insulation tips

[garvi]

hi all,

I would like your opinion about the insulation of the oven.

Given that the problems that arise when you do not insulate well are significant (uneven cooking, fall fast temperature lines, etc.) I was wondering what techniques to use:



For example, I've built a oven, using it to insulate the stove a mixture of vermiculite and refractory cement. But I read about a hob isolated with a mixture of sand and salt.



What do you think. What impact on the implementation of a good cooking surface.

Thank you for your answers

[matthewcove]

This is definitely a complex topic. The behaviour of the oven depends on a number of factors, including the amount of thermal mass, insulation and how you fire the oven. For example, if I want to have a stable temperature for a number of bakes, I can heat the oven to hotter than I require, leave to equilibrate overnight so that the hearth is about 380 and the dome a little hotter. I can then leave the oven open a little, allowing cool air to circulate, cooling the inside surface of the oven to baking temperature of 300. Deeper in the bricks the temperature is still up at 380 and acting as a heat supply to the inner surface. like this I can bake 3 or four loads of bread without the oven temperature declining at all. With an oven with only limited insulation, you could use this technique to stabilize the temperature for a time.

Clay ovens here in Poland are insulated with a type of straw in clay mix and provide a good working temperature for about an hour on a single firing.

Matthew

[tonyb]

The don't necessarily agree that lack of insulation per se is a problem, it all depends on what and how you are cooking. If you are only using the wfo occasionally for pizza, then you can get away without any insulation, (though I wouldn't recommend it), the 2 issues you mention are not relevant to this style of cooking (ie with a fire). However if you want to bake multiple batches of bread then lots of thermal mass and insulation are essential to be successful at retained heat cooking.

The key question wfo builders need to be clear about is what they intend to cook which should then drive the design.

[dannii]

I am currently building a pompeii style WFO using entirely firebricks and fire cement, what insulation would you recommend I use in addition to this? I'm planning to render and paint the finished dome, to my understanding you shouldn't use thermal blanket and render on top as you will squash it so it doesn't work adequately.

Dannii

[dirtlover2005]

Dannii

Most go with thermal blanket covered with chicken wire then a layer vermicrete (vermiculite mixed with cement at up to 10:1) then render. If you are going solely for blanket just use a smaller gauge mesh. It boils down to how much room/ money you have plus as above what you want to use the oven for.
I would aim for 50mm blanket with 100mm vermicrete - but good ovens have been built with more and a lot less!!

[dannii]

Thanks so much for your reply, so the thermal blanket will be adequately protected by the chicken wire and its easy enough to render on top of? does this work out to be a more effective\cheaper method than just a large layer or vermiculite/concrete mix? 


Dannii

[garvi]

Thank you very much!

[cobblerdave]

G'day
with modern product for insulation, if you don't use them your doing you project a disservice. With Cal Sil and Ceramic insulation sometimes hard to get and a bit expensive, most people can access and afford either vermiculite or pearlite mixed with Portland cement it makes an excellent hi temperature insulation. Pearlite/vermiculite cement mix is roughly 1/2 the insulation value of the expensive stuff. That's to say 4ins of 'pearl-crete' will have the same insulation value as 2 ins of ceramic fibre.
A box of Ceramic blanket seems to work out about 2 ins of insulation over a 42 in oven, its stiff and the results can be a bit lumpy so it seems that a lot of folk follow up with a pearl-crete layer, using it to smooth things out, and add some more insulation without having to buy another box of ceramic. Ceramic insulation is stiff and can be rendered over without the pearl-crete layer. My own oven has 2 ins of ceramic and 4 ins of rockwool over that then straight render.

Regards Dave

[edinburghman]

Interesting points on here; I am considering a Pompeii oven built with firebrick, then a layer of vermiculite as a render, then thermal blanket, more vermiculite and k rend to finish.

For me insulation is very important - I want the dome to not got to hot on its external surface, and I want the oven to stay hot.

Thoughts?

[tonyb]

I have a 50mm thermal blanket covering the firebrick dome then about 75mm of vermicrete and a render coat and the dome is barely warm on the outside. I'm not sure why you want 2 separate layers of vermicrete, rather than a single one though I don't see any material disadvantage, its funny stuff to work with and not easy to get a thin layer.

[chas]

Jul 18, 2014 22:19:18 GMT tonyb said:

The don't necessarily agree that lack of insulation per se is a problem, it all depends on what and how you are cooking. If you are only using the wfo occasionally for pizza, then you can get away without any insulation, (though I wouldn't recommend it), the 2 issues you mention are not relevant to this style of cooking (ie with a fire). However if you want to bake multiple batches of bread then lots of thermal mass and insulation are essential to be successful at retained heat cooking.

The key question wfo builders need to be clear about is what they intend to cook which should then drive the design.

Encouraged by this post while still in some turmoil, I thought I'd ask about my build, and more particularly, its insulation:

dunno what its 'style' is - sort of cross between barrel vault and pompeii - it's a 'dome behind a front brickwork fascia' type made from old red half-bricks set in clay for the dome over a starter ring of 2 courses of lime-mortar brickwork. I'll post pics as it progresses, unless it's already well documented here somewhere.

This is a style promoted by a course arranged by Essex Council I went to in 2010 - I've held off building my own due to lack of a final decision on siting and the finding of an old oven door, now bought.

We only plan to cook focaccia and pizza in there, but will, no doubt extend our cooking ambition.

So, the insulation questions:

1) I was planning to make the floor from 2" pamments bedded in lime mortar over the concrete slabs that form the platform base as I was shown, but I've seen suggestions to more massively insulate the floor. Is that for the benefit of the baking - or to protect the slabs - and if dry sand is used, how much is enough?

2) the basic soft brick dome is 4.5". With a bit of clay tidying and a lime-mortar render, say 5" tops. I don't really want it massively thick, but understand the usefulness of insulation. As I plan to make a protective heavy canvas 'beanie' cover for it when not in use, I have the idea that if I made it double skinned lined with insulation it could act as a sort of tea cosy... but how hot does the outside of a 5" dome get in use?


Again sorry if this is covered elsewhere, but I couldn't find it in 'search'

Cheers, Chas.

[cobblerdave]

G'day

www.fornobravo.com/forum/f51/bacteriums-2nd-build-17614.html#post130292

Attached is a link by an FB member who has built an oven before with an un insulated hearth floor.

Regards dave

[albacore]

I'm not an expert, but I'd say you definitely need underfloor insulation. I'd go for 40 - 50mm of calcium silicate board - not cheap, but worth it in the long run. Then the usual on top, ie 50mm fibre blanket and about the same of vermicrete.
I went with thermalite blocks underneath and now wished I'd used calsil.

[cobblerdave]

G'day
My own oven has 50 mm of thermalite block our local name hebel, as underfloor insulation. This is what I had free so I used it . 4 years on its still going strong . My oven appears to have basically the same performance as other forno type ovens.
As cement based products are effected by heat over 300C I would always recommend to protect such cement block products from the worst heat by a layer other insulation .... Just in case.
Regards dave

[chas]

Dave, Lance, thanks for the info, and that seems like the floor mod is sorted: instead of pamments directly set onto the concrete slab platform, I'll inset the pamments within a circle of bricks on edge, level with the bricks and filling the 2.5" void with an insulating/isolating fill of vermiculite lime mortar - better than nothing and costs me nothing. I love to build from stock and can't justify a biggish spend. I have got the best part of a bag of vermiculite somewhere...

That just leaves the likely temperature at the outside of the brick dome: will it set fire to my canvas cover if it's lined with fire blankets (have a few of those tucked away somewhere too)?

Cheers, Chas