So I built the base using the plan and have the void filled with a perlite, vermiculite, Portland blend. I then have bricks on their side around it. Did anyone ever think to add a layer of calcium silicate board before sand and cooking bricks?
Welcome to the BrickWood forums, Chris!
I’m sure someone has thought of it before, but can’t find any posts on it here.
While I’m sure it sounds like a good idea to add more insulation to the insulated base, I don’t think you’d be adding much to the performance of the oven. The material in the void is there to provide a thermal break between the very hot firebrick hearth and the very cold concrete. Vermiculite or Perlite are sufficient to prevent conduction between those materials; without one of them (or a commercial insulating castable product, which is one of the specified alternatives but adds cost) the concrete would tend to draw heat energy from the hearth.
I don’t see a benefit to it for two reasons:
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You’re reducing the space available for sand, which is the leveling surface for the firebrick. The firebrick “ride” on the sand as they expand and contract, and need some play underneath.
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Adding a calcium carbonate board will add about $50 (in really rough terms) to the cost of your build. I’d rather put that $50 into the insulation and materials above the hearth, where it will count, or else keep it in my wallet.
Thanks for the question, Chris, and good luck continuing your build!
Thanks for such a quick reply. I did order an extra box of fibre to double my layer of insulation over the entire oven, so it just got me thinking…If I am adding double the insulation in the back and over the top…Should I also do the bottom.
I had the same thought. I did four in of insulation on the outside but did not add the ceramic board. After making pizza and closing the damper and putting on the door, my oven is between 600 and 550 the next morning. 350 around 2pm.
It seems intuitively true, but physically no need. I did the four layers over my oven as well, because that’s where both flame and heat will be directed and because I’m located in a colder place.
That’s right where you should be with this design. This is an oven that encourages overnight guests. 
I’m envious of this heat retention! I also put on 4 layers of insulation, and have closed the damper (although it doesn’t close completely - there’s still some smoke coming out), but it’s only about 200 degrees the next morning. @Newman, how much is your front closed off? We’re putting more insulation on the standard door today, so we’ll have to see how tonight’s bake goes. Maybe that will help?
I guess I should clarify I did four inches #8 kaowool. Front is completely open (want to be able to fit a big turkey for Thanksgiving) with the standard door but added the gasket. Also, we’re averaging 100 F daytime temp outside right now. I expect this to be different come winter. Without the door, I was getting about 225-250 F the next morning.
As far as still having smoke come out. The damper should let some airflow through. It just limits it.
Your damper won’t choke the oven completely, so some smoke is fine.
What have weather conditions (wind and temperature) been like in your area? As @Newman says, it really helps to have ambient temperatures hovering around 100°F!
When you say “more insulation on the standard door,” are you talking about a gasket, or an actual layer of insulation on it? The latter might help a bit, but it’s really the convective air flow you want to stop. The chimney damper does not stop air flow entirely, by design, so a draft coming in around the edges of the door is like turning on an exhaust fan in a hot room. I can definitely see it cooling your oven down quickly.
Thanks to you both! Took a bit to track down that high-temp gasket, so we’ll have to put it on sometime this week and test it all out next weekend. And it’s good to know that not everyone closed off the fronts.
It’s nowhere close to 100 degrees here in Minnesota - more like 70!
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