Floor Thickness & Insulation

[Wellsy]

Hi all,

First time posting here after spending a lot of time reading through the forum over the years.

I’m finally getting started on building a small oven and was hoping to get a bit of clarification on a couple of things.

My current plan is to use 50mm calcium silicate board with 75mm fire bricks on top. I’ve read a lot of mixed opinions on whether 50mm is enough. Some say it’s fine and others say 100mm is the better option, so I’m a bit unsure whether 50mm will do the job or whether I’d be cutting myself short just to save about $100.

I’ve also seen a local builder use 50mm calcium silicate board, then 25mm of castable mix on top, followed by the fire bricks. Just wondering what people think of that method as well.

We’re hoping to use the oven for more than just pizza. My wife is keen on doing roasts and bread too, and ideally we’d like to get a few days of cooking out of it, so heat retention is pretty important.

Any advice would be much appreciated. Thanks.

[SableSprings]

Welcome to the forum community Wellsy! For most of us, 50mm (~2") is the recommended amount of the CalSil board beneath the cooking floor. When properly insulated over the dome (100mm ceramic batting) and with an insulated door, you can expect to do at least 2 days of cooking from retained heat. As with all things, there is a diminishing return on the amount of insulation applied to your build. We have had several members, super-insulate by using twice the normal amount of board & batting. They have been able to retain baking capability for 3-4 days (albeit, you're looking at lower temp bake items). Most important (no matter how much insulation you have) is keeping that insulation dry. There are some render and mortar/cement additives that are pretty good at keeping the water/moisture out, but ultimately the best option is a roof over your oven area. Also, having a covered area makes using the oven during rainy & cold weather a lot more fun (and if you screen the sides, less flies/yellow jackets in the summer ). Just as an example, for a bread run (3-4 oven loads), I would build my fire late at night and dampen it down with the door. In the early afternoon, I'd plan my baguettes (2 loads of 10) to go in at ~285-290C, followed by a load of 4-5 large whole wheat rounds (~230C), finishing with some nice light loaves/buns (205-210C). Obviously, once you've cleared out your coals to bake breads, you are working with a limited amount of stored heat...the more items to be baked that you put in...the more heat you transfer/lose from your cooking floor. If you do pizza for a dinner then close up the oven...the next day you'd easily be able to do some breads followed by a roast chicken/pork/beef with roasted veggies. It all becomes about heat management and planning for the succession of temperatures in the cooking chamber over time. The nice thing is that a relatively small fire will boost a still warm oven up pretty quickly.

As to the CalSil board with castable mix+firebricks - it adds to the amount of thermal mass and thereby will require more wood/longer burn time to bring it up to temp. That design would definitely be one I'd recommend installing the higher amounts of top & bottom insulation since you'd be targeting large bake runs with small daily "refresh burns". I hope that helps a little. Looking forward to your progress posts!