#21  
Old 02-02-2010, 08:35 AM
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Default Re: Terraced enclosure

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Originally Posted by Tscarborough View Post
Sand. On the hearth, all that matters is compressive strength, which is not a problem. When you use it in a vertical application, shear, flexural, and tensile strength matter. Realistically, you will need 4" of dirt to grow anything. Searching, I get approximately 2.66 pounds per SqFt of hydrostatic pressure for 4", so coated with surface bonding cement you should be good (rated at around 2880PSF in the vertical).
Great, I understand your concern. I had similar concerns, not about shear as you point out (thanks for that, I can visualize the problem you are suggesting), but just about crumbling along the corners. I am considering walling the vermicrete in 2" concrete slabs...or hardibacker...or perhaps wrapping the whole thing in chicken wire before I apply the SBC...or perhaps embedding some rebar in it.

Thanks again.

Cheers!
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  #22  
Old 02-02-2010, 09:35 AM
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Default Re: Terraced enclosure

Okay, cool, so going from there we have:

Code:
Cement   Vermiculite   28-day PSI   K-factor
1        8             70-125       .6-.65
1        7             125-140      .65-.69
1        6             135-175      .69-.73
1        5             175-225      .72-.75
1        4             225-325      .79-.81
R-value is simply 1/K so I'm disregarding it for the moment (they are functionally equivalent). I plotted these values to see how it might extrapolate to 12:1. I didn't do a curve fit or anything like that, but eyeballing it, it looks like K-factor is virtually linear in the P-V ratio. Therefore, 10:1 should be about K=.55 and 12:1 should be about K=.5. If my understand of K-factor is correct, it is linear, i.e., half the K-factor transmits (or loses) twice the heat and could be compensated for with twice the thickness. Therefore, a unit of 5:1 vermicrete (in thickness) is equivalent to about 75% unit thickness of 10:1 or 67% unit thickness of 12:1.

The scaling-factor appears to be approximately .75. In other words, as demonstrated above, 10:1 vermicrete is not twice as insulating as 5:1, i.e., X inches of 5:1 is not equivalent to X/2 inches of 10:1. 5:1 is a helpful baseline since it is established on FB and in the Pompeii plans that 5:1 is approximately half the K-factor of the various insulating blankets (disregarding for the moment the nuances of 4#, 6#, and 8#, on which I am unclear about the details).

As a final note, the plot clearly shows that PSI, unlike K-factor, is not linear!

[EDIT: Followup: a number of sources state loose vermiculite's K-factor as .48-.6 and its R-value as 2.0-2.1. So, the discussion above suggests that 10:1 and 12:1 are nearly optimal.]
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Last edited by kebwi; 02-02-2010 at 07:33 PM.
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  #23  
Old 02-02-2010, 03:47 PM
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Default Re: Terraced enclosure

You're an engineer, aren't you? Name:  cool.gif
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  #24  
Old 02-02-2010, 03:51 PM
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Default Re: Terraced enclosure

Computer scientist, not a hardware engineer, which was why I have been so panicky and obsessive about this project. I have no confidence in my ability to handle a hardware project since I spend my entire life typing.

And yes, I need to stop doing this so much...I'm driving half of FB crazy. I think people are going to black-list my threads if I don't relax.

Cheers!
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  #25  
Old 02-02-2010, 04:10 PM
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Default Re: Terraced enclosure

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Originally Posted by kebwi View Post
I'll keep this question in this thread since it arised from my various enclosure designs, but it is somewhat of a tangent to the original subject of the thread:

I am trying to determine what I will *lose* by molding the terraces out of vermicrete instead of building an enclosure shell and filling it with vermiculite. In either case, I would apply an outer coating of some sort of stucco (surface bonding cement being the most likely candidate), along with perhaps other surfaces to increase water resistance. That's not my point here though.

What I really want to know is the comparative insulation value (R value?!) of loose vermiculite vs. the common ratios of vermicrete, 5:1 to 12:1, although I admit that for an upper enclosure I would probably do something between 8:1 and 12:1.

Does anyone know how much better loose vermiculite is vs. 8:1 or 12:1 vermicrete? Obviously for a given thickness, that goes without saying.

Thanks.

Something else just occurred to me - if you make the terraces part of the oven structure you'll need to insulate well enough not to over warm the soil. Warming it mildly won't hurt - might even help (will mean extra vigilance in the watering department) but if the soil gets too hot you'll have a lot of dead plants. Few plants can tolerate hot soils - soil rarely gets that hot outside of the desert. A well insulated oven probably won't get too hot but if there's a hot spot the plants nearby will be toast.

Just a guess, but watering before firing is probably a bad idea. Water conducts heat better than soil if I recall correctly. Plants that can tolerate high air temps won't necessarily tolerate getting their feet hot.

How hot is too hot would vary by plant but I've honestly never seen a discussion of soil temp that would help here. Most soil discussions I've seen were on when is the soil warm enough rather than when it is too hot.
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  #26  
Old 02-02-2010, 04:16 PM
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Default Re: Terraced enclosure

Yes...if I had a clearer picture of oven external surface temperatures, I could incorporate that into my design. My oven will have 2"-3" of InsWool HP Bulk. I have no idea how that translates to the various blankets most FBers use. It will then have some form of vermiculite, either loose fill under a true enclosure, or more likely, vermicrete forming a solid structure comprising the terraces. In either case, the vermiculite will probably be a few inches thick. I could try to make it significantly thicker, but that will raise the terraces up so far no one will be able to see them.

What would be very helpful is a report of external temperatures for a variety of FB igloos.
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  #27  
Old 02-02-2010, 04:17 PM
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Default Re: Terraced enclosure

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Originally Posted by kebwi View Post
Computer scientist, not a hardware engineer, which was why I have been so panicky and obsessive about this project. I have no confidence in my ability to handle a hardware project since I spend my entire life typing.

And yes, I need to stop doing this so much...I'm driving half of FB crazy. I think people are going to black-list my threads if I don't relax.

Cheers!
Nah, you're fine - I've monopolized 'really stupid questions' around here.

Just remember two words: small scale. You can always try it small to see how it works. I read about one guy who tested various cob mixes by building tiny ovens out of each on pavers and baking muffins to see how well each worked (an idea I have every intention of stealing). You can scale down the structure and build it just to get the kinks worked out before going large scale (note: nobody does this - most just work it out as they go along and things turn out fine ).
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  #28  
Old 02-02-2010, 04:21 PM
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Default Re: Terraced enclosure

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Originally Posted by kebwi View Post
...

What would be very helpful is a report of external temperatures for a variety of FB igloos.
That's a Dmun question if I ever saw one. (He's gonna kill me now ) Dmun is one of the most knowledgeable folks around for just that sort of info.

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  #29  
Old 02-02-2010, 04:36 PM
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Default Re: Terraced enclosure

worry less, cook more
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  #30  
Old 02-02-2010, 04:38 PM
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Default Re: Terraced enclosure

In the context of the present discussion, I assume you are referring to cooking the plants growing on top of my oven.
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