Ben's 39

[captkingdom]

Hi everybody,
After my year of lurking, I am well underway.
I plan to post pictures soon but as I am taking this week off to work on my oven I have a couple questions.

1. As you may know I have determined that Roxul rockboard 80 is NOT a good sub-hearth insulation board. I am trying to do this oven on the cheep and need a good alternative. I am planing on using the Insblock 19 (locally available and cheaper though probably inferior to FB Board) for the top layer but would like 4 more inches of insulation below. For this I propose to use polyisocyanurate heavy duty, which has a compression rating of 100 psi and operating temp limit of 250F. From my reading here it seems to be a bit of a consensus that under the first layer of higher tech insulation temps should not really exceed 160, right? I had considered the vermiculite mix but its insulative properties are quite a bit lower than polyiso and other products. So what do you all think?
2. On morter, I was intrigued by using the Fire clay Calcium aluminate mix as it is more genuinely refractive, and I am not to intimidated by its difficulty, having experience with rapidly setting products, but I have not found a local source. I like the fact that its strength is in large part catalytic as opposed to an air dry pre-mix that will not strengthen until heated. The portland mix stumps me because I thought that regular cement did not hold up in high heat and I could not understand why replacing aggregate with fire clay obviated that problem. I turn to the great amassed wisdom of the group for a recommendation on morter. If I go with the pre-mixed how much am I likely to use? I will not be doing nearly as much cutting to fit as many of the other fine ovens builders have done. Is the ease of premixed worth the extra cost?

[captkingdom]

FYI you can vote for an insulation option and a morter option

[dmun]

I have two and a half inches of insblock19 under my cooking floor, and my support slab hardly gets warm during cooking. You may not need much more than insblock19 layer, if anything. It's not an inferior insulation: it's made by Harbison-Walker, and used widely in pottery kilns.

As for mortar, I use and recommend Heat-stop. Sure it's expensive, but it spreads like peanut butter, and dries rock hard and strong. If I had to go cheap, I'd look into one of the waterglass-flyash (Mortar choice issue - need help to decide) blends.

Your poll mentions pre-mixed mortar. If that means the wet stuff in buckets, that's a proven problem. Most of those aren't waterproof, and they take forever to dry.

[wlively]

I used Inslblok 19, would not hesitate to use it again. It sits on 3.5 inches of vermiculite concrete and my oven holds temp very well.

I used Heatstop 50 and would use it again. Would have loved to try refmix but could not get that in my budget. I was very liberal with it, as I got it at $32 a bag and used 3 bags. if you are going with the tight exact cuts I would think you could get by with 1 bag and definitely not use more than 2.

[captkingdom]

Thanks.
Here's another question:
I found a source for treated perlite, but have not found Vermiculite yet. Is there any reason that I could not set my insblock on top of a 4" layer of lose perlite?
I am pouring my concrete slab with a recess for insulation so that would hold it in.
Thoughts?

[dmun]

Is there any reason that I could not set me insblock on top of a 4" layer of lose pearlite?

Have you seen perlite? It's a bag of little white fluffy balls. It makes a fine insulating concrete, and a loose fill on top of your oven, but to build on loose perlite would be like building on shifting sands.

[captkingdom]

Sounds like one strong vote no.
Actually I am a bit familiar with it, I just thought that when contained it would consolidate and support.

[Lars]

You would definitely want to mix w/portland.

Vermiculite, Menard's loose insulation dept. 9.99/3 cu. ft.

Lars

[christo]

I like the FB board I used under my floor. After long periods the underside of the oven gets barely warm and I did not have to build forms or mix vermicrete and level it.

I scrounged much of my materials and I splurged for the FB Board. I got hooked by one of James' posts about a solid state oven.

If my budget was tight, I would not hesitate to use the vermicrete. Try not to get freaked out when it comes out feeling like cork, from what I can tell that means you did it right.

I cannot give an opinion on mortar as I used the FB and Heatstop 50 on my build.

Christo

[captkingdom]

So as I mentioned I found a source for treated perlite but no Vermiculite.
How do they compare for insulation value? Is it close enough to call it good and get the perlite or should I keep searching for vermiculite? Anyone know conductivity ratings?