The thicker the walls of a wood fired pizza oven the greater the thermal mass the oven will have. The thermal mass of a traditional oven in conjunction with the effectiveness of the insulation determines how long the oven takes to heat up, the amount of thermal energy that can be stored, and the duration that will take the oven to cool down. A long cool down duration will allow for the cooking of different food items sequentially after the fire has expired. For example an oven brought to temperature for cooking pizza may then be used for roasting, and baking of bread, and then finally drying of fruits such as tomatoes at the later stages. Generally it is recommended that that for a personal home use oven the dome have a thickness of 4 inches (10 cm or 1 fire brick) and the insulation also be 4 inches (10cm) thick.
The material used for the walls and for the insulation additionally affects the oven effectiveness along with the thickness. A medium such as fire brick which has a very high bulk density and heat conductive properties requires less thickness than a less effective medium such as cob. Similarly with the insulation, fire blanket is generally more effective than insulating fire brick or vermiculite cement, and thus the thickness chosen will depend on the material chosen. When choosing the wall and insulation material the efficiency of the material, the initial costs and ongoing operation costs, and the types of food to be cooked should all be considered. The below table gives example thicknesses, heat up and cool down times for an oven constructed out of fire brick and vermiculite cement as insulation.
I believe the measurement in cm's on third line has to bel 5 cm dome, 10 cm insulation. Am i wright?
That's right. The conversions were incorrect. These are just estimates however generally the outcome is that oven wall thickness will slow heat up duration and increase the time it takes for the oven to cool down due to thermal mass. Insulation will multiply this effect.
newbie question; can the dome be made with the full length of the brick with the small end facing inside the dome, and can the large spacing of brick on the outside of the dome be filled with fire brick cut into wedges with a thin layer of mortar or is this unnecessary overkill.
Yeah that's the normal approach. You should have the length of the brick towards the inside and outside of the dome so that the small surface area of the brick is visible internally. If you're concerned about the long bricks being difficult to stack you can use a mallet to break the brick in half and use half bricks to create the dome. Display the clean ends of the broken bricks internally for a nice wall. The cut ends of the brick will be covered by the insulation.