Oven floor design: thickness & insulation

[dbhansen]

I've been reading multiple posts discussing the costs/benefits of adding insulation under the oven floor and/or making the floor thicker, but I'm still confused. I'm designing a 42" WFO mostly for bread for family/friend use, but pizza parties too. My wood sources and time are limited. Can anyone explain to me:

(a) Is it beneficial to have more than 2" of FB Board under the oven floor? Can you never have too much insulation under the floor (or over the dome)? I was thinking of using 1 layer of FB Board between the cement hearth and oven floor, but would it be beneficial to put 2-4" of vermiculite concrete in there as well, or 2 layers of insulating board?

(b) In combination with more insulation, what is the effect of adding thickness to the oven floor, such as turning the bricks on their side or adding splits? Longer firings but longer heat retention? What about temperature? I am hoping for relatively quick firings for relatively small quantities of food (a few pizzas and 6-10 loaves of bread). It sounds like the standard 2.25" floor should be fine. Will a thicker floor contribute to scorching or do anything else to the cooking process itself? Should I consider it if I may want to do 2+ batches of bread?

I wish I could just "try out" both and see what works, but it seems there's no turning back once it's built.

[dmun]

Re: Oven floor design: thickness & insulation

2 inches of cal-sil board and flat 2.25 inch fire bricks are fine for domestic ovens. There should be no problems doing two sequential batches of bread, particularly if you plan the bake so that the loaves that like hotter temperatures go first. Keep this in perspective: draw a 42 inch circle on a piece of plywood, or lay down a string on the rug that size, and put a loaf of bread there. That's a big oven for domestic use.

What do I find? It takes a long time for my oven to cool off to bread baking temps, and I've burned loaves of bread because of impatience. More mass would keep me cooling my heels even longer.

On the other hand, a herring bone floor of bricks laid on their edge would look REALLY cool...

[Frances]

Re: Oven floor design: thickness & insulation

On the other hand, a herring bone floor of bricks laid on their edge would look REALLY cool...

I'm sooo glad I didn't read that one before building my oven...

[james]

Re: Oven floor design: thickness & insulation

I was talking with Dave in DC the other day, and I think we came across of good way of describing just how much heat a 2 1/2"-3" oven floor and dome can hold. Think of your oven as a sponge, soaking up water from a spray bottle. You keep straying the sponge on one side, and that side gets wet, but the other side of the sponge is bone dry. If you spray and wait, spray and wait, at some point, the whole sponge will get wet -- or fully saturated.

That is basically what is happening with our wood-fired ovens. You put a lot of heat into the inner face with a large fire, but outer edge of the floor and dome are still cold until the oven absorbs more heat. Just think how much fire and heat it would take to fully saturate a 2 1/2"-3" oven to where it was 800?F all the way to the outer edge. And think about how much heat would be stored for baking.

My point is that there are no residential cooking requirements that cannot be addressed with standard a FB oven (pre-made or Pompeii). If you need more heat, you can fire the oven longer, and it will store more heat.

Back to my sponge analogy. If you spray a thicker sponge with the same amount of water, that water will just be wicked to the outer edge of the sponge, so that none of the sponge will be wet -- it will all be damp (or in oven terms, warm, not hot).

The fatter sponge, or thicker oven, cannot retain water (heat) that you have not put in.

What do you think? Does the analogy work for you?

I know I have said this before, but I think the idea that thicker is better can really get folks off on the wrong foot.

James

[Xabia Jim]

Re: Oven floor design: thickness & insulation

I believe that that is a lesson I've learned here at FB. Too much mass can just wick away your heat.....Thinner dome is ok with insulation and so is a single brick hearth with insulation board under it.....especially for the home oven.

and if you have space and money, double the board under and the blanket over.

[dbhansen]

Re: Oven floor design: thickness & insulation

What do you think? Does the analogy work for you?

Yes, James, that's a great analogy. Thanks. I believe I read some of that in another of your posts a while back but had forgotten about it. I hope to pay you back by purchasing my insulation board from you later in the year (among other things)!

Continuing the analogy ... do you think 4" of insulation board would help that sponge hold much more water than 2"? Or are we only talking about a few drops?

[dmun]

Re: Oven floor design: thickness & insulation

I have 2 1/2 inches of insblock19 under my brick floor, which is a less effective insulator than cal-sil, and my slab never gets more than warm to the touch.

More won't hurt, but after all, heat rises. I'd go for more blanket on the top if you have spare change in your budget.

[dbhansen]

Re: Oven floor design: thickness & insulation

Thanks dmun, I appreciate the input and feel more confident in my design decisions.

