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  #11  
Old 10-20-2013, 11:43 PM
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Default Re: planning questions

sorry, i'm new here, and i'm sure this is boring for old hands, but i don't understand. if you don't have a vent cap in the top of your structure, and it was a humid night so the bricks absorbed some moisture. then you heat your bricks to 900F. where do you think the steam is going to go? presumably it doesn't build up pressure and blow your igloo to pieces. vermiculite and stucco are not vapor barriers.

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  #12  
Old 10-21-2013, 12:12 AM
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Default Re: planning questions

About half the water in a concrete brew, (assume the same for mortar) is taken up in the hydration process as the concrete cures. So there is still plenty to remove. Many builders also wet their firebricks, either prior to, or as they are laying them. Vermicrete takes about four times the amount of water that is taken up in the hydration process. So there is usually a large amount of water to be removed. As the water travels away from the fire in an effort to escape it gets trapped in the insulation layer. Water expands some 1600 times it's volume when turned to steam and can easily crack the outer shell of an igloo oven through the increase of pressure.if you see steam then you are pushing the process too fast.

Last edited by david s; 10-21-2013 at 12:16 AM.
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  #13  
Old 10-21-2013, 01:15 AM
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Default Re: planning questions

Hi David

So wouldn't a valve as in cobblerdave build and others I presently cannot recall alleviate that somewhat.

Until the stucco is sealed, one sealed I could see a problem.
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Old 10-21-2013, 01:54 AM
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Default Re: planning questions

If doing an igloo I think it is a good idea to drive out the water before doing the render/ stucco outer shell to reduce the possibility of cracking it. A vent will help reduce the steam pressure should the oven get wet after it is finished.if an igloo oven is exposed to the elements, even if the outer shell has been made waterproof, water will still get in, it needs an exit.i generally give a new oven about 10 decent cooking fires, after it has been cured, before making the outer shell waterproof. If your outer shell keeps water out, then it will also keep water in. A vent will allow that insulation layer to breathe and the waterproof coating will keep out most of the water.
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Last edited by david s; 10-21-2013 at 02:07 AM.
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Old 10-21-2013, 08:54 AM
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Default Re: planning questions

Quote:
Originally Posted by david s View Post
About half the water in a concrete brew, (assume the same for mortar) is taken up in the hydration process as the concrete cures. So there is still plenty to remove. Many builders also wet their firebricks, either prior to, or as they are laying them. Vermicrete takes about four times the amount of water that is taken up in the hydration process. So there is usually a large amount of water to be removed. As the water travels away from the fire in an effort to escape it gets trapped in the insulation layer. Water expands some 1600 times it's volume when turned to steam and can easily crack the outer shell of an igloo oven through the increase of pressure.if you see steam then you are pushing the process too fast.
sorry, still not making sense to me. is there some place this vent is documented, or is this just local list practice? it seems to me that it's simply placing a 3 sq. in. hole in the insulation layer for little or no practical purpose.

the concrete and bricks are wet. then they cure. during this curing the concrete shrinks in volume, and it is then that cracks commonly form. when you start heat cycling it is these fractures you're exposing. vapor forced through the concrete passes through, and does not increase the volume of the concrete by 1600x.

the concrete is cured, then additionally it is forced dry by firing. at this point there is no appreciable water left. if this forced drying cracks the concrete then you've developed the vents you seek. afterwards a stucco barrier is applied which is also not a 100% vapor barrier, but should repel water. a liquid barrier and a vapor barrier are not the same thing. at this point the oven will maintain an equilibrium w/ the environment until it is fired, when it will again be forced dry depending upon the length of the firing. but we're talking small amounts of moisture here. these domes are not large enough to develop their own climate. and even if they do have a small amount of moisture and steam migrating through the insulation w/ every firing cycle, so what? you can't stop it from happening, and any venting is at best just providing a preferred direction for the micro-fractures to form, while allowing the heat to conduct right out the top.

