Base insulation

[jakew]

Hi guys,

So glad to be part of this forum. Just started my first build. I’m building a dome shaped pizza oven. Approx 1m diameter. I’ve got a base made from solid concrete blocks, laid flat supported by steel for the base of my oven.
I was going to lay 100mm of vermiculite mix ontop of the base as the insulted base to lay my fire bricks on top of. Will this provide enough insulation?
Do you lay the walls of the oven ontop of the 100mm insulted base or around the edge?
Any advice welcome. Thanks

[oblertone]

100mm of vermicrete will provide sufficient insulation but you'll have to let it dry out first and it's not too strong in compression. An alternative would be thermolite building blocks laid on their sides over an area big enough to let your dome insulation come into contact with the base insulation.

Your oven floor needs to be at the elbow height of the main user otherwise you'll be constantly stooping; work backwards from this point to determine how high your base should be.

[downunderdave]

Jun 7, 2020 13:31:23 GMT oblertone said:

100mm of vermicrete will provide sufficient insulation but you'll have to let it dry out first and it's not too strong in compression. An alternative would be thermolite building blocks laid on their sides over an area big enough to let your dome insulation come into contact with the base insulation.

Your oven floor needs to be at the elbow height of the main user otherwise you'll be constantly stooping; work backwards from this point to determine how high your base should be.

If casting or pouring your own vermicrete mix for underfloor insulation it needs to be strong enough to adequately support that which sits on top. If made stronger the insulation value is reduced. The recommended mix is a 1:5 vermicrete (see attached table) For over the dome a much leaner brew can be used which has higher insulation value. 1:10 is about as lean as you can make it while still having adequate workability and resulting strength to act as a firm substrate to take a rendered layer over the top.

[jakew]

Perfect, thanks for the reply guys. I’m all excited as my firebricks, clay etc...is all due to be delivered tomorrow and can crack on with the build. Just out of interest when you say it needs to dry out. How long would you suggest?

Thanks

[woodywun]

I've just contacted Forterra who manufacture Thermalite blocks and they told me that they do not recommend them being used on their flats for loading reasons.  I can't really see this being an issue with having just a cooking floor/dome on top of it but it's something to bear in mind.  The blocks would certainly need carefully bedding in something. 

The other thing that they said was that they do not recommend them being located next to strong source of heat due to potential cracking but I guess it's 'buyer beware'.  Thermalite has a better heat conductivity value (U value) than Vermiculite concrete and the blocks are bargain basement as far as costs go. 

I've just put in my sub-cooking floor insulation and I've opted for Vermiculite cement, which is in the process of drying now.

Has anyone had any experience of using Thermalite blocks located under the cooking floor, recently?

[jakew]

I’ve got solid concrete blocks as my subfloor. They will provide some insulation as well as a solid base. They’re 7n blocks and suitable for block and beam floors so I’m sure a pizza oven not going to kill them. I’m going for vermiculite mix as the floor insulation. How long did you let it dry for?

[samuraidave]

Hi there, my build has thermalite blocks on their sides under my fire brick floor. It is 9 years old and I have noticed no problems. I also used the blocks as a thermal break between the vault and chimney and they are also OK. I used them as a door also and yes they did crack after about 5 years but they still held together OK a d I think that as long as they have no load bearing duties they should be fine. Good luck and pop dome pics on if you can.

Oh I forgot that I also used thermalite blocks in the chimney which is also holding up well.

Dave

[lorettaf]

I think i’m going to go with thermalite blocks on their sides for the base insulation. Bedded in sand, over rubble plinth filler. There seems to be divided opinion on here and another forum as to whether there should be something else between the thermalite blocks and the firebrick hearth. Some are suggesting still using CaSi board, which defeats my cost conscious build.... i was going to bed the firebricks on a little sand to get them flat. i can use a thicker layer of sand but is that more of a conductor than insulator?
It’s going to be a clay/ perlcrete/ cob oven. So i can put a layer of cob between the firebricks and Thermalite, or perlcrete. However if i do that i feel i might as well cast a 100mm perlcrete base which i was trying to avoid due to drying time/ cost. I can be persuaded....
Thanks for your help.

[nilly71]

I cut the thermolite blocks lengthways to give 4.5" insulation.

My build ukwoodfiredovenforum.proboards.com/thread/3477/pizza-smoker-grill-essex-london

[lorettaf]

Fantastic build and great photos - thanks