Insulated Pad question

[Eddie4]

I am not a mason at all. But am going to attempt building the "oven".
when mixing the vermiculite with the portland cement for the support pad, the instructions say mix 5 parts vermiculite to 1 part cement.

Did I say I wasn't a mathametician also?

How many bags of 80lb cement are needed for the pad and how much vermiculite will be needed?

Eddie

[dmun]

I assume the volume of the vermiculite concrete is about the same volume as the vermiculite. I tried to find an easy answer for you but to no avail, maybe one of the recent builders will jump in.

Just a quick note: The support slab is quite different from the insulating layer, made from vermiculite of perlite. Ovens are heavy and need support, and the vermiculite concrete has no structural strength. The reinforced concrete support slab goes under the insulating layer.

And in the US, portland cement comes in 94 pound bags, concrete mix comes in 80 pound bags. You'll notice that extra fourteen pounds when you pick the bag up.

[gjbingham]

One of the small issues with the 5:1 ratio in the plans - it is not stated whether it is by weight or by volume. I used volume because the vermiculite is so light. I assume that was correct.

Eddie - need to know the size of the oven you're building. I used 3 large bags of vermiculite to pour the insulation layer (not really a support pad) directly under my oven. If you take that layer out to the edges of the hearth, probably somewhere around 6 - 8 bags. Check out the plans again. Make sure your terminology matches my description for the step you're trying to accomplish.

[Eddie4]

I think that were the confusion set in.
I forgot that the support pad had to be poured then the insulating pad.
If you skip the insulation and just pour a support pad, do you really lose that much heat from the oven surface?

Ed

[dmun]

If you skip the insulation and just pour a support pad, do you really lose that much heat from the oven surface?

Insulate! Insulate! Insulate!

If you put your bricks directly on the support slab, it will wick all the heat out of your floor, and you will never get your oven to pizza temperatures.

You can't skip this step! If you do, you will have an oven that will not cook properly.

Be glad you asked this question first. It's so sad to tell people who have already built their domes, that there's nothing much they can do except start over.

[Dutchoven]

I wholeheartedly concur with dmun...I created the oven hearth slab in the manner of Alan Scott and at the time he suggested bonding the insulating slab underneath the regular concrete slab to support the oven...it works but not nearly as well as the inverse would have...insulate with the vermicrete and if you can add a layer of FB board insulation over that and then lay the floor bricks...that is what I am doing on my next build...as for the mixture it is by volume...you could be really specific and build yourself a box with the inside dimensions of 12" by 12" by 12" and measure your parts that way(would be exactly one cubic foot)...portland and vermiculite is good for the hearth insulation but I would recommend mixing vermiculite with "Type N" masonry mortar for the outside of the dome...type N is a bit softer and sticks better to vertical surfaces...
Dutch

Originally Posted by dmun

Insulate! Insulate! Insulate!

If you put your bricks directly on the support slab, it will wick all the heat out of your floor, and you will never get your oven to pizza temperatures.

You can't skip this step! If you do, you will have an oven that will not cook properly.

Be glad you asked this question first. It's so sad to tell people who have already built their domes, that there's nothing much they can do except start over.

[mfiore]

Originally Posted by Dutchoven

. . . you can add a layer of FB board insulation over that and then lay the floor bricks
Dutch

I know it's come up before, but if you use the FB board insulation, isn't the vermiculite overkill?

[gjbingham]

Mfiore - yes - to a point. If I did another, I'd do the equivalent of 6 inches of vermiculte under the floor, or 3 inches of board. Two inches of board on top of the concrete hearth should suffice though.

Eddie - I thought you were off track. You definitely need insulation on top of the concrete hearth. Don't skimp. This is apparently where most ovens loose their heat fastest, or have trouble maintaining temps. Follow the plans, you will be pleased.

[Dutchoven]

Yes to a point is a good answer...2-3 inches of vermicrete with 2 inches of FB board on top is pushing but a REALLY well insulated oven is just going to be so efficient
Dutch

Originally Posted by mfiore

I know it's come up before, but if you use the FB board insulation, isn't the vermiculite overkill?

[gjbingham]

Hey Dutch, good answer. I like the way you think! I beat you by a couple of seconds I think.