adobe+vermiculita insulation

[cynon767]

Re: adobe+vermiculita insulation

I dry-mixed about 6.5 or 7 to 1 for the hearth insulation, and it worked fine. A little crumbly at the edges if I'm not careful with it, but otherwise totally solid (so far!).

[JoeyVelderrain]

Re: adobe+vermiculita insulation

if i use a blanket, and then the perlcrete, won't the perlcrete crush the blanket and flatten out making it useless?
and if i do use a blanket how do i attach it to my dome so it stays put whilst i coat the perlcrete on top of it?

[kebwi]

Re: adobe+vermiculita insulation

And to be precise, would an answer to JoeyVelderrain's question vary with the density of the blanket, 4#, 6#, 8#?

BTW, I had at least one manufacturer tell me that higher density insulation is more insulating than lower density, flying the face of the thermal conductivity data. I think he was just clueless though, right?

[papavino]

Re: adobe+vermiculita insulation

8# blanket is going to insulate better than 6# and 4#. The higher the density of fibers, the more little air pockets and tortuous the air path will be. It holds the heat back more efficiently than a less dense mesh of fibers.

As far as perlcrete crushing the blanket, I don't think I'd be too worried about it. I think the blankets will still insulate effectively with a little compression. The perlcrete mixture isn't that heavy.

[kebwi]

Re: adobe+vermiculita insulation

Ah, okay. In another thread there was some dancing around the issue of which is better, low density or higher.

Incidentally, why does this not jive with the thermal conductivities, which are lower for lower density blankets? My understanding is that lower thermal conductivity would imply lower transmission of heat, i.e., better insulation. I accept that the intuitive rationalization is incorrect, but I still don't understand the error in the logic.

Thanks.

[papavino]

Re: adobe+vermiculita insulation

Where are you finding the thermal conductivity values?

[kebwi]

Re: adobe+vermiculita insulation

Oh, I don't even remember. Most of the websites for the *manufacturers* of these materials (not necessarily the suppliers, who seem borderline clueless in some cases) either have data specs posted online or PDFs you can download. I honestly don't have them bookmarked, I've been skimming and skipping mostly.

[JoeyVelderrain]

Re: adobe+vermiculita insulation

8# blanket is going to insulate better than 6# and 4#. The higher the density of fibers, the more little air pockets and tortuous the air path will be. It holds the heat back more efficiently than a less dense mesh of fibers.

yeah, this makes total sense to me.
thanks!

[voon]

Re: adobe+vermiculita insulation

When you say it works fine, dmun, do you mean that if you use the described method (drill mix, then add vermiculite last) then up to 10:1 can replace the standard 5:1 even for structural support, like the hearth...or would you not recommend going over 5:1 for the hearth and only using higher ratios for the dome?

I'm a new member, and no expert on oven building or masonry, but my understanding from reading the free Pompeii oven plans on this site is that vermiculite or perlite are not added to the concrete that forms the structural support for your oven. You make a form on top of your stand and pour normal concrete with 1/2 inch rebar inside for the structural support layer. The insulating layer can be FB board or vermiculite/perlite cement mix that is poured on top of the structural layer. Then you build the floor and dome on top of that.

Again, I'm no authority, but I don't believe that a vermiculite or perlite cement mix is ever intended to be "structural", especially underneath the oven. It's for insulation.

[dmun]

Re: adobe+vermiculita insulation

Again, I'm no authority, but I don't believe that a vermiculite or perlite cement mix is ever intended to be "structural", especially underneath the oven. It's for insulation.

This is exactly right. Insulation layers have high compressive strength, which means you can pile a lot of weight on top of them without their squishing, but no tensile strength which means you need to support this layer from below.