Trouble? with insulating board

[Irv Mac Dowell]

I've completed the block stand for my 90, and today poured the reinforced oven floor. As instructed in the manual, I laid the insulating boards on the top of the concrete. Almost immediately it became apparent that the boards were sucking the water out of the concrete. I lifted one board and the concrete was virtually dry underneath. I removed all of the boards, re-wet and screeded the floor and replaced the boards. I'm concerned about the cure under them. Is this a problem?

Also, the boards are an inconvenient size, at least for my 90, since one has to be cut to achieve the correct size and shape. Perhaps they could be made in a wider form to facilitate mounting the floor and dome.

Thanks for any thoughts.

Best regards,

Irv Mac Dowell

[krosskraft]

I remember having problems with that board as well. The men that poured the hearth for me scrunched it down in the concrete and then put some rope or something over it to keep it from popping up. It was a tense moment. They actually cracked some of the board, so I took some pieces of the board and wedged them into the cracks, then I think I filled it with refractory mortar. Hopefully some folks will share some other ideas, but just keep going in your quest for the oven.

[gjbingham]

I think that you need to wait to install the ceramic boards until the concrete has cured. The plans call for pouring a vermiculite/concrete insulation layer at the same time as the concrete hearth pour. This is not really necessary either. Hope they're OK!

Irv, the shape is determined by the manufacturer and not soley intended for use in WFOs. Just like any building material, you have to take whatever material is available and make it fit your situation, even if that requires stacking, cutting, shaping, grinding etc.

[Irv Mac Dowell]

I ran into the same problem with the boards sucking water from the concrete when I set the hearth floor yesterday. Within less than a minute, whatever concrete I placed was sucked dry by the boards. I finally had to pour water onto the boards until they were soaked enough that they didn't pull water from the mix. I wish the instructions that came with the oven were more forthcoming about this drawback to the boards. I think I might have gone with the Perlcrete instead.

Best regards,
Irv

[ikhan42]

I too thought the boards get used after the cement floor is set. Am I missing something ?

Imran

[SpringJim]

why not use a vapor barrier between the concrete and the board?

plastic sheet?, wax paper?

[Irv Mac Dowell]

I guess waiting until the concrete is set is the way to go. The problem arose for me because the picture in the instructions shows the board on clearly wet and unset concrete.

The second problem arises when setting the floor, because the board sucks all of the water out of the leveling refractory cement.

Irv

[dmun]

why not use a vapor barrier between the concrete and the board? plastic sheet?

BINGO! This way you can set the boards flat and secure into the wet concrete without the water sucking problem.

As a side note, I originated the boards-into-wet-concrete technique, but I was using Insblock19, which is a rigid mineral wool product, not Cal-Sil, and doesn't have the water sucking properties of the more efficient product.

As far as the leveling material, if your boards are flat, you most likely don't need it. If you do, use it dry and sparingly.