How much perlite for dome insulation

[Give_Me_Spices]

Hi all,
I have purchased 2 in ceramic blanket to wrap up my dome with. How thick of a perlite layer do I need to put on top of it? Is 2in enough or should I go 4in?


Thanks,
GMS

[Gulf]

Three inches of ceramic fiber blanket is optimum. But, you will be ok with another three inches of perlcrete imo.

[Give_Me_Spices]

Thank you. I have read here a lot of different ratios of cement to perlite and I'm not sure which one to go with. Also is the ratio by volume or by weight?

[UtahBeehiver]

It is by volume. FYI, different ratios apply to where it is being used, ie floor or dome.

[Petter]

The 2" additional perlcrete will reduce the dome thermal conductivity to about 75% of its original value. Since the floor insulation is unchanged, I would estimate the oven thermal conductivity to be in the 85% range of the original. Worth it or not is your call.

[Give_Me_Spices]

UtahBH, what an excellent data table. I don't have an idea about the workability of any of those mixes, in an earlier post you had suggested to me to use a 8-10 to 1 ratio for the dome. I assume that 7-1 is a workable mix as most folks here seem to mention it.

Petter, a very good point. Meticulousness goes hand in hand with overengineering. Realistically both options will be fine, although to me 15-25% improvement is a significant change and it's worthwhile. There are so many great approaches proposed in this community, people here go over and beyond to ensure optimum functionality and us average bears are left with the task of parsing out the necessary work from the triple-over-ensured space tech that will allow you to make your pizzas under 3 ft of water and still produce an excellent curst.

Cheers,
GMS

[david s]

Even a 10:1 mix contains more mass from the cement proportion than the vermiculite or perlite. Anything leaner makes placement more difficult. A little trick that works well is to add a handful of powdered clay for every litre of cement added. This greatly enhances the workability and placability of the mix- The thicker the layer the longer drying is required. I find a layer of around 35 mm, allowed to dry for a week before applying a second layer and allowing a further week to dry, is faster than waiting for a thicker single layer to dry. Apart from adding additional insulation the requirements of the layer are to restore the hemispherical form and to provide a firm enough substrate to apply a render/stucco onto. I find just one single ~ 35mm layer sufficient. Correct water addition is also vital. If using medium grade vermiculite or perlite (~6mm grain size) it is 3 litres for every 10 of perlite or vermiculite if using a finer grade (~3mm grain size) it requires 3-5 litres water/10 litres vermiculite or perlite. This will make a more workable mix, but take longer to dry because of the higher water content and tighter packed mix. For these reasons I opt for the medium grade. Wear gloves for placement, or you’ll regret it, mix in a barrow not a mixer, it degrades the grains. I find a 50/50 mix of vermiculite/perlite creates a better mix than either alone. So a 10:1 mix would be 5 parts perlite, 5 parts vermiculite, 1 part cement, powdered clay, 3 parts water. Mix dry ingredients well before adding 1/3 of the water, mix well add 2nd 1/3 water, mix well again, knocking down any lumpy bits with the back of the spade. Then finally adding last 1/3 of water. Slightly too much water will pool in the bottom of the barrow tending to wash the cement off the grains, This can be balanced by adding a little more dry material in approx same recipe proportions.Hope this helps. After 10 years of this method I have mixed literally tons of this stuff, works for me.

[Give_Me_Spices]

Thank you david s for this detailed response. This is exactly what I needed.