cracks in outer insulation

[kscott]

the exterior of my dome has cracks in it. I would like to fill them with something and then cover the whole dome with more insulated concrete. I'm looking for some suggestions.

[david s]

I presume you mean cracks in the vermicrete insulation. This is usually caused from firing the oven too hard before the vermicrete has dried enough. Because the vermiculite, or perlite if that was used instead, is so water absorbant it takes up more than double the amount of water required for the hydration process. This free water needs to be eliminated slowly otherwise sudden steam expansion creates cracks. If this is your situation just fill the cracks with some of the same mix, leave it for a week to allow sun and wind do much of the water elimination the fire it gently until moisture has gone. To test for water still in the layer, throw some sheet plastic over it to see if it condenses on the underside. Only until all the water has been eliminated, should you proceed to stucco the outer shell.

[kscott]

David, thanks for the reply. You are correct about this being the outer dome with vermiculite. I have routed out the cracks and filled them with a high temp elastomeric material than can handle up to 3000 degrees F. My plan is to cover the outer dome with another layer of either concrete or some type of insulated concrete. Do you have any suggestions on the best way to add this other layer of insulation? What are your thoughts on covering the dome I just fixed with tinfoil first and then adding this other layer?
Any help would be appreciated.

[david s]

David, thanks for the reply. You are correct about this being the outer dome with vermiculite. I have routed out the cracks and filled them with a high temp elastomeric material than can handle up to 3000 degrees F. My plan is to cover the outer dome with another layer of either concrete or some type of insulated concrete. Do you have any suggestions on the best way to add this other layer of insulation? What are your thoughts on covering the dome I just fixed with tinfoil first and then adding this other layer?
Any help would be appreciated.

I certainly would not recommend adding a foil layer as it locks moisture in. i did this with my oven and was a bit worried so I perforated the foil in two places. Now when the oven is drying the outer shell gets quite hot in those two places, meaning steam is finding its way through the perforations. Even if your oven is totally waterproof the porous refractory will pick up moisture from the atmosphere in humid weather. Adding some kind of vent to allow steam to escape is a good idea IMO.

Remember also that foil will not work as insulation if it is hard up against conductive material. It needs to be surrounded by air or insulate material to work.

I'm also interested in any material that can remain elastic at 3000F. Everything I've tried except HT silicon (good for around 300 C at tops) goes hard What is the name of the product you used?

[kscott]

I like the vent idea. What about that extra layer of concrete or insulated concrete? Should I use insulated concrete or regular? I wonder if it it really makes a difference at this point. Any thoughts on that?

[david s]

My feeling is that the outer shell needs to hard and strong. Adding vermiculite or perlite to the mix will make it more insulating, but weaker. By all means use it as a first layer to provide high build to get the form nice and spherical. Lime in the mortar apparently gives the hardened mix some flexibility and self crack healing. But an elastic acrylic render over the top of the cement render does a better job.