Scratch build thread - 36-inch inner diameter dome oven

[mnagy]

Finally finished with the outer bricks. Here’s how it looks today. I poured an extra beam of concrete behind the arch and filled the back gap with concrete as well. The sides maintain a 1/2” gap. I’ve got some mosaic tiles to put upside down on top and my vermiculite just arrived today. Gonna fill this thing with 5:1 vermicrete and then give it a week before starting on the dome.

I’m a little weirded out about the wetness I’m seeing in this back corner of the oven. After every rain this area seems wet for days afterwards. I wonder if it’s a constant moisture coming up from poor drainage in my yard? I guess I’ll monitor it more closely and see if this section ever dries out.

[UtahBeehiver]

A week of drying 5 to 1 vcrete may not be enough. Once you cover the vcrete with the floor bricks the moisture in the vcrete is really hard to remove. David S uses a cheap soil moisture meter to check v/pcrete. It may look dry on the surface but the core has a ton of water in it.

[mnagy]

Thanks for the tip! I’ll slow my roll and really let that stuff dry.

Here’s the timeshare tile I’m using. They are a little slippery, and I’m worried about them shifting during the pour. So I might put a dab of glue on a few tiles per sheet just to be sure. I’ll also put some mesh drywall tape on the seams.

[mnagy]

And of course I meant mesh tile. I’m personally offended that Siri thought I would ever consider using “timeshare” in a sentence.

[david s]

Attached is a little experiment I did on drying a vermicrete slab that you may find interesting and informative. If there is moisture trapped under the floor it will eventually dry out, but it could take months. The process of eliminating underfloor moisture can be assisted by drilling some holes up through the supporting slab. The idea of the tiles is to create some pathways to drain holes in the supporting slab. If you pour vermicrete over the tiles these pathways get blocked off. I suggest you lay some weed mat over the tiles before filling over it with vermicrete.

Vermicrete insulating slab copy.doc.zip

[mnagy]

Thanks David. I thought the mesh backing for these tiles would be enough to keep the poured vermicrete out of the tile cracks. I can certainly add some additional screening material if you think it’s needed.

[mnagy]

I’ve laid all the mesh tile (face down) on the hearth slab (which has several 5/8” holes in it). I actually put 4 dabs of glue on the tile face to prevent it from sliding around. Next I used fiberglass mesh tape for the seams. The tape is normally used for cement board seams. Finally, since I had a bunch more tape, I just went over the whole darn thing with a layer of mesh tape. Between the tile mesh and the mesh tape, it should keep the vermicrete away from the channels.

[david s]

Yes the mesh on the tiles should be enough to hold up the vermicrete from the drain channels. It is important to get the correct amount of water in the vermicrete mix. Too much water will wash the cement off the grains of vermiculite. The finer the grade the more water is required. The correct amount is when it just starts to pool a little in the bottom of your barrow. Medium grade is probably best. Do not use a mixer as it degrades the grains and sticks to the sides of the mixer and blades. Just fold it in gently with barrow and spade.

[mnagy]

Question for the pros… I’m about to start pouring a 5” thick layer of 5:1 vermicrete. I’ve got good drainage, but would there be any benefit to making this pour in two stages? Say pouring 3” today, and then the remaining 2” next week?

[mnagy]

Yes the mesh on the tiles should be enough to hold up the vermicrete from the drain channels. It is important to get the correct amount of water in the vermicrete mix. Too much water will wash the cement off the grains of vermiculite. The finer the grade the more water is required. The correct amount is when it just starts to pool a little in the bottom of your barrow. Medium grade is probably best. Do not use a mixer as it degrades the grains and sticks to the sides of the mixer and blades. Just fold it in gently with barrow and spade.

Thanks! I was going to do 5 parts (A3) vermiculite, 1 part Portland, and 2 parts water. I’m guessing your suggestion is to be more reactive with the water, rather than pouring in a measured amount.

[david s]

That sounds about right. Doing it in two stages is a good idea because there will be some drying of the first layer before placing the second. At least a week between the two is better.

[mnagy]

Another question for the pros:

What should I use to attach the oven floor bricks to the vermicrete layer? I was thinking about applying some refractory mortar with a notched trowel. I know some people just use leveling sand/clay (allowing them to easily replace floor bricks as needed) but my dome is sitting on top of this floor and I’m concerned that it needs to be as solid as possible.

[UtahBeehiver]

The floor bricks should be free floating. Sand/clay mix is best and just enough to level the bricks.

[mnagy]

Need some advice…
About halfway through my 2nd vermicrete pour, I realized I wasn’t going to have quite enough vermiculite to make it perfectly flat. So I continued pouring and packed it down so that there is about 3/4” all the way around/across for one more pour. The question is whether I buy more vermiculite and pour a thin layer of the same thing; or if I pour something else instead.

[mnagy]

UtahBeehiver,

You mentioned that the dome floor bricks should be free floating… What about the first row of dome bricks? Should those be mortared to these free floating floor bricks? Or should this row (of half-brick soldiers in my case) just be mortared to each other but not the floor? Thanks!