Pumice and Rockwool as insulation on a homebrew cast?

[hughjamton]

I'm waiting for materials to be delivered to start my base with the intention of building a 800mm homebrew cast oven.
Like everybody I have questions.
I have to build it on a budget, so it may take a while to source the materials at the right price.
I have, very kindly, by a member on here, been gifted 120 kgs of pumice stones that I intend to use as a large part of the insulation along with rockwall.
My intention is to give the cast dome a coating, maybe 40mm of pumice, then 75mm of rockwall, then more pumice with a rendered topcoat.
david s in a previous posting you recommended that the pumice particles should be no larger than 6mm, can I ask, is that for superior insulation or ease of use?
The stuff I have is stones, 20/30mm with rounded edges, if I have to crush it to less than 6mm I'm going to end up with mostly sand. Do you think it would be ok to crush it a just a bit, A, to give it sharper edges so it "interlocks" better and B, to make it a bit easier to work with.
You also said that you had used rockwall as an insulation once but it was difficult to use as it crushes too easy.
My plan is to use half inch weldmesh over the rockwall instead of chicken wire, stiffened with some strategically placed stainless 5mm helibar. Not sure if it's called weldmesh in Oz? Square galvanised welded mesh, I think 19g?, much stiffer than chicken wire, I use it on my coops, a fox will bite through chicken wire.
I'm going to use thermolite blocks for base insulation, I could make room for 30mm or so of pumice as we if you think it would help, under or over the thermolites?
What do you think?

[UtahBeehiver]

Thermolite alone does not have the temperature rating to go directly under the fire brick. Another UK builder is using this material but place a sheet of CaSi board between Thermolite and firebrick.

Rockwool is not good choice for dome insulation. Refer to postings by David S who has tried it in the past.

[hughjamton]

Thanks UtahBeehiver, Casi board is out of the question for me due to cost. On a uk forum lots of people report using thermalites directly under the fire bricks, apparently without problems, however, as I said in my post I will have room to put 30mm or so of pumice between them and the floor, do you think this will be of help?
I think Dave S reported that the rockwool worked but was difficult to use, hopefully he'll confirm one way or another.

[david s]

20/30mm pumice will be very difficult to apply if used as an aggregate in a wet mix with cement. Crushed up a bit will work much better. Reducing the pumice aggregate size will not change its insulation value.
1/2" weld mesh will be very difficult to bend over a dome form, but maybe not so bad if you are building a half barrel.I don't use any chickenwire over the blanket as it is time consuming to apply and I see little point in having a highly conductive material like steel in the middle of an insulation layer. If you cover the blanket with a lean vermicrete mix which is very weak a chicken wire or weld mesh layer will do little to strengthen it.
Rockwool is a cheaper option than CF blanket for over the dome insulation, being slightly less insulate but a bit cheaper. After getting some and using it on two ovens I wouldn't use it again as the small gain in cost was outweighed by the physical qualities of the stuff. It's much harder to cut and apply, floppy and springy, making it difficult to render the vermicrete layer onto. I also used foil backed roof insulation on my first oven using free waste offcuts from a building site scavenging the stuff from a skip, surprising gems to be had for the right price there. It worked ok, but same problems as with the Rockwool with the added problem of excessive compression with any weight applied against or on top of the stuff.

[hughjamton]

Thanks David, I can get rockwool for free and I'm going to try and use weldmesh to stop it compressing too much rather than for strength. I do expect it to be awkward to handle.
What do you think about this, a fair bit down the line yet, but, when it comes to the moment of the first small fires, using a small de-humidifier first?
I know it will draw water inwards which is the opposite of a fire, but I think initially it would remove a fair amount of water.

[david s]

Free is a great price, go for it. The difficulty I had with the rockwool was mostly on the sides of the somewhere you are asking the lean vermicrete to stand vertically on its own. I suggest you go about 6” high and leave it to set for 24 hrs then go higher. Once it starts to slope in it is much easier to apply. If the brew is pretty lean (10:1) it won’t compress the rockwool much if made around an inch and a half thick.
Not sure how easy it will be to get your weld mesh to conform to a compound curve.Be sure to allow the vermicrete to dry well before driving the water out by fire slowly, then do the outer rendered shell.

[hughjamton]

what do you think about my idea of using a de-humidifier in the dome?

[david s]

I think it will dehumidify the chamber, but take forever to dry the refractory and the insulation beyond it. Moisture moves away from heat so gentle fires would be far more effective than dehumidifying. Having said that I also maybe wrong. We had a flood in our house from a burst flexihose and they use 4 big dehumidifiers going for two days to dry things out. They did a pretty good job.

[hughjamton]

My idea is that it would take the moisture out of the dome whilst the pumice is starting to dry naturally on the outside for a week or so before I started any fires, I thought there would be less chance of the cracking?
Also I have a thick metal tray from an old microwave that I was thinking of propping up on bricks with the initial fires beneath to deflect concentrated heat from the top of the dome.

[david s]

That’s a pretty good idea although it may make keeping the fire going more difficult. When the oven is wet it will smoke and sometimes go out. The tray might make it worse, but worth a shot.