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Thinking about putting sand under a pizza oven cooking floor reminds you that there are two reasons to insulate something—either to keep heat where you want it, or to keep it from getting to somewhere you don’t want. Sand is the later case.

There are some very old, and out of date pizza oven installation plans that tell you to put a layer of sand, maybe 2-4″ thick under your cooking floor. They said to build a concrete heath, and then create an outer ring of brick that you fill with sand, and the place the cooking floor on that thick layer of sand.

Just say no. Don’t do it.

Sand is not an insulator, it’s a thermal mass, and it will absorb the heat you are putting in your oven with its fire, wicking it away from the oven chamber where you want it. Because the sand layer is so thick, it will protect your concrete hearth from getting too hot, and breaking down, but that is only because it’s a big heat sink—a big sponge soaking up the heat from your oven floor. After a long time of firing your oven, the sand will eventually get hot, but only after it has made your oven heat up time much longer.

A better plan is to design and build your oven with a cooking floor with a thickness that you want, that provides the balance of fast heat up time and heat retention that works for you. And then aggressively insulate that cooking floor, preferably with CalSil board, and optionally vermiculite or perlite concrete, to keep the heat inside your oven, where you can enjoy cooking with you. Not a design where your heat is constantly wicking into a sand heat sink, away from your oven chamber.