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Old 07-21-2009, 05:54 PM
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Default Concrete base direct on earth?

Hi, due to limited space, I am building my oven into an enbankment, directly off the back patio. This saves both space and time, but also means that the concrete base will likely sit directly on the earth, with a minor brick wall surround to support and contain the setting concrete. My issue is this : as the concrete is not suspended above the ground (like generic ovens), but will sit directly on the actual earth, I suspect it will be a bigger than usual heat sink. Should I therefore put some insulating layer (of what?) below the concrete base. Or simply make the insulating layer above the concrete thicker. Any suggestions would be welcome. We live in Melbourne Australia, so the climate can be cold in winter, so want to ensure the oven is as efficient as possible. Thanks for your advice.
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Old 07-23-2009, 05:00 AM
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Default Re: Concrete base direct on earth?

Steve , I would also be a little concerned about the amount of moisture that will be drawn up into your oven in times when you are not using your oven. Rising damp could be a problem if the area is damp. I don't see a problem with losing heat if you insulate properly, your insulation will only stop the heat leaving your oven and any heat that does escape will be absorbed through your slab and then through your soil.
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Old 07-23-2009, 06:10 AM
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Default Concrete base direct on earth?

Thanks Rodney, Agree on the moisture. Spoke to someone today who suggested some drainage or blue metal below the concrete, to take the moisture away from the slab. I have also been assured that so long as i have enough insulation, heat loss should be ok. But moisture will be the main concern.
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Old 07-23-2009, 07:54 AM
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Default Re: Concrete base direct on earth?

Quote:
moisture will be the main concern.
Wet insulation is ineffective insulation. A moisture barrier may be in order. Also, if you have frost, a layer of well drained crushed rock will help prevent frost heave.
Quote:
the concrete base will likely sit directly on the earth, with a minor brick wall surround to support and contain the setting concrete.
This retaining wall should be well supported, if it's bearing any of your oven's weight.
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Old 12-20-2011, 12:27 PM
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Default Re: Concrete base direct on earth?

So moiture can actually be drawn up into the oven? What is the result of this?
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Old 12-20-2011, 01:39 PM
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Default Re: Concrete base direct on earth?

The result is that the insulation gets wet and therefore doesn't work too well until it is dry again. If I were in your position I'd be painting some Bondcrete, or similar product, over the supporting slab to try and prevent the rising damp getting into the insulation. Also some sheet plastic, as is normal for concrete slabs, under the supporting slab will also assist in preventing rising damp.
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Old 12-20-2011, 04:51 PM
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Default Re: Concrete base direct on earth?

Stephen -
I am taking a similar approach and facing similar challenges with my oven build. You might want to take a look at the thread I started

http://www.fornobravo.com/forum/f6/s...ild-16994.html (Starting new 36" build)

I have gotten lots of good feedback.

Bottom line seems to be
1) keep water away from the base with a good layer of gravel
2) use a vapor barrier
3) additives in the concrete can prevent wicking

For good measure, I am probably going to use an additional inch of CalSil as well to ensure sufficient insulation from the base pulling cold up from the ground.

Hope that helps

Dennis

Oh - also forgot about the potential for differential settling with part of the slab supported by a wall and the rest laying on grade. I am going to drop a couple sonotubes to a clay layer to avoid this.

As I said... lots of good feedback here.
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Old 12-20-2011, 08:37 PM
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Default Re: Concrete base direct on earth?

Quote:
Originally Posted by Stephen View Post
But moisture will be the main concern.
Put a layer of concreters plastic below the slab, problem solved.

Saying Melbourne is cold to these boys who regularly get to -40 c will make them laugh.....
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