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Old 10-21-2013, 02:12 PM
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Quote:
Originally Posted by tikidollracer View Post
sorry, still not making sense to me. is there some place this vent is documented, or is this just local list practice? it seems to me that it's simply placing a 3 sq. in. hole in the insulation layer for little or no practical purpose.

the concrete and bricks are wet. then they cure. during this curing the concrete shrinks in volume, and it is then that cracks commonly form. when you start heat cycling it is these fractures you're exposing. vapor forced through the concrete passes through, and does not increase the volume of the concrete by 1600x.

the concrete is cured, then additionally it is forced dry by firing. at this point there is no appreciable water left. if this forced drying cracks the concrete then you've developed the vents you seek. afterwards a stucco barrier is applied which is also not a 100% vapor barrier, but should repel water. a liquid barrier and a vapor barrier are not the same thing. at this point the oven will maintain an equilibrium w/ the environment until it is fired, when it will again be forced dry depending upon the length of the firing. but we're talking small amounts of moisture here. these domes are not large enough to develop their own climate. and even if they do have a small amount of moisture and steam migrating through the insulation w/ every firing cycle, so what? you can't stop it from happening, and any venting is at best just providing a preferred direction for the micro-fractures to form, while allowing the heat to conduct right out the top.

-SM-
Purging moisture to prevent steam explosions is a problem. Many folk who have built cob ovens have discovered this. Likewise every potter knows that his wares are subject to this problem if the first firing is rushed. Usually the problem occurs at round 200-300 C where water turns rapidly to steam and expands quite violently. It is a mistake to think that because you are over 100C that all the water must be out. Manufacturers of castable refractory include fibres in the mix that burn out at low temp (around 160 C) and a network of tiny fibres is left for the moisture to find its way out more safely. This technology was actually developed in conjunction with concrete to give concrete buildings some protection from fire damage.i have found from experience that rushing the early fires of a new oven will definately crack the outer shell (I've done it a couple of times) I developed a system whereby moisture can vent around where the flue pipe comes out of the outer oven shell, about four years ago, and it works pretty well. If you believe that a vent won't work or is unnecessary then don't put one in, many builders don't.
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  #17  
Old 10-21-2013, 02:32 PM
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Default Re: planning questions

Gudday tikidolracer in answer to post 15
Yes in theory that sound good . But my oven operates in a real world.
With a sealed dome any moisture end up in the insulation and yes it will be there insulation just loves to wick up water, it only has one way to get out. Back into the brickwork of the dome. Slowly.
A vent at the top of the dome tips the balance and allows me to dry my oven effectively
Equilibrium could be achieved in the Simpson desert I suppose but my oven lives at the bottom of the garden. It gets wet from time to time, and you dry it out.
It never had a vent to start with, but a few dome cracks mainly a the rear of the chimney. It now has vent and a brick veneer (unsealed I might add) and a outer rain door and a spoon drain.
So in a way I have achieved an equilibrium. It get damp I dry it out and cook in the open air the way I like too.
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Last edited by cobblerdave; 10-21-2013 at 05:32 PM. Reason: I-phone fat fingers
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  #18  
Old 10-21-2013, 07:01 PM
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Default Re: planning questions

i'll let it rest, but i'm still unclear where or why this practice is needed. pottery is a different beast, and has far more moisture. good kilns rest just above 100C because of this. concrete cures, and removes much more water. if the curing is rushed, or the firing is too abrupt before complete curing, sure i can see problems happening. if you've somehow added a vapor barrier to your stucco and water cannot escape, ok. otherwise i can imagine more plausible reasons why a masonry structure breaks than steam explosions from natural humidity.
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  #19  
Old 10-21-2013, 07:30 PM
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Default Re: planning questions

Quote:
Originally Posted by tikidollracer View Post
i'll let it rest, but i'm still unclear where or why this practice is needed. pottery is a different beast, and has far more moisture. good kilns rest just above 100C because of this. concrete cures, and removes much more water. if the curing is rushed, or the firing is too abrupt before complete curing, sure i can see problems happening. if you've somehow added a vapor barrier to your stucco and water cannot escape, ok. otherwise i can imagine more plausible reasons why a masonry structure breaks than steam explosions from natural humidity.
To put it a little less dramatically than steam explosions . Maybe, just so that your oven dries out and gets back to clearing a little faster after periods of unuse IE: inclement weather etc. They can always be plugged, filled in, and rendered over if they do not prove usefull. For my build, the vent has been the easiest/cheapest eccentricity that I have included.

Just Say'n
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  #20  
Old 10-21-2013, 09:46 PM
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Default Re: planning questions

It seems popular to have a vent 'just in case' but it is a personal choice and you have some time to ponder the quandary before you have to commit. It certainly got some air anyway.

You might be the one to put the theory to the ultimate test. Without the sceptics we would still have a flat earth. Good luck with that!
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