Insulation Thickness

[wisnoskij]

Here goes again, this did not seem to submit last time.

I have been trying to figure out how to calculate how much insulation to use.

If I use Insulating Fire-brick for example, What thickness is thick enough to fully insulate a wood-fired oven?


Specifically I am thinking that I am not really interested in heating up a few hundred pounds of bricks, so am thinking of the internal just being straight insulating fire-brick. I understand the point of the heat absorption/retaining, but would rather just have direct fire heat.

And particularly for the dome roof, it would be sooo much simpler, lighter, and cheaper to just go one straight layer. And I love the look of brick. If I just went one layer of IRB How thick would I want this, specifically for short burnings? I am not running a commercial operation here, I am really thinking a normal use case would be a few hours.

Also, it is a lot of a softer product than dense FB, so would that mean that I would not want it as my oven floor, as all the metal tools and wear and tear would damage it a little too fast? I think it is still supposed to be about normal brick strength, so it should not be too bad.

the big bonus of just fire-brick insulation is the price, if I put anything else, not only do I have to pay for that, but than probably also some a layer of normal brick (or whatever to cover it and protect it form the weather). And then it is like 3 times heavier, and I need to increase the foundation.

I understand that they can be a lot more expensive, and cost is a huge issue, but I am thinking because I might be able to significantly cut down on the amount of materials needed, it is so elegantly serves what I what for a oven, that it might work out the best for me.

what are your opinions on this?

[boerwarrior]

Re: Insulation Thickness

Hi Wisnoskij

I'm not sure I understand your question...

are you basically asking if you can build your oven out of insulation? I'm not sure why you would want to do that - it would be like cooking food in a regular fireplace with the only heat coming from the fire. If you want to do that you are probably wasting time and money by following the plans and advice here - you could simply build a brick "box."

Forgive me if I have misunderstood!

[Tscarborough]

Re: Insulation Thickness

As stated, the entire point of a wood fired oven is it's ability to retain heat. You can make the mass thinner, but not really less dense. IFB are 4 to 5 times the cost of low duty firebrick, and are very soft. The firebrick are cheap, and you can use cheap insulation, too (perlite).

[wisnoskij]

Re: Insulation Thickness

"cooking food in a regular fireplace with the only heat coming from the fire"
yes, but there would be insulation holding the heat in.

I am not some fancy chef, all I want to the ability to cook with a wood fire and for it to be as economical and efficient as possible. So it would use the same basic design and materials, but without the heat holding ability. I think this can be as efficient longterm and even more efficient for short burns than using a large mass of heat storing FB. But I might be wrong.

And it would be far far harder to maintain a steady heat. But for the most part 350 degrees is a pretty arbitrary temperature, 400 or 300 are close enough.

Am I being completely crazy?

[wisnoskij]

Re: Insulation Thickness

Also I have no idea how you could ever use Perlite as "Perlite is a type of volcanic glass that expands four to twenty times its original volume and becomes porous when heated."
Umm, so if I put perlite in a wall that wall will explode outwards the first time that it gets hot?

As 1 inch of insulation will become 4 to 1 foot 8 inches thick when heated???

[rsandler]

Re: Insulation Thickness

"cooking food in a regular fireplace with the only heat coming from the fire"
yes, but there would be insulation holding the heat in.

Holding it in where, exactly? The air in the oven isn't going to stay put. You need somewhere to store heat for the insulation to do any good. Part of the point of a brick oven is that you get all three kinds of heat--radiant heat from the dome, conduction from the brick directly into the food, and convection as the hot air blows around the dome. Without some kind of thermal mass, you lose the conduction and radiant heat.

If what you're really getting at is an oven that heats up quickly, and don't care about heat retention, a thin-shelled dome may be the way to go. FornoBravo sells a couple of small, fully assembled, cast domes that might fit the bill. A steel dome might also suit. Theoretically you could build a thin dome out of dense firebricks, but I'm not sure how sturdy this would be--Tscarborough or someone else who actually knows about these things would have to chime in.

That said, you mentioned wanting simple cooking, and some of the best food I've made in my oven is by taking a big hunk of meat, throwing it in the oven after it's cooled down below 300F, and coming back the next day to cut it up for dinner.

[rsandler]

Re: Insulation Thickness

Also I have no idea how you could ever use Perlite as "Perlite is a type of volcanic glass that expands four to twenty times its original volume and becomes porous when heated."
Umm, so if I put perlite in a wall that wall will explode outwards the first time that it gets hot?

As 1 inch of insulation will become 4 to 1 foot 8 inches thick when heated???

As far as I can tell, it is typically expanded perlite that is sold in stores, so you wouldn't have that problem. In any event, perlite undergoes expansion at over 850C, and there is absolutely no way you'd get the outer surface of a brick oven that hot. Perlite-concrete ("Perlcrete") is frequently used as insulation on brick ovens. Same idea as using vermiculite.

[Tscarborough]

Re: Insulation Thickness

" But I might be wrong."

You are wrong. Fire works by consuming oxygen and wood, thus, you will be venting the hot air and drawing in cold air. Just buy a weber grill and call it good.

[wisnoskij]

Re: Insulation Thickness

"Holding it in where, exactly? The air in the oven isn't going to stay put. You need somewhere to store heat for the insulation to do any good. Part of the point of a brick oven is that you get all three kinds of heat--radiant heat from the dome, conduction from the brick directly into the food, and convection as the hot air blows around the dome. Without some kind of thermal mass, you lose the conduction and radiant heat."

