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Do I need to add extra thermal mass under my oven floor?

No. You don’t.

It’s a shame that oven kit producers and pizza oven plans recommend this wasteful step. To understand why, consider how a pizza oven works. The oven floor acts like a battery, absorbing heat from the oven’s gas or wood fire, and then releasing that heat into your pizza, bread and cooking pans. At first, your floor is a sponge absorbing all the heat to fully charge, or load itself with heat. The floor becomes hot at the inner edge, and over time, high temperature reaches the middle, and ultimate the outside edge of the floor’s thermal mass.

At that point, the heat is stopped by the floor’s insulating layer.

The real question is how much thermal mass do you want, or need in your cooking floor. And for a vast majority of cases, you can choose a floor material (firebrick or Biscotto stone) that is thick enough for you. For example, almost all residential pizza ovens work extremely well with a 1.6-2″ cooking floor. That strikes the perfect balance between fast heat-up times and heat retention. Mid-size (24-32″) refractory ovens can heat up to pizza making temperatures in 20-30 minutes, and easily hold enough heat for unlimited pizza making in a row and more bread than you could ever eat. Similarly, large commercial pizza ovens typically have a cooking floor that is 2 1/2″, which can readily provide the pizza making throughput required by the largest pizzeria.

There is only one case where additional thermal mass is required under the cooking floor—a commercial bread oven. In that case, the 4.5″ cooking floor (firebrick standing upright on its long side) is set on an additional 3 1/2″ concrete thermal layer, which is in turn sitting on a layer of vermiculite concrete.

Please, don’t waste your time or money building a brick or metal border and filling it with sand or refractory concrete. It isn’t necessary. Simple make sure your oven floor is thick enough for your needs (2″ is almost certainly enough) and place your cooking floor directly on your CalSil floor insulation.