Advice on thermal mass / insulation confusion, please

[simonj]

Hi, 

I'm about to take the plunge and buy a modular oven kit. Although I'd love a beautiful brick dome, I'm keeping my eye on the prize of cooking so going for the quicker option.
(Probably from everyone's favourite at Kiln Linings, although the veneto model is substantially bigger than I thought I wanted.) 

I'm focussed in baking and slow cooking.

But I'm pretty confused about some elements of the construction of these ovens and of the use of insulation and thermal mass (terminology?).

In my mind, long heat release means a requirement for a large amount of thermal mass. Soak up tons of heat, then release back into the oven cavity.

The kit will come with the cast dome, firebricks for laying underneath with depth 25mm, an insulation board under that, insulation blanket and render.

Is 25mm really enough underneath, should I buy deeper blocks or more than one layer?

On top, i see builds on here with loads of mass loaded outside of the layer of insulation blanket (in the built-in structure). Can someone explain to me the purpose of putting thermal mass outside of the insulation layer? If the heat doesn't get there, there will be nothing for it to store, and it certainly won't subsequently release it back into the oven.

Perhaps I've got completely the wrong end of the stick.

But why is the mantra not mass, mass, mass, more mass, then finally insulation if necessary to protect the outer finish materials and hands touching the outer?

Thanks, I appreciate any help.
I'm in a bit of a rush to make my decisions because I currently have builders in and they want to know dimensions for the 'cooking station' so they can complete laying paving, raised beds etc.

[chas]

Apr 16, 2015 9:05:53 GMT simonj said:

Hi, 

I'm about to take the plunge and buy a modular oven kit. Although I'd love a beautiful brick dome, I'm keeping my eye on the prize of cooking so going for the quicker option.
(Probably from everyone's favourite at Kiln Linings, although the veneto model is substantially bigger than I thought I wanted.) 

I'm focussed in baking and slow cooking.



Is 25mm really enough underneath, should I buy deeper blocks or more than one layer?

On top, i see builds on here with loads of mass loaded outside of the layer of insulation blanket (in the built-in structure). Can someone explain to me the purpose of putting thermal mass outside of the insulation layer? If the heat doesn't get there, there will be nothing for it to store, and it certainly won't subsequently release it back into the oven.

Perhaps I've got completely the wrong end of the stick.

But why is the mantra not mass, mass, mass, more mass, then finally insulation if necessary to protect the outer finish materials and hands touching the outer?

Thanks, I appreciate any help.
I'm in a bit of a rush to make my decisions because I currently have builders in and they want to know dimensions for the 'cooking station' so they can complete laying paving, raised beds etc.

Hi Simon, can't help much with the floor insulation spec but as fas as the dome's concerned, the 'mantra' has some merit in self-builds:  I'd say increase the mass first and insulate after if slow cooking is an ambition. I suspect if a pre-cast dome shell is thin/light enough to transport and then becomes unbearably hot to the touch, it ain't thick enough.

You say "In my mind, long heat release means a requirement for a large amount of thermal mass. Soak up tons of heat, then release back into the oven cavity." Bang-on, but you don't want it leaking away.

As an example, the outside of my 4.5" brick dome doesn't get too hot to touch (suggesting the mass might just be sufficient)  but I've come to the conclusion I still need to insulate it as the whole thing cools down too quickly for slow cooking, or indeed for multiple pizza baking.

All that said, Kiln Linings are no slouches - if they say "HAS BEEN SPECIALLY PRODUCED FOR THIS APPLICATION AS IT KEEPS THE HEAT INSIDE THE BLOCKS & COOKING AREA" I guess we must believe them...if it keeps all the heat inside, it won't get hot outside, will it?

Chas

[simonj]

Chas,

Thanks for the response.

It occurred to me that heat stored must be thermal mass times temperature differential.
So having insulation on the outside of the cast refractory allows that layer to get hotter rather than losing its heat to the outside or to the next layer of material.
Keeping the heat energy in the inner layers means it is available to be poured back in to the oven.
The insulated solution may be able to store the same amount of heat energy as the solution with double the thickness of thermal mass. But I guess the release profile will be quite different.


I then had a further question on the use of layers of tinfoil. I see it used to wrap round the insulating board under the oven
1. to keep the board waterproof - fair enough, but given the roll width will be less than the board, there will still be joins, so hardly an optimal solution, and
2. to help reflect heat back in - what?? tinfoil doesn't keep heat in; try using it for oven gloves! While it would reflect infrared radiation, the heat that's being absorbed through the firebrick structure of the oven won't be in that form. Surely there's little point having the insulated board when the heat will simply whizz along the layer of tinfoil with its hugely rapid heat transfer ability.

Likewise, over the dome, I can see it might help to keep moisture from the render out of the blanket, but it won't keep heat in. In fact, it will allow heat to move more easily around the dome so more can be lost from any weak points as it will aid heat equalisation.

S

[cobblerdave]

Gday
Wow lots of question... Never a bad thing... I'll try to answer as many as I can remember.
Thermal mass. The thickness of the caste refractory in most cast ovens is about 50 mm thick. But it's a higher density and then has the ability to store and finally let back the heat from its mass into the oven. That's if it's insulated. The insulation encapulates the dome and floor and prevents the heat from escaping out. Having only one way to go its back to the inside of the oven.
25 mm floor firebricks. It's a good thickness for your oven. It doesn't need to be too thick. Heat goes up and even thought the fires on the hearth brick it's the last thing to be heated. That's why an active Fire ( with flame) in the oven when cooking pizza as the reflected heat replenishes the heat to the floor as well as cooking the tops of the pizza. My own oven has 75 mm lite duty firebrick as the hearth floor and 115 mm of brick on the dome. Because the densities the same it's a good balance. The density of the caste refractory dome and the floor differ in yours but still balance. 
The extra thermal mass placed over some ovens isn't. It's most probably cement/ pearlite mix, an insulation put over ceramic blanket insulation to.... Add extra insulation and to provide a stronger surface to render.
Most oven manufacturers have a thin metal door. Leaks heat. A full insulated door will prevent the escape of heat from the oven and will complete the full encapsulation of the oven and extend those cooking times.
Damp proofing... The more the better. Regardless of what "s  supplied. Add a waterproof membrane under the slab and under the insulation layer like insulation you can never have to much.
Regards dave
 

[simonj]

Thanks Dave.

Lots of questions, yep, but all boils down to about 1!

Yes, I'll certainly be looking to insulate the door. And also plug / insulate the chimney once the fire has died, as the chimney's in the main chamber.

It will be easy enough for me to get an upgrade from the 25mm fire bricks for the hearth to 30-something, 50-something or 76mm. Are you saying I shouldn't do that because of the balance?

I was thinking of putting an extra layer of cement immediately outside the dome, inside the insulation blanket layer as well. An inch or so.

[cobblerdave]

G'day
My first thoughts were WOOO if he mucks this up he could ruin a working oven. So I decided to have a bit of a think about it.
Your priority is for Baking and general cooking so you would be better to have more mass. You understand that you can get the floor to thick in comparison to the dome, great. I recon you'll get an oven that will give usuable temps over a weekend which is what most people want. Youll have to increase the insulation thickness as well to handle the extra mass of course but that's doable. A rethink of stand side due the the extra thickness of course. I recon Go for it
Regards Dave

[simonj]

Dave,

Thanks for taking the time to think about it.
Of course, I hadn't realised I could get the floor too thick compared to the dome until you mentioned it!
So I'd also never thought about it in terms of ruining a perfectly good oven!

I reckon I'll add some, but in moderation.