even think about using fibreglass bats under the floor, they will just squash flat and therefore not insulate.
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Old 05-19-2011, 03:23 AM
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Default Re: For experts and/or experienced builders!

I really fail to see how glass broken or otherwise is a good insulator?
One hot piece of glass with transmit its heat to the next piece of glass etc.

It just doesnt make sense.
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Old 05-19-2011, 03:46 AM
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Default Re: For experts and/or experienced builders!

The thermal conductivity of glass is 1.05 compared to dense brick 1.31, concrete and mortar 1.7 so it's not that bad. It is the air trapped between the material that creates the insulation value, after all glass and clay are transformed into fibres to create ceramic fibre insulating blanket and fibreglass insulating batts. What seems the important factor is the resulting density of the material you end up with. With pieces of curved glass from broken bottles, the contact points are reduced significantly and the spaces between them are maximized. The addittion of cement to perlite or vermiculite also reduces the insulating value too. I'd still not use broken glass if I had access to better materials.

Last edited by david s; 05-19-2011 at 03:57 AM.
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  #20  
Old 05-19-2011, 03:53 AM
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Default Re: For experts and/or experienced builders!

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The thermal conductivity of glass is 1.05 compared to dense brick 1.31, concrete and mortar 1.7 so it's not that bad.
So its just slightly better than dense brick? I can now see how it would be a good insulator.....
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