planning questions

[david s]

Re: planning questions

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.

[cobblerdave]

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.

Regards dave

[tikidollracer]

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.

[Gulf]

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.

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

[Greenman]

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!

[david s]

Re: planning questions

K

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.

in using the term "steam explosions"
I was talking more generally about excess water in the refractory itself, in an effort to explain the importance of water removal to avoid damage to the oven. As the water is driven off it moves out wards away from the fire and into the insulation. The vent works in much the same way as the hole in a saucepan lid to reduce the pressure build up. In my build because I use vermicrete rather than blanket the careful removal of moisture is more important.

[mirassou]

Re: planning questions

Isn't the vermicrete over 3" of ceramic fiber insulation more than is necessary? The Pompeii plans indicate vermicrete over 1" insulation, but I'm planning cladding the exterior with 1/2 to 1" heat stop, then adding 3" of fiber blanket. Isn't this enough?
Thanks,
Paul

[Gulf]

Re: planning questions

Isn't the vermicrete over 3" of ceramic fiber insulation more than is necessary? The Pompeii plans indicate vermicrete over 1" insulation, but I'm planning cladding the exterior with 1/2 to 1" heat stop, then adding 3" of fiber blanket. Isn't this enough?
Thanks,
Paul

Paul,

3" of blanket is enough insulation. But, it must be kept dry. For igloo builders, that means, some type of water resistant render over the insulation. Some render straight on top of the blanket. But, blanket insulation can be a little lumpy to render over for some builders, like me . The vcrete layer can be used to help "even out" those hills and troughs. It also has the extra benefit of being extra insulation. In this discussion, it does add more water that must be removed. I beleive, like David S. , the best way to get it out is to get it out before it is trapped in. I really thought that I had given it enough time. But much later, seeing the moisture coming out of my vent hole, has made a believer out of me .

The heatstop cladding applied to the outside of your dome brick will be extra thermal mass, not insulation.

Joe

[mirassou]

Re: planning questions

Got it...whew..thought I was going to have more work ahead of me, but I'm building an enclosure around mine.

[cobblerdave]

Re: planning questions

Got it...whew..thought I was going to have more work ahead of me, but I'm building an enclosure around mine.

Gudday
Think again ..... Gulfs build has full oven head cover. I mean full, big,huge.
Check his build out.
You might want to add to your soffits
Regards dave

[Greenman]

Re: planning questions

Slapping a bit of vcrete and mortar around can be quite therapeutic, loads cheaper and quicker and very satisfying if you stand back and see a dome of great beauty there is a little personal magic! It works better if you have no snow to contend with and builds of the ilk of Gulf, Stonecutter, Utah and too many more to mention are works of art.

The guaranteed good bit is the inner glow when the keystone is placed on the dome and the absolute knowledge that the first pizza cooked in your oven is superior to every other pizza that has ever been made. That is also magic.

House/no house - vent/no vent. Vexing questions but it seems you have it sorted now. Onwards and upwards!!

[mirassou]

Re: planning questions

Oh that's right! So Gulf, why the vermicrete?

[Gulf]

Re: planning questions

Oh that's right! So Gulf, why the vermicrete?

If I understand the question correctly, here goes:

This is just my thinking, so take it with a grain of salt .

The blanket placed between the firebrick and the vcrete is the primary insulation. But, (if you think about it) it is also one heck an expansion joint. In my build, I was wanting to accomplish two things. The first was to preserve the look of the perfect sphere that I had worked my as off to accomplish. The second was to have a beefy/structural layer to support a stucco shell on which to apply brick flooring splits. (The splits give the illuson of the real deal .) The vcrete did both of those, plus it gave the added benifit of more insulation.

[cobblerdave]

Re: planning questions

Gudday
Why the v-Crete . I didn't bother with the stuff it was not worth mucking around with, especially as the constant rain at that time it would never have dried. 4 in of rockwool over the 2 in of ceramic , chicken wire and render (stucco) and the oven was sealed in. Just in time for the one in a hundred year flood as it turned out. I've since helped out pearlcrete on oven and am still not impressed with that stuff . If I built again it would be, dry the dome ,3 in of ceramic then render over that. No mucking around and waiting for water to slowly work its way out. Dry and sealed in minimum time. Anyway that's how I see it
Regards Dave

[mirassou]

Re: planning questions

What's used for the render? That's probably what I'll do as I'll be enclosing mine.