I have 2 1/2 inches of insblock19 under my brick floor, which is a less effective insulator than cal-sil, and my slab never gets more than warm to the touch.

More won't hurt, but after all, heat rises. I'd go for more blanket on the top if you have spare change in your budget.

[james]

Re: Oven floor design: thickness & insulation

One more thing I would like to add on floor thickness and keeping the cooking floor hot. I've read lots of postings, talked with people and experienced first hand the challenge of keeping the cooking floor hot when you cook lots of pizza for a party.

Along with keeping your floor the right thickness -- about 2 1/2" and insulating it well, I want to mention how important it is to keep a good fire going the whole time you are cooking. A serious flame lapping into the top of the dome or more. You should never have to stop cooking and drag the fire back over the oven floor to recharge it.

If you picture the Modena G gas-fired oven, it helps make the point. That oven has a monster burner that allows it hold 800?F in the dome and floor, so that it can bake 90 sec - 2 minute pizza all day long, without stopping. What I think it interesting is that the floor is fully heated by heat reflecting off the dome and bouncing down onto the floor. It never has a fire started in the middle of the floor and moved to the side. For me, that really underscores the importance of having a good fire while you are baking pizza.

So next time you have a big party -- keep a good flaming going the whole time you are cooking.

James

[dmun]

Re: Oven floor design: thickness & insulation

I saw a video, I forget where, of That Famous Pizzaria in Naples. Under the oven there was a pile of dried wood chips, which of course would flame up much more quickly than logs would.

I've been surprised how much heat comes from above. You can put chops in the oven and grill them on top just like in a broiler. The bottom, on a rack, hardly browns at all.

[RTflorida]

Re: Oven floor design: thickness & insulation

James, good point about keeping a good fire going throughout your pizza bakes; I always have, but lately I have kept a bigger fire (basically because my current batch of logs are larger). On a couple of my earliest multiple pizza bakes, I had to do exactly as you mention - pull the coals back over the cleared area to recharge.
Another point I've learned - letting my oven "charge" a little longer after it burns white. Initially, I waited 10-15 minutes; now I go about 20. No more issues (as of late) with the hearth cooling down before all of the pizzas are done.

RT

[gjbingham]

Re: Oven floor design: thickness & insulation

Great thread!

Dmun says heat rises. I think statement is entirely correct wrt heat in air. My oven's stand gets warm by the next day, so heat certainly sinks towards cooler areas as well. Ever sit on a big cold rock? It's not the rock that makes your butt cold, but the fact that heat flows from warm to cold, and you actually loose heat to the rock, and your butt gets cold.
In a well insulated and closed oven, the heat has to go somewhere, otherwise the oven would stay hot for eternity. While I didn't build a perfectly thermodynamically perfect oven, I'm still not happy with the way the oven floor cools so much faster than the dome. I'll keep working on moving my fire to the side so that it keeps burning hot. Seems like when I move it over and add a log, it burns hot for about 10 minutes then afterward, is really hard to keep the fire going really hot. Probably just poor oven management.
Comments, recommendations? As I said, gread thread! Thanks!

[james]

Re: Oven floor design: thickness & insulation

Hey George,

You should be able to add a smallish piece (2-3") of wood every 15-20 minutes, and have it combust pretty much on impact. That way you add flame and heat, without damping the fire until the new piece catches.

What do you think? A bigger fire? Of perhaps slightly smaller wood, so that it is ready to catch fire? More aged wood that is ready to catch?

Good thread.
James

[RTflorida]

Re: Oven floor design: thickness & insulation

Keeping a bigger fire (not huge or scary) has been a big help to me. I started out letting my fires die down too much ,I think. I've never had issues with logs catching once I 've reached the high temps, they DO catch almost instantly. My problem was not keeping at least one good, fresh log on the fire...doing so has made a big difference recently.
I would also like to add, if you have the opportunity to get your hands on some seasoned citrus, jump on it. There is a pretty big learning curve because it burns so long and hot (my first fire I thought the whole oven was going to melt), but my wood consumption is literally 1/2 of what it is when I use oak or hickory. Maybe the citrus is the reason I am having no problems with hearth temps dropping after several pizzas...probably a combination of the changes I've made, I'm not sure.

RT

[Xabia Jim]

Re: Oven floor design: thickness & insulation

I would also like to add, if you have the opportunity to get your hands on some seasoned citrus, jump on it. There is a pretty big learning curve because it burns so long and hot
RT

Interesting, my spanish friends told me the best wood for Paella was orange wood....I'll need to try that!