Alan Scott 36 x 48 oven Insulation board under hearth?

[TJ Renk]

I am building the Alan Scott oven and need some advice on using FB insulation board on top of my hearth slab. The plan does not call for it but Insulation board seems to be the standard now. Can this go directly on my hearth slab and then set my firebrick hearth on top of the board? Confused on how to integrate the Insulation board into the hearth sandwich. Thanks, everyone.
TJ

[david s]

An Alan Scott oven with its huge thermal mass is suited to multiple batches of bread on a daily basis and if used in this way does not get a chance to cool down. If this is your purpose then fine, however if you want to cook half dozen pizzas on a saturday night or the occasional Sunday roast it is not a good choice as time taken to reach temperature as well as fuel consumed hardly makes it a versatile use. You should find builds on this site as there have been a few who've followed this design. It has a suspended concrete slab which acts as a giant heat sink so works in a different way to most of the ovens we build here which try to prevent heat from conducting to the supporting slab.

https://community.fornobravo.com/for...oven#post13997
https://community.fornobravo.com/forum/pizza-oven-design-and-installation/other-oven-types/413913-alan-scott-oven-project-begins#post413913

[JRPizza]

TJ Renk you could place your board directly on your hearth slab but if there is a chance you are going to get water on the slab most folks put down tile, foam glass, or something else first that won't wick water so their board does not get soaked. Some even drill weep holes in the hearth to drain any water that gets in. You also need to not only put your floor bricks on insulation but your barrel vault bricks also should not touch the hearth to avoid the major heat loss issues david s discusses above. Any brick that is going to retain heat for cooking should be isolated from either the slab or outside air.

[TJ Renk]

An Alan Scott oven with its huge thermal mass is suited to multiple batches of bread on a daily basis and if used in this way does not get a chance to cool down. If this is your purpose then fine, however if you want to cook half dozen pizzas on a saturday night or the occasional Sunday roast it is not a good choice as time taken to reach temperature as well as fuel consumed hardly makes it a versatile use. You should find builds on this site as there have been a few who've followed this design. It has a suspended concrete slab which acts as a giant heat sink so works in a different way to most of the ovens we build here which try to prevent heat from conducting to the supporting slab.

https://community.fornobravo.com/for...oven#post13997
https://community.fornobravo.com/forum/pizza-oven-design-and-installation/other-oven-types/413913-alan-scott-oven-project-begins#post413913

Thanks for the advise. I am all set on the AS. A neighbor built one and its awesome and it is up to temperature in under two hours.. He did recommend the insulation under hearth, he did not do this as it was built years ago, but he was not sure where to place it so I guessed someone did it here.

[TJ Renk]

TJ Renk you could place your board directly on your hearth slab but if there is a chance you are going to get water on the slab most folks put down tile, foam glass, or something else first that won't wick water so their board does not get soaked. Some even drill weep holes in the hearth to drain any water that gets in. You also need to not only put your floor bricks on insulation but your barrel vault bricks also should not touch the hearth to avoid the major heat loss issues david s discusses above. Any brick that is going to retain heat for cooking should be isolated from either the slab or outside air.

ok, thanks. I would think getting water in a BO would be catastrophic. How would that happen? I will look into the foam glass- not familiar.

[JRPizza]

The original AS design, at least from "The Bread Builders" did not have insulation between the oven and hearth interface but did isolate the hearth some with the suspended design. If you want to enjoy multi-day cooking you need to have your heated bricks in a cocoon of insulation, including the arch bricks.
As far as water goes it is difficult to waterproof an oven and if rain hits the hearth it will try to find it's way under the floor. Best bet is a roof to keep all the rain away, second best is a dog house design where all you have to really worry about is rain getting in past the door.

[TJ Renk]

Thank you. I like the idea of the Ceramic fireboard under the hearth bricks. Is 2" thickness enough? I found this product: Will it work?

[JRPizza]

The properties they list look similar to what I used, but you should shoot them an inquiry about compressive strength. If it is supporting the weight of your dome you want to make sure the bricks don't sink into it

[david s]

Cost is always an important factor and many builders opt to cast their own perlcrete or vermicrete slab largely because of this factor. It’s disadvantage is the time and energy required to remove excess water, both during initial build and also during service should it pick up significant amounts of water from weather.
An option that would help is to use the silicon coated perlite which reduces water absorption. This product is not available to me so I can’t tell you to what extent it reduces absorption or the additional cost above the standard perlite. If I could get my hands on the stuff I’d consider making some tests to assess if the performance and cost saving were viable.
Another option that you might like to explore is to use waterproof cal sil board.

[RJB]

An Alan Scott oven with its huge thermal mass is suited to multiple batches of bread on a daily basis and if used in this way does not get a chance to cool down. If this is your purpose then fine, however if you want to cook half dozen pizzas on a saturday night or the occasional Sunday roast it is not a good choice as time taken to reach temperature as well as fuel consumed hardly makes it a versatile use. You should find builds on this site as there have been a few who've followed this design. It has a suspended concrete slab which acts as a giant heat sink so works in a different way to most of the ovens we build here which try to prevent heat from conducting to the supporting slab.

https://community.fornobravo.com/for...oven#post13997
https://community.fornobravo.com/forum/pizza-oven-design-and-installation/other-oven-types/413913-alan-scott-oven-project-begins#post413913

Hello David. I'd like to jump in here if I might and ask a question as I'm not sure I'm understanding completely. You mention that Allan's oven has a lot of thermal mass and that his floor would act as a heat sink. But from what I understand reading his book and going over the photos of his build it appears he has an insulation layer under his hearth floor consisting of 40mm of calcium silicate. Is that not enough insulation, would more be better?
TIA for any input you can share.

Rick

[david s]

For those following an Alan Scott oven, the plans or their interpretation have a number of variations. If the insulation is placed under the supporting slab, the heat will conduct into the supporting slab making it a heat sink. The more common approach is to place the insulation between the firebrick cooking floor and the supporting slab, thereby isolating the supporting slab as a heat sink. The former configuration will be slower to heat up, but once heated will retain heat for longer. Thus making it more suited as a bread oven which does not require the much higher floor temperature needed to cook pizza. You don’t indicate what your main cooking use will be, but this is an important design issue to address before you launch any further.If your intention is to bake multiple batches of bread for the village, then the Alan Scott design may suit you. If you want an all round good performer the FB Pompeii is a better choice. It’s just like boat design pros and cons and compromise in between.