But that is the same problem that your oven will have when fully heated? The only difference between yours and mine is that yours quickly absorbs the heat out of the air and take a while to heat up, while mine does so quickly and requires a fire to keep warm. And specifically what I was thinking was to have the out vent below the ceiling hight, so it was more of the slightly colder hot air that escaped, than the warmest top layer.

"You are wrong. Fire works by consuming oxygen and wood, thus, you will be venting the hot air and drawing in cold air."
That is how all wood fire ovens work...

But I think you have convinced me, if no one has ever heard of a successful insulation only oven, it is probably because it was a stupid idea.

OK, so say you have convinced me. But if I still think that my normal use case is rather short (I am not opening a commercial pizzeria or a bakery), that does mean that I should use less heat storage mass, right? I would not want a foot of FB, I would want an inch or two?


And thanks everyone for your help so far.

[david s]

Re: Insulation Thickness

IFB is used successfully as both the inner refractory and insulation for kilns fired to stoneware temperatures (1200+ C) at 4" thick walls.I can't see why they wouldn't work in an oven. The won't store much heat, but if your intention is to use the active fire to cook the top of the pizza I think it would work ok. The downside as has already been stated is that they are expensive and abrade easily. They would not be suitable for the floor though. You really do need heat storage there to cook the base of the pizza as well as having a more durable surface. Dense firebrick is the better solution here.

[wisnoskij]

Re: Insulation Thickness

The main difference, I imagine David S, is that a wood oven needs venting for cold air in and hot out. I guess the heat absorption of the walls is so important because it can quickly draw the heat out before it escapes out the chimney.

SO maybe it is important.

[shuboyje]

Re: Insulation Thickness

Not only have I not seen an oven successfully built this way, I've seen one built this way that did not work.

A Few years ago a very respected member of another forum with multiple wood fired ovens and lots of wood fired oven knowledge had a small insulation only oven built to see how it would work. He had it built by a friend who had a business doing poured in place insulating chimneys using a very high quality insulating castable refractory. The refractory he used was a better insulator and stronger material then any other I have seen before or since. The resulting oven was beautifully built. It just didn't work. Problem being with no mass in the dome to store heat the floor quickly looses temperature and cannot regain it fast enough. That is why wood fired ovens have more mass in the doem then the floor. Heat rises, that heat is stored in the dome's mass. Heat also moves from hot to cold, so as the floor looses temperature the stored heat in the dome reheats it. The heat from the fire reheats the dome, because again, heat rises. With mass, insulation and fire you can keep this going perpetually. Without the mass it falls apart fast.

Don't reinvent the wheel. Build a low mass oven and insulate well. Instead of bricks halved making 4.5" of mass cut them in thirds for 3". For insulation most hardware stores now carry mineral wool batts with Roxul being the most common. It's cheap, efficient and able to handle the temperature. Use homebrew mortar to save more money. Insulate under the floor with perlite mixed with portland cement. I'd bet you can build the functional part of a 36" oven for about $250 this way, it will heat fast and work perfectly. Obviously the stand and finish will drive the cost up or way up depending on what you do, but you will need those with any oven.

[wisnoskij]

Re: Insulation Thickness

Great post shuboyje.

Not sure if I should just make a new thread, but on to another topic, alternate designs.

A barrel arch oven, because I like brick arches and domes sound a lot harder, with a trench in the middle, so you would put the food on some sort of slab over top of the trench, and the fire could be very easily tended and kept separate??

Not exactly sure what the difference is between dome and barrel, I am reading some things where people are listing all these difference that simply do not make any sense. I am thinking if you make them the same size, and mass that they should be identical except for shape.

Is this another stupid idea?

[david s]

Re: Insulation Thickness

The main difference, I imagine David S, is that a wood oven needs venting for cold air in and hot out. I guess the heat absorption of the walls is so important because it can quickly draw the heat out before it escapes out the chimney.

SO maybe it is important.

No difference really, I was thinking of LPG fired kilns using IFB's and they are the same, cold air in, hot out. The mass of the wares inside has the effect of storing thermal mass which you wouldn't have in an oven. I think Shuboyje has given you the definitive answer. It has been tried and doesn't work well. You would not be the first (me included) to think that you can improve on 2000 yrs of Roman ingenuity. Stick to the tried and proven that works.

[stonecutter]

Re: Insulation Thickness

Great post shuboyje.

Not sure if I should just make a new thread, but on to another topic, alternate designs.

A barrel arch oven, because I like brick arches and domes sound a lot harder, with a trench in the middle, so you would put the food on some sort of slab over top of the trench, and the fire could be very easily tended and kept separate??

Not exactly sure what the difference is between dome and barrel, I am reading some things where people are listing all these difference that simply do not make any sense. I am thinking if you make them the same size, and mass that they should be identical except for shape.

Is this another stupid idea?

A barrel vault is simpler to build than a dome, certain domes are easier to build than others. Like David said, you get the mass to temp and it doesn't matter much what shape you have.

What it comes down to is how you would like to use the oven. But for strictly pizza, low dome, thinner mass (2"-3") would be the most efficient. Insulation is a must for any oven that isn't used on a daily basis, which most home ovens are not.