Insulating dome

[fluxia]

Hello

I have nearly finished to build my first oven but I am unsure on how to insulate the dome. I am going to use vermiculate mixed with cement but I was thinking to put a layer of loft insulation held by a chicken mesh and then pour the verm - cement mix on top of it, and then finish it with a layer of cement.

Should I apply the mix directly on the thermal mass instead? (considering that the averall thicknees can be a max 20 - 22 cm)

I am also unsure if it's best curing it before I insulate it (so the moist evaporate easily and if there are scrack I can do something about them) or after having insulated the dome? (with less risk of cracks)

Is there any advantage in adding foil?


Thanks for your help!!

Andrea

[peterlyn]

Re: Insulating dome

Hi, London (I'm originally from Stanmore; hubby from B'ham),

I have some details posted on our thread that you might find useful:
ttp://www.fornobravo.com/forum/f8/pompeii-nanoose-bay-14633.html

(in fact, that section of the forum is probably a better place to post. I think this section refers to managing fires for curing and cooking. But I could be wrong!)

Our build was 'wet' too but you can continue to insulate. You really want to lessen the amount of rain getting to the oven anyway. I know my husband was very determined to get the structure waterproof ASAP.

As with us, the curing will be crucial to get rid of all the moisture. Slow, low and long () will be the best way to dry the oven with the least amount of cracking. And don't forget, hairline cracks should not be a worry at all and are almost impossible to avoid.

Good luck!

Karen

[dmun]

Re: Insulating dome

Is there any advantage in adding foil?

No foil! It's useless. It rots out in contact with the caustic portland cement, but before it does it creates a vapor barrier that prevents you from drying out your oven. Stomp out the foil meme.

loft insulation held by a chicken mesh and then pour the verm - cement mix on top of it, and then finish it with a layer of cement.

If "loft insulation" is anything like our domestic fiberglass insulation you don't want it anywhere near your dome. It has an organic binder that will burn at oven temperatures. It will smoke and stink. If you want a layer of insulation under your vermiculite concrete, you should get a refractory insulation blanket. Plain vermicrete is fine, though, if you use enough of it.

I am also unsure if it's best curing it before I insulate it (so the moist evaporate easily and if there are scrack I can do something about them) or after having insulated the dome? (with less risk of cracks)

We've never come to unity on this issue. People do it both ways, for the reasons you state. If you are insulating with vermiculite concrete, you will need to cure again anyway to drive off the large amount of water in the mix, so my opinion is to cure both at once.

[Neil2]

Re: Insulating dome

"People do it both ways, for the reasons you state."

Agree with Dmun. You can do it either way and achieve the same results.

[fluxia]

Re: Insulating dome

Thank you for your replies,

unfortunately I have now already applied the insulation blanket and covered it with vermicrete. On the pack it says it's mineral wool. It's a roll that I bought from "B and Q". Do you recommend to remove the all thing before it dries properly ( I applied the vermicrete yesterday) or should I try and see if it smells when I fire the oven up?

Andrea

[fluxia]

Re: Insulating dome

well.... I had to take the drastic decision to remove the blanket: I checked it online and it did turn out it is made of fiberglass with a binding agent which will decompose at 200 degrees C. So thanks for the your advise. Fortunately I was able to remove it in less than 15 minutes as the vermicrete was not properly dry yet.

I now have to start the isulating process from the begining. I am considering to use a ceramic fiber blanket or to apply vermicrete straight on the thermal mass. Do you think 4 inches of vermicrete is sufficient if I decide for the second option?

Is anyone aware of any health issues about ceramic fiber? I have notice from a seller on ebay that it comes with a warning on the box saying that the banket is Carcanogenic. I wonder if this can be an issues in case there will be cracks in the oven in future.

[dmun]

Re: Insulating dome

I have notice from a seller on ebay that it comes with a warning on the box saying that the banket is Carcanogenic.

None of this stuff is any good for you. Once it's encapsulated in masonry the mineral fiber insulation is harmless. None of it can get out of the hairline cracks you are likely to get.

Cutting and installing it is another matter. Protect your lungs. Use a real respirator, not one of those ten cent paper masks. This is true of all masonry materials, btw. Even ordinary concrete dust can cause silicosis years down the road.

well.... I had to take the drastic decision to remove the blanket: I checked it online and it did turn out it is made of fiberglass with a binding agent which will decompose at 200 degrees C. So thanks for the your advise. Fortunately I was able to remove it in less than 15 minutes as the vermicrete was not properly dry yet.

You will not regret this decision.

[fluxia]

Re: Insulating dome

Thanks Dmun. I must admit I made many mistakes... This is the first time I attempt to build something with bricks that is bigger than a barbeque, so I am not surprised. Initially I put too much thermal mass. Than I removed the cladding in excess as soon as I realized it was a mistake. I also didn't put insulation under the oven floor (however, I have not cemented the bricks so I can remove them - at least the ones in the central part of the inner surfice - to insert some sort of insulation later on if needed). My oven is built (see picture) with non-refractory bricks, as I had about 300 good solid and heavy bricks and firebricks were too expensive for my budget. I used fireclay with a bit of cement to do the cladding on the dome. Now thanks to your advice I have fortunately repaired my latest mistake by removing the fiberglass insulation blanket and vermicrete.

I hope I am going to get the next step right! - I bet my neighbours are having a laugh - seeing that oven building up during the day and unbuilding itself at night ....

I decided to forget about the blanket and to apply a coat of about 5" or 6" of vermicrete mix (5:1 vermiculite to cement).

I also understand - see previous post from Karen - that it's important to make the dome waterproof as soon as possible (once it has been cured). Any suggestion on this? I live in rainy England so I suppose it may be fairly important to protect it from the rain ....

[peterlyn]

Re: Insulating dome

Fluxia, you can continue your build while you are curing; drying out the total build may take weeks, if not months, in the winter. Our structure is pretty much waterproof but we have many more curing fires to light. Until we got the stucco/plaster coats on, we used a tarp to keep much of the rain off.
One of the reasons Dave wanted it waterproof early on was because the fire-clay mortar we used would not dry, even re-emulsifying in high humidity.

Karen

[fluxia]

Re: Insulating dome

HI
I have did the big fire today and it seeems it's going well, fire is still burning now it went all white inside and no cracks as yet...
thanks for your support and I ll keep you posted

[fluxia]

Re: Insulating dome

haa!!!! here we go.... I have got a crack !, strangelyenough I had made a big fire on Saturday night and all went well, so yesterday I did it again but I may have exagerated. the result is a crack on the centre of the dome with some smoke coming out of it and another crack on the side which seems less concerning.... I am going to post more details in tool, tip and tecniques which it's probably a more appropriate forum to post.