Structural Hearth poured. Next steps?

[agrasyuk]

Poured my counter top slab on few days ago. Really happy to see the project take shape.
before I pour vermiculite-portland mix as insulating base I had few questions that I cannot find unambiguous answer for.

is there a benefit having a waterproof membrane (foil sheet?) Between the concrete and vermicrete to prevent moisture being wicked up?
Is there a need/benefit using concrete sealer ?

I cannot purchase ca-sil boards unfortunately. Planned on having about 6" of vermicrete. With that I got hold of rock wool matts. These are stiff , but not rigid as board and will compressor under load. I'm thinking whether its possible to use those as base and pour the vermicrete on top

Thanks.

[UtahBeehiver]

V-crete or P-Crete does not have very good shear load capacity so if the rockwool compresses there is a possibility of cracking your V-crete. That said, there was a recent build that used Roxsul under their V-crete but I believe David S suggested installing piers of Vcrete at critical load areas, IE, perimeter of dome. The jury is still out on this since the build is very recent. V-crete in the 8 to 1 ratio has pretty good thermal value as shown on the attached doc that David S has found.

[Gulf]

.......................I cannot purchase ca-sil boards unfortunately.................

If the reason availability, Here is one source near Chicago.................. Here is another one.

[david s]

V-crete or P-Crete do not have very good shear load capacity so if the rockwool compresses there is a possibility of cracking your V-crete. That said, there was a recent build that used Roxsul under their V-crete but I believe David S suggested installing piers of Vcrete at critical load areas, IE, perimeter of dome. The jury is still out on this since the build is very recent. V-crete in the 8 to 1 ratio has pretty good thermal value as shown on the attached doc that David S has found.

If the vermicrete is under load don't use anything under 5:1 as it won't be strong enough. For my first oven, ten years ago, I used5:1 vermicrete piers supporting the large firebricks and dome, with fibreglass ceiling bat insulation in between. It worked pretty well and when I replaced the oven with a better and larger one the insulation was surprisingly still in good condition.
regarding sealing between the supporting slab and the underfloor insulation, a concrete sealer is probably the better solution. Any vapour barrier there will prevent water wicking up from the stand but also prevents moisture from escaping down. My solution for this is to have one or two small (12mm) holes in the centre of the supporting slab (they can be drilled, but easier to cast them in when laying the slab) that can act as a drain. This helps remove moisture from the insulation layer much faster than if not there.

Also there is an error in that table. The 5:1 vermicrete should read 175-225 not 175-125

[agrasyuk]

Thank you guys.
a support ring of 5:1 v-crete under the wall + rock wool and thinner layer of v-crete under the floor sounds like great idea!
I will be drilling a hole or two into the slab for drainage /drying as well, hope I don't hit rebar ).

Joe, thank you for the links. I will be contacting these guys.

[david s]

Start with a smaller diam hole to guard against hitting rebar, you can more easily fill it.

[Gulf]

Here's where chronicling a build with pics can come in very handy. If you have one of your rebar placement, some measuring and dead reckoning will help a bunch .

[agrasyuk]

I do keep a detailed photo log of the build, although I don't post every single picture here to not be to annoying )
this one is the only slab i do not have "ready to pour - rebar map" image, storm was moving in fast and everything was happening in "burning ground" mode to finish the pour before it landed.

Thanks again!