Insulation vs. Wall thickness

[das]

I am looking to build a 36" Pompeii oven, and I am curious as to whether I could help with the heat time by cutting the fire bricks to say ~3.5" instead of the common 4.5" (halves) but then bulk up the exterior insulation to 4 or 4.5" and still get maintain the heat retention properties for next day roasting.

Also do you guys think upping the floor insulation to 3 or 4" of ceramic board would be beneficial. Seems many have had issue getting the floor up to temp in under 90min. going with the standard 2" or vermicrete base.

Does anybody have experience with the FB Premio which uses a 3" Ceramic board base, 3" refractory dome, and 4" of insulation. Curios how much of a difference the added insulation makes and if the Pompeii's can benefit equally?

Lastly, how does the insulation FB sells compare to Unifrax Duraboard and Durablanket, as well as other options from Thermal Ceramics, is the FB stuff imported from China? From looking around it seems there are more expensive as well as cheaper options, which is the best stuff for the money, without compromising long term stability etc.? If I can use cheaper stuff them adding extra insulation would not cost much more.

Thanks for the help.

[Mike D]

Re: Insulation vs. Wall thickness

I am still building my oven, but I did double my floor insulation (2" board and 3.5" of vermicucrete). I don't know what a 3.5 thick dome would do. I would say that you can never have too much insulation.

Mike

[das]

Re: Insulation vs. Wall thickness

Thanks for the reply Mike, your build is looking great, did you buy the kit or source locally?

I called FB this afternoon and they said the amount provided is optimized, and any additional is overkill. Yet, what I find interesting is that on the Premio, which uses the same 2.5" firebrick floor tiles as the other modular ovens and pompeii kits, comes with 3" base insulation.

[casper]

Re: Insulation vs. Wall thickness

Thanks Das for your perceptive post. I agree with Mike, never too much insulation so how much room do you have? Leave 6" and fill with loose vermiculite or wrap in Ceramic blanket to 3" but insulation is exponential so doubling thickness will halve the RATE of heat transmission. The ceramic blanket at my local refractory construction firm comes in 1"thick rolls. The box is ?70 but enough to wrap three times. The board they use is good for under the floor brick, takes compression and 1000+degC. It's dear, ?50/m for 2" but I will blag offcuts and use sand to fill the gaps. They have 1 and 2" only so I will sandwich.

My first mark 1 oven never got hot floor so yes this is vital to insulate well, 3" of the proper silicate board. There are insulating damp proof magnesium silicate boards suitable from builders merchants too but there is no cost saving.. I am using a sheet of this as my base layer under the insul because I'm building on a slab laid directly on the ground ( cut into the hillside).

[casper]

Re: Insulation vs. Wall thickness

Can you help me decide on the refractory brick thickness? Earlier plans at fb use the halfbrick but I note the cast ovens are 2". I read somewhere they revised the advice and suggest turning the brick sideways to nimimise the thickness fromp 4.5" to 3 or less where possible.

As I understand it the ref brick is a good conductor but holds thermal energy. So the question must be how long do I want it to hold that heat? Pizza, suckling pig/turkey followed by stews and bread? In theory the lower thermal mass heats up much quicker but cools down too quickly.
I suppose it's like choosing between gas or charcoal